The Bunny Tanks Up

Part 3: Fuel Tanks

Visitors= 

I'd considered building extra large tanks, and got a lot of information from RV-list members regarding that. Van's of course wouldn't support the idea. Now that I've built part of my wings, I wouldn't recommend that idea to anyone who needs to read this Guide. There's a lot of sheetmetal design work involved, and you'd be very much on your own.

Joe Larson <jpl@showpg.mn.org> posted this handy "LR Tank decision-making guide" to the RV-List:

  1. What kind of range will your ordinary main tanks give you? ____
  2. How many hours is this in the air?                          ____
  3. How long can you fly without needing to stop and pee?       ____
  4. If the answer to 3 is less than the answer to 2, check this box [  ]; and if you will be the only pilot of this aircraft, stop here. You don't need aux tanks.
  5. How much will the aux tank system weigh when empty?         ____
  6. How much does one pound of weight reduce your range?        ____
  7. Multiply line 5 by line 6.                                  ____
  8. Subtract line 7 from line 1. This answer is the amount your normal range will be after installing the aux system.  ____
  9. If line 8 is shorter than your common destinations, and line 1 is not shorter than your common destinations, check both of these boxes  [  ] [  ]
       If you continue to fill out this form, you will be audited.
  10. How much fuel will the aux tanks hold?                      ____
  11. How far can you fly on one gallon of fuel?                  ____
  12. Multiply line 10 by line 11. This answer is the amount your range will be extended if you add aux tanks.           ____
  13. If line 12 doesn't get you any place line 1 already reaches, check this box          [  ]
  14. Will you be flying over moutainous terrain? If NOT, check this box  [  ]
  15. Will you be flying over vast stretches of uninhabited country? If NOT, check this box  [  ]
  16. Will you be flying over large bodies of water? If NOT, check this box  [  ]
  17. How much time will it take you to design and install an aux fuel system?                                            ____
  18. Multiply line 17 by $20.                                    ____
  19. How much will you spend on parts for the aux fuel system?   ____
  20. Add lines 18 and 19. This is the cost of the aux system.   ____
  21. If line 20 is greater than $800, check this box [  ]
  22. How many hours per week have you spent on construction?     ____
  23. No, honestly, how many hours per week, really?              ____
  24. Divide line 17 by line 23, and multiply the result by 1.3, because you're still lying. This is the number of weeks adding an aux fuel system will add to your construction.    ____
  25. Count the number of boxes you have checked in lines 4, 9, 13, 14, 15, 16 and 21.                                      ____
  26. Subtract line 25 from the number "8"  (8 - line 25)         ____
  27. Multiply line 26 by 3: ____  If this is greater than the number in line 24, check this box [  ]
  28. If line 27 is checked, add one to line 25 and write the result here, otherwise just write line 25's value here.     ____
  29. Each box checked represents a strike against installing aux tanks. If line 28 is 3 or less, you have a strong case for installing aux tanks. If line 28 is 6 or more, you'd be installing aux tanks for very little gain. If the answer is somewhere in between, flip a coin.


RV-List message posted by: "Brian Eckstein" <eckstein@net-link.net>: With all the talk about extra fuel capacity, I thought I'd chime in since I did something about it. This is something I wrote up for the occasional inquiry. Note that there is no extra plumbing.

Long Range Fuel Tanks for RV6

First, why would someone want to deviate from a well proven design for the few times that more than 3+ hours of fuel is needed?  I like to fly into small grass airports that don?t sell fuel and I always want to return AND I plan on long cross country flight. When I drive, I often go for over 5 hours without stopping, except for gas. So, for me it makes sense. The challenge becomes designing it in a safe way that doesn't compromise a well designed airplane. I consulted with an aviation professor who teaches at a major aviation university about adding weight to the wings, and his reply was that distributing the weight out the wings will result in a smoother flying machine. Just don't exceed gross weight.

Vans fuel tank design uses a rib spacing of 9.1 inches. I don't know why he chose 9.1, but I'm comfortable that is provides a solid structure. Vans doesn't like to discuss modifications to his designs, so I didn't ask - why 9.1". Starting from this basic dimension, the next decision became: Should I extend it 9.1, 18.2, 27.3,...  I chose 18.2. This will increase fuel capacity from 38 to 53 gallons. The baffle plate that the ribs attach to must be custom bent to accommodate the new, longer tank. In my case it had to be 64.95 inches long.  The original is 46.75, add to that 18.2 and you get 64.95. I bought the sheet stock from Vans and had them bent locally. The extra ribs are just stock items from Vans.

One of the very nice features of an RV is the pre-bent leading edge skins that fit so nice. One could buy sheet stock large enough to cover the entire tank and bend them yourself, or one could add an extension to the provided tank skin. I chose to add an extension. So, my tanks have two piece skins with a seam. I consulted with a military and commercial A&P mechanic about the seam and he said that seams are common in big airplanes. Just be sure to proseal properly. Keeping Vans skin meant that I only had to bend the 18.7 inch extension.  Why 18.7? Because I had to cut .5 from the original to accommodate the seam. It was hard enough to bend the 18.7 inches, I would not want to try to bend 66" skins without special tools that I don't have.  Under the seam are back to back ribs riveted together. On top of the back to back ribs is a strip of .016 aluminum. I chose to use the Vans supplied skins for the outboard of the tanks and put the extensions inboard covering the first three ribs. The reason I chos [something lost here].

On a RV6, the tank is mounted to the spar attaching to both the rear facing large flange and a smaller forward facing flange.  The length of this smaller one is determined by the length of the tanks.  If you lengthen the tanks, you must lengthen the mounting flange.  I had 18.2" extensions bent locally to accomplish this. These extensions were riveted right into the spar along with the other spar components.

The leading edge needs to be shortened by an equal amount.  This will require cutting the leading edge skin, placing the undersized rib (W-408-1R/L) at a new location. This will require an additional hole in the spar flanges. I asked Vans about this hole and they said: not a problem where I wanted it.

I advise anyone who decides to lengthen their tanks by this way to carefully draw out the way they want it to go together.  There are details I have not covered here which should be clear if you think about it and put the dimensions on paper.


Tank Construction

Drilling stiffeners to tank skin (27KB)The notes say to ensure that the reinforcing ring attach holes aren't too close to the tooling hole in rib 1. Since they're pre-drilled in the ring and panel, and the panel can only go on one way, you don't get a lot of choice.

Marking and drilling the ribs to rear baffle is done pretty much the same as drilling the main skins to the main spar... mark one end-rib centreline on the rear baffle, then clamp the baffle to the skin to mark the other rib centrelines through the skin's pre-drilled holes.

Be extra careful when drilling ribs to baffle... the tank rib layout isn't symmetrical. Ribs 1 through 5 are only 9 1/8" apart, whereas ribs 5 and 6 are 9 3/4" apart. Bottom line; if you drill your ribs onto the baffle upside-down (or inside-out or however you want to think of it), you'll end up with a baffle/rib assembly that'll match the *other* skin. Anyone wanna buy a spare right-tank baffle. :-(

I did a whole lot of measuring and eyeballing to try to centre the fuel filler flange under the prepunched hole. In the end, it was easy -- leave the cap inside the flange when fitting. The prepunched hole in the skin is only just larger than the cap, so this will ensure the flange is centred. As noted by WC, be wary that the flange is assymetric -- the top surface curves fore-and-aft to match the curve of the skin.

Similarly with the drain flanges -- I enlarged the drain hole very slightly, so that the drain could fit through and screw into the flange and hold it in place whilst drilling. Note that the drain flange attach holes should not be dimpled. WC says this, but for some of us, reading it once isn't enough!

Having completed the tank skeletons, I would like to caution you to do some measuring when your main spar is in the jig. If I remember correctly, there was a mention in the instructions that measurements relating to ribs had to be made from a reference where the web curved under between the straps.

Be extra careful with rib preparation and later with drilling alignment - you don't want to deal with edge problems here. I used an ice pick to get spot-on alignment.

Study the drawing carefully as there are some really critical alignments and edge placements you want to achieve.

The tank skin fit to the rear baffle and doubler flange is critical. They really don't give you a lot of leeway here. If the skin is too far forward, the rearmost screw holes will interfere with the spar stiffeners and/or the front screws won't hit the doubler correctly and/or the rivets will be too close to the baffle web. If the skin is too far aft, the front screw holes will hit the doubler web and/or miss the rear baffle.Drilling tank skin to rear baffle (21KB)

donspawn@juno.com (Don R. Jordan) wrote to the RV-list: You want your baffle supported with 2- 7/8 inch wood blocks (cut a dado for the rivet heads). If your LE section is right, the tank skin should fit.  Main thing is: does the LE & tank leading edge end up straight?  The edge of #6 rib should butt up against the W-623 strip. If there is a taper between the tank & LE, file to fit. After the tank lines up right , you can file tank, LE, or main skins to fit. Roll your edges that need it just prior to drilling.

I used a couple of strips of wood 4' x 1/2" x 7/8" as spacers. The edges needed rounding off so that they'd fit snugly into the corner of the doubler. I also made some notches so that they'd clear the bolt and rivet heads in the spar. These spacers could then be shimmed to get the right spacing between the tank baffle and the doubler.

Test-fit the tank skin and rear baffle on the spar and check out what screws go where. Mark the rivet and screw drill lines on the baffle so that you can see them through the skin pre-drilled holes. Also, mark the screw line on the W-623 flange strips. And note where the screws will go into the doubler flange, and check where that is relative to the spar stiffeners. Note that there's no screws into the spar inboard of main rib 5.Drilling tank skin top to ribs (31KB)Drilling tank skin bottom to ribs (17KB)

When drilling the skins to the ribs, it would be extremely useful to have an assistant to push the ribs around from the other side. I didn't, and did a lot of to-ing and fro-ing to the other side of the wing while working on my first tank. On the second, I tied strings to the ribs through the vent holes and fed them out the filler hole. That allowed me to pull the rib into alignment and hold it there while I drilled. Even then, I must have got in and out under the skins at least a dozen times.

RV-List message posted by: Kevin Horton <khorton@cyberus.ca>: I found that I could lift up the bottom aft edge of the skin a tiny little bit (even with the straps done up pretty tight) and push one end of a steel carpenters square up between the skin and the baffle and use it to prod the ribs sideways as required. Once you've got the first three or four holes aft of the leading edge done, the ribs are pretty much fixed in position, so its critical to have things well lined up when you do those.  The corners of the end of the square that I stuck up between the ribs were covered with duct tape to keep from scratching up the ribs.

Someone else: I don't think I pushed down the tips of the ribs on LE and tanks enough. Me either -- when it came to attaching the T410 doublers there was a bit of space between the front of the rib and the skin. The PROBLEM IS ..that you can make your LE stick out maybe 3/32 too much and this is transferred to tank as tank rests on the W-623 strip. Anyway...the rivets that attach my tank skins to baffles are real close to the flange bend.

Make careful note of the plans regarding which holes get dimpled and which get countersunk. It's not what you would always think.

Tips:

Make sure your end ribs are flat; .063 stiffeners are attached and mine were slightly convex.

RV-List message posted by: Bob Haan <bhaan@easystreet.com>: We found the following ideas very helpful.
  1. Before you dimple the tank skin and the rib flanges, scuff the inside surface of the tank skin and the outside surface of the rib flanges where the rib flange and tank skin contact so that the proseal gets a better grip. This is much easier to do before the dimples get in the way.
  2. Mark the location of all the rib flanges on the inside of the tank skin and then mask off all the rib locations where the proseal will go using masking tape. This is not so much to have a very neat inside but rather to make application of the proseal easier, quicker, and cleaner. This masking is easiest to do for all the ribs before any ribs are riveted.
  3. Cut paper towel into about 3 inch squares; about 30 squares needed per rib. This does not sound significant. Try it you will like it.
  4. Put a piece of scotch tape on the outside of the tank skin over every rivet hole for the rib that you are about to assemble.
  5. Put on disposable gloves.
  6. Get an artist's spatula at an art supply store for about $4.00. From the side it looks "z" shaped. If it is the right shape, it can be set down on the handle with the spatula and the material to be applied suspended up in the air. Use this spatula to apply a thick coating of proseal to the area between the masking tape where the rib flanges contact the tank skin. The shape of the spatula handle definitely helps keep the proseal from getting on your knuckles and then transferred all over the work area. The scotch tape on the outside of the tank skin keeps the proseal from coming out on the skin.
  7. Slide the rib into place. We did not put any proseal on the rib flange.
  8. Insert the clecoes through the scotch tape. Do not remove the scotch tape until you are about to insert a rivet.
  9. Pull a cleco, drop it in a jar of lacquer thinner, remove the scotch tape, insert a rivet, set the rivet, etc.
  10. If any proseal gets on anything, immediately remove it with the paper towel squares and lacquer thinner. Use a paper towel square once and immediately put it in a paper bag.
  11. When all the rivets have been set, remove the masking tape from inside the tank.
  12. Put proseal on the shop heads inside the tank using the spatula.
  13. Clean the proseal off the clecoes that have been soaking in the lacquer thinner.

RV-List message posted by: rimbold <rimbold@ntr.net>: After sealing one of the fuel tanks on our RV-6A, and allowing it time to cure, it was time to clean up the "extra" sealant. There was plenty of it in areas where the fuel tank skin needs to fit flush against something else. Putting masking tape on these areas before sealing, and then removing it while the sealant is still wet is an idea that I hadn't had before I applied the sealant. Hindsight is a wonderful thing. :-)

I started out using a sharpened hobby (aka popsicle) stick to scrape away the hardened sealant. I then used some lacquer thinner to clean away the residue. This would have taken hours.

I decided to try a Dremel (hand-held rotary tool) and the small wire wheel that comes with it. It works like magic, removing the dried sealant in an instant, but not doing more to the aluminum skin than a Scotchbrite wheel. It's easy to control in curved places, too.

The wheel is made of very fine stainless wire, and leaves the skin smooth. Best of all, there's no need to mess around with solvents.

RV-List message posted by: "Alex Peterson" <alexpeterson@prodigy.net>: There is a conflict in the plans regarding this (Van chewed us out for this one a couple years ago in an RVator, saying we need to read the plans; I confirmed that indeed the plans conflict). Anyway, I stuffed the nutplates between the 1.25" spaced rivets.

> Started dimpling tank skin and past experience said to check EVERYTHING
> first so I looked for the wing root fairing details- found on dwg.46
> that the fuse bottom skin goes to the nutplates on the bottom skin/main
> rib and so does the fairing on top.(!?) What nutplates on the TOP of the
> root rib? Mentioned nowhere in the plans (I looked ALOT), looked in
> Frank J. and V.'s notes and Will C.'s, nothing. Asked the archive and
> found where one builder a few years back had to drill out every other
> rivet on the top of the root rib to make the attachment.
>
> Is this correct? Do not install every other rivet in top AND bottom of
> root rib and install nutplates?

I only saw this message after I'd completed my tanks (including rivetting *all* holes). However, it does seem likely. When I get to attach wing fairings, I may have to fix this problem; until then, I won't worry about it. Proceed at your own risk!


Tank Sealant

Van's has switched to a different brand of sealant (Thiokol MC-236 B-2 instead of Proseal). Maybe some of the comments regarding Proseal don't apply to the new sealant? The new sealant seems to be stickier than Proseal, as far as I can tell from the video. I'll use the term Proseal in this document, even if that isn't strictly accurate. Bill Freeman wrote to The AirCraft Rotary Engine NewsLetter: There is one solvent made that slowly swells it and breaks the bond to the substrate, used by most airlines to remove when resealing their wing tanks. I think its called Skysolve or Sky-something.

RV-List message posted by: "Brian Holman" <bholman@fullcomp.com.au>: Most of the difficult to fix tank leaks are on rivets, especially along the back of the tank. One of the reasons for this is that we all go to great lengths to make sure we clean all surfaces very thoroughly,  but how many have thought to clean the RIVETS. It appears the dies used to make them are lubricated with oil and they are very oily. Place a couple of hundred in a SMALL GLASS jar, put in some clear solvent and watch the solvent change color after you shake it. Then put the ends of a few cleaned and uncleaned rivets in some Proseal and let it set. I found that the uncleaned ones pull out clean and easy while the clean ones are stuck. I recommend you clean them three times with clean solvent each time. And then keep oily fingers off them.(All fingers are oily!)

The black death/Proseal is not that bad.

Don't try to be a perfectionist on the inside of the tank (seal it up, then show your friends).

For New Zealanders, what George O refers to in the video as "Coleman lantern fluid" is known here as "white spirits". Available from most petrol stations... but make sure they give you the right stuff -- in the past, I've been given kerosene and other stuff. Ask for Pegasol (Mobil) or Shellite (Shell).

 I bought a cheap cake icing set, but didn't use it -- the sealant is so thick I doubt that the plastic icing set would stand up to the pressure needed to force it out. Tongue depressors and popsicle sticks are only marginally strong enough for the mixing -- a kitchen knife would be better. Also don't use too deep a cup for mixing... my first session I used a cup maybe 3 inches deep. That meant that sealant was spread over about 3 inches of my spreading tool. Next session, I cut the cup in half, and it is much easier to mix, plus less messy.

spencer <dspencer@kiva.net>: We get big syringes (~1" dia.) at a veternary supply place and put the mixed proseal inside them. They work great. AND... we found that you can use compressed air to blow out the unused portion after it has cured, and get several uses out of each syringe. We use both techniques along with tongue depressors and popsicle sticks -depending.

I made up a set of scales for measuring the sealant -- knew that that old screwed-up HS-610 would come in handy one day! The distance from the pivot to one end is 10 times the distance to the other. The black gunk goes on a tongue depressor at the long end, the white gunk in a plastic cup at the other. Balance the whole rig (including the containers) by putting clamps in the right place on the arm. I found that a clothes peg was a good handy moveable weight for the final balancing.10:1 sealant measuring balance (25KB)

How much to mix was a bit tricky... I took a guess and it worked out near enough -- just enough to do the stiffeners and tank filler flange, but none left for the drain flange. As with epoxy-priming, it's a good idea to have a couple of items ready for sealing that you don't need to do just yet, so that if there's a bit of sealant left over you can do those too. However, think ahead! I attached one of my fuel vent angles before rivetting that rib to the skin... it was then extremely difficult to rivet the rib on.

Put plenty of sealant on the stiffeners -- it's much easier to clean up a bit of excess that squeezes out than to try to add more. Clean up any sealant that ends up where the ribs are going to go. Better yet, put masking tape on those areas.

Vans callout of 3-4 rivets to attach the filler flange is hopelessly optimistic -- better to go for 3-5.Stiffeners and filler flange rivetted on (21KB)Fuel filler flange (9KB)

I went through several latex gloves on the first session. (NZers -- latex gloves and tongue depressors can be bought at your local pharmacy; also available from your local hardware store (Mitre 10). The lady didn't even blink when I said I wanted a box of 100 gloves ($17.00 or so). Makes you wonder what other people in this small town are up to!) The gunk seemed to always stick to the tip of my left index finger (I'm left handed), so I'd have to change that glove every time I spread sealant on something. After the first couple of stiffeners which I did one at a time, I went to two at a time with a consequent glove saving. Second time around, I taped all the rivets in and then rivetted, then spread the sealant. Much more efficient use of time too.

George O recommends wearing two pairs of gloves at a time. I started with that, but found that it was hard to pick up the rivets (Blokes: imagine wearing two condoms at once!). If one of your gloves gets sticky, you can just peel it off and put another one on.

Keep wearing gloves until you've finished cleaning up, or you'll end up with black hands and black-rimmed fingernails. Except that lacquer thinners dissolves latex; I'll buy vinyl gloves when I need the next box. In fact, cleanup seemed to be the messiest part of the whole job.

The sealant was noticeably stickier/drier at the end of the session, two hours after I mixed it. Don't try to do too much at once -- stick (pun intended) to GO's and WC's 4 sessions. Or maybe even more. I ended up doing 2 ribs at a time, although I reckon now I could comfortably do 4 in a session. Leave a couple or three days between sessions on the same tank so that the sealant can set good and hard. This is one area where working on two tanks at the same time is a big plus. Whilst waiting for sealant to set, go on with doublers and other stuff (see next section).

The stiffeners and filler flanges are all rivetted on by back-rivetting. To me, this seems like about enough to do in the first Proseal session.R tank rib 1 with fittings in place (16KB)L tank rib 1 with other rib 1 parts (28KB)

When you put rivets in holes, just put the tip in... it'll stick to the sealant that's oozing out. Then use a tool of some sort (your rivet gun or squeezer or bucking bar) to push the rivet all the way in before setting it. This way, you won't get sealant on your finger tips. Similarly, use the same corner of the bucking bar on the rivet, otherwise the glove on your bucking hand will get covered in no time.

Rib 6 and the drain flanges (only just) are rivetted to the skin using a handsqueezer. I rivetted the T-410 nose plate to rib 6 with AN426AD4-5 rivets. I used flush rivets because the plans call for AN426AD-4-7 rivets to attach T-410 to rib 1 and the attach angle, but it turns out that that isn't really important.

There's also the -6 rivet in the tooling hole (cut down one of the 6-7s left over from the spar) and a -3 rivet in the small hole in the middle of the reinforcing circle. These need to be driven (I used the Avery C-frame for the -6 rivet). I was *very* generous with Proseal round the nose of rib 6. This seems to me to be a likely leak point.

OTOH, don't be too generous with it round the drain flanges. When I rivetted them on, some sealant squeezed out into the threads inside the drain flanges. I didn't clean it out properly at the time, so spent over an hour digging it out 3 days later with a scriber. It might be worth installing a temporary screw whilst rivetting them on to prevent this.

Rivetting ribs 2 to 5, I found that I needed a narrower-than-normal face on my trusty bucking bar to do the rivets near the front of the stiffeners. Also, the 4 rivets nearest the front required me to use the side of the bar rather than the end. To minimise sealant-spreading via my bucking hand, I did all the 'normal' rivets first, then the nose rivets, then finally the two near the stiffeners.Ribs 2-6 rivetted on (26KB)

SCOTT R MCDANIELS <smcdaniels@juno.com> wrote to the RV-list: The plans call out AN426AD4-7 rivets to attach T405/T410 to the root rib. This is called out because some builders install the bracket to the rib before installing the rib in the skin. If you do it that way the rivet heads can get in the way of squeezing the rivets in the skin/rib flange. The flush heads go on the outside of the bracket. The T405 angle is so thick that *it* (rather than the rivets through it) will interfere with squeezing the skin/rib rivets. I rivetted it on after the rib is in place -- be careful not to put too much pressure on the gun when you do this, or you could bend the nose of the rib). I used AN426AD4-8s here.

I sealed the reinforcing ring when I rivetted it to rib 1 (WC says it doesn't need to be, but I wanted to minimise the amount of fuel getting to the inside of the cork gasket). That was by far the messiest job so far. Sealant seemed to go *everywhere*. After the parts were rivetted together, I sprayed the screws with CRC and screwed them through the holes and nutplates... the idea was to force the sealant out, without having it stick to the screws. The next couple of days, I took the screws out and cleaned and CRC'ed them. That worked fine -- they screw in just fine.

Once the sealant had set properly on rib 1, I fitted the access plate, stood the tank in the cradle, added a couple of tubes to the vent and fuel lines, and filled it with water. That should preliminary test showed that none of the skin rivets is leaking before the rear baffle is attached. Both of my fuel caps leaked until I'd tightened them down a little. This water test isn't totally conclusive (fuel will leak out of smaller apertures than water), but was a great confidence boost. I then did a further check with some white spirits (about 2-3 gallons), tipping the tank this way and that to check for leaks. None found!

The rear baffle wasn't particularly difficult to fit... I had a helper ease the edge of the tank skin outwards and slotted it into place. As well as the wooden clamps, I used 1/8" clecos to hold the skin to the baffle whilst the sealant sets. Again, I CRC'ed the clecos to make sure the sealant didn't stick to them. Be sure to clean up excess sealant on the inside of the skin, and the inside of the baffle down to the rivet line so that it can lay down tightly on the spar.L tank rear baffle in place (20KB)

Patrick Kelley <patk@mail.ic.net> wrote to the RV-list: If you check out my website you will find how I clamped the rear baffle and skin. The objective is to put pressure along as much of the trailing edge as you can, so I borrowed an old trick and laid saran wrap over the spar, with wrap hanging over the web flanges. Once the rear baffle was riveted and the rivet heads sealed, I placed the tank on the spar and clecoed it in place. I used side grip clecoes at the inboard end where the second row of screws don't continue. When the proseal cured, I removed the tank and peeled off the saran wrap. The trailing edge conformed perfectly to my spar and there was no excess proseal to remove between the spar and tank.

Fred Hiatt <hiatt001@gold.tc.umn.edu> wrote to the RV-List: For those of you using the tank sealant that Vans now ships, here is more information than you may ever want to know.

After reading the note from Wayne Bonesteel about fuel tank sealant made by Advanced Chemistry & Technology, Inc., I called Nick Champlin (if desired, contact Tel: (714) 373-2837 Fax: (714) 373-1913) at the company to learn more (us engineers are ever curious).

He gave me permission to forward the following info regarding the Thiokol MC-236 B-2 Fuel Tank Sealant:

The material is made to pass Mil-S-8802 specification which among other things requires a hard cure time of 40 hours maximum. (This stuff is used on the Stealth Bomber (B2) fuel tanks). Over time (years) the cure time may be longer, HOWEVER, the mechanical properties (read strength) do NOT change. Working temperature affects the cure time also. For example, the rule of thumb is that if the working temperature is 90 deg. F, (70 degrees being nominal) the cure time can be 1/2 of the 40 hour spec., conversely, if the working temperature is 50 degrees F, then the cure time can be 2X the 40 hour spec.

Storage temperature affects the nominal 40 hour cure time, that is, it will extend slightly with time if stored at room temp. If the material is stored in a freezer or refrigerator, the added cure time will be small. They had some material returned to them that had been on the shelf for several years and the user had lost the accelerator and wanted to know if they had accelerator that would work with the old stuff. They mixed up a batch using the accelerator that matched the batch number, and they detected no change in the mechanical stability or strength of the cured material.

The older Proseal had some additives in it which did affect the mechanical properties over time. In addition, it would skin over in the can because of these additives. By the way, Nick Champlin used to work for the company that made(makes) Proseal.

The above info comes with no written or implied guarantee by myself or Nick, however, after 25 years in the business he knows what he is talking about.


Rib Doublers and Inspection Plates

Do these parts whilst waiting for tank sealant to set. Since the doublers for rib 6 are needed before the next Sealant session, do them first.

On my kit, the T-410 doublers are all pre-cut to shape, inside the inspection plate. Cut them out, grind off the little tabs that held them in place, and you're almost done. I spent a long time with a light shining down the length of the tank, trying to get my doublers to exactly match the curve of the nose of the skin. I reckoned it was good enough when there were several places where the doubler contacted the skin, and the gaps were all less than 1/32" or so. Some of the flutes in my ribs are *way* deeper than that. I made sure there was *plenty* of sealant around these ribs.

RV-List message posted by: GHLX34A@prodigy.com (MR GEORGE T KILISHEK): The hole for the tank sender/access plate reinforcement ring in the tank root rib should be about 4 3/4"... smaller than the reinforcing ring i.d.

When drilling the inspection plate ring to rib 1, it's (I think) impossible to get proper edge distance between the tooling hole and the rivets that attach the reinforcing ring to the rib. I think the best solution might be to use the tooling hole as the screw hole. I'd already drilled both the rings on before I discovered this. My fix was based on someone else's suggestion for fixing skin holes: Cut a short (less than 1/8") section from the shank of a -6 rivet. *Lightly* countersink (with deburring tool) both sides of the tooling hole. Set the rivet shank piece in the hole. It'll be flush with the rib surface, allowing the cork gasket to mate tightly on one side, and the reinforcing ring to be rivetted tightly on the other. Note that the 3-4 rivets called out for attaching the nutplates to reinforcing ring and rib are too short -- I'll use 3-5s.

Sam Buchanan has some good ideas (and photos) relating to tank construction. I particularly like the idea of adding gussets to the rear corners of the outside ribs.

Check out the (I think) January 98 RVAtor -- it includes an article on modifying the T405s by cutting a slot in them rather than just having a hole.

Note that for an inverted fuel system, you'll be drilling a hole through the doublers and rib 1 of your L tank.



Tip from an RV-4 builder I visited -- use sealed nut plates to attach the inspection plate. He didn't give me a part number, but showed me one. They have a kind of 1/2" deep can crimped on the back of them for the tip of the screw to fit into.

With ordinary nutplates, fuel can migrate round between the threads of the screw and nutplate (there's only 75% contact in the thread), and therefore gradually seep out.

Bob Steward <unistar@mindspring.com>: There are some sealed nutplates which have an O ring around the face which is riveted. The entire area around the threads is covered with a small metal "dome". When using these, don't get any Pro-Seal down in the holes. When you tighten the screw the hydraulic pressure will pop the dome off the back of the nut plate and you'll have a leaker. Should be able to get them from ACS, Skybolt, all the usual suspects.

I won't be using these sealed nutplates myself -- the company I work for builds petrol pumps, and I'll use the same thread sealant they do.

Inverted Fuel

The flop tube is fairly heavy -- several ounces. You might want to think about whether you'll ever be doing negative-G aerobatics (and have an inverted oil system and carburettor). I think I might, so it's worth the small amount of extra work to install the flop tube now.

The guides suggested in the sketches and plans look a bit flimsy to me. I'm making mine out of 3/4x3/4x.032" al angle.L tank -- flop tube, guides, fuel sender (30KB)

You'll need a large drill bit (14mm or so) to drill the fuel feed hole through the T410/rib 1/T405 area. There's not a lot of leeway for choosing a location for this hole and maintaining edge distance for the attachment rivets.

From RV-list discussions, I believe that in many (all?) flying RVs the O-ring on the flop tube has come off. The brass weight then dings the tank skins. When I looked at mine, that certainly seems likely. So I've removed the O-ring and replaced it with Proseal.

After I finish building the tank, I don't want the flop tube banging round inside the tank without the resistance of fuel. Therefore I'll either wrap the flop tube in foam or remove it until I'm ready to fly.

RV-List message posted by: "Scott A. Jordan" <SAJ_SLJ@compuserve.com>
The plans are vague. The drawing of the trap door is not bad, don't think I can describe it verbally any better.

In short:
If you are familiar with aerobatics you are probably familiar with the "Aresti dance". Put your flop tube in the tank with stiffners and ribs installed (only inboard bay needs to be in) and the rear baffle removed. Do your Aresti dance and watch the flop tube. If there is any way that the flop tube could get hung up anywhere, find a way to block it.

What the plans show / my modifications:
Picture in your mind (or better yet put the flop tube, stiffners and rib in place and simulate) a vertical down line where the tube will fall forward of the stiffener. Because of the amount of flexibility in the flop tube it could end up in front of the stiffner and against the rib. Now put the tank into a steep climb. The tube will be hung up in front of the stiffener against the rib. If you look closely at the plans, you'll see an anti-hangup guide going from the rear stiffner to the inboard rib. This will prevent the flop tube from getting hung up in front of the stiffener. Use a piece of any scrap and clamp it in position with cleko side clamps and test its effectiveness.  Mine goes from about 2" out on the stiffner to about half way up the rib just in front of the fuel transfer hole.

I also saw a small posibility (knife edge flight) of the tube getting hung up in the fuel transfer hole (that was cut out of the center of the wouldbe lightning hole) and put a small strip of alluminum across it to be sure.

The plans also show a guide across the access plate on the inboard rib. I couldn't see any possibility of the tube hanging up on the platnuts but put a guide in there to prevent the flop tube from continuously rubbing and chaffing on them. The plans vaguely show a strip extending from one platenut on the top to one on the bottom, extended out to protect the tube from the hardware (picture a sideways "U" with legs extended out from the tips). I was concerned about the possibility of the tube getting hung up above (or below) the guide as shown so I brought mine up near the top of the tank before bending it down (picture the "U" with legs extended inward). It was more difficult to rivet into position but eliminated one more possibility.


Fuel Vent Lines, Pitot Line

My vent lines are 41" long. Vans provides *plenty* of 1/4" al tube.

SCOTT R MCDANIELS <smcdaniels@juno.com> wrote to the RV-list: To straighten soft Al tubing, I take the roll and unroll it against the floor or a table top. This will leave a slight curve in the tube. To remove this you put a small rag around the tube and then your hand on the rag. Draw the tube through your hand a couple of times while you put bending pressure on it in the direction to straighten it, and you can make it look like it was never rolled.

Dick Martin <martin@gbonline.com> wrote to the RV-list: To straighten soft Al tubing, simply lay the tube on any large flat surface, (table, floor etc) and then lay a long board (3/4" x 6") parallel to the tube and move the board back and forth at right angles to the tube and press down at the same time. After a couple of these motions you will notice that the tube will be almost perfectly straight.

Stephen J. Soule <SSoule@pfclaw.com> wrote to the RV-list: Re the length of the inboard end of the pitot line: I ran it to the side of the fuselage and installed a bulkhead fitting on the fuselage. I flared the inboard end of the aluminum line and attached it to the fitting.  I will run a plastic tube from the airspeed indicator to the fuselage fitting.

Re flaring the outboard end of the pitot tube: I pushed the tube out into the inspection hole and flared it in there. I think you could pull it through to open air above the skin, but watch that you don't crimp the tube in the process.


Tank Attachment

Do this whilst waiting for the tank sealant to set.

As per WC's notes, initially drill all #8 screws holes to #30. Then use a nut plate and 1/8 cleco to locate the nut plate rivet holes. FINALLY enlarge the holes to #19. If you do drill them #19, then use a #8 screw and nut plate to locate or they will not be centered.

The W-623 splice strip is thin (.032"), so I dimpled the #40 nutplate attach holes and used my deburring tool to countersink the K1100 recessed nutplates correspondingly.

The aft row of nutplates in the main spar are K1100s (recessed). Dimple the flange (#19 hole), countersink the #40 holes. You will need Avery's "close-quarters dimpling tool" (p/n 5450 -- a #8 male die with hole in the middle, plus a #8 hex-head screw) to dimple the inboard 6 screws on upper & lower flanges. At the same time, the corresponding holes in the tank skin can be drilled out to #19 and dimpled. Do the same to the tank skin holes that attach to the W-623 splice strips.

The forward row of nutplates in the tank attach doubler on the main spar are K1000s (flat). No dimpling required. Countersink the #40 holes and install the nutplates.

Bob Skinner <bskinr@trib.com> wrote to the RV-list: When machine countersinking the fwd row of #8 screw holes, it is very helpful to make a pilot guide out of thick aluminum that you can clamp to the inside surface to hold the pilot steady. If you try to machine countersink for a #8 screw in the thin metal without this aid, you'll tend to end up with an enlarged, oblong countersink hole.

After the tank is assembled and sealed, the tank's skin/baffle will be countersunk. I used my #30 C/S cutter, followed by a touch-up with my hand deburring tool... when the countersinking is complete, I found that the hole had been enlarged to about 5/32". Go too deep with the machine C/S and you'll make the hole too big.

There's lots of screws -- you'll want an electric screwdriver to attach the tank to spar!


Fuel Senders

I like the idea of a capacitance-type fuel sender rather than the Smith-Warner float type. I've bought two MIL Spec Capacitive liquid level sensors.  They were originally designed to be Air Force KC-135 Stratotanker fuel quantity probes.

Each is a black anodized aluminum cylinder, about 15" long and 1.25" outer diameter. They have mounting ears at one end, and a sliding, rotating ring with attachment ears. A two-conductor wire comes out of the side at the top, terminating in a multi-pin plug. The plug has an integral O-ring seal; the unit can be mounted inside a fluid tank. (Unfortunately, getting the connecting plugs was going to cost me $1000 (!!!!) or more, so I'm going to cut the plugs off and replace them with Cannon ones).

Measured capacitance in the air is 126 picofarads, and changes proportional to how high a liquid level is on the cylinder assembly.

Mounting these senders took several hours... if I was starting again, I'd just go with the standard S-W senders. Also, there's the cost of the connectors -- they can be *very* expensive.R tank fuel sender brackets (21KB)R tank with fuel sender in place (22KB)Setting up L tank fuel sender (25KB)R fuel sender and access plate (29KB)L tank -- fuel sender in place (27KB)

The senders are too long to fit in one bay, so a hole needs to be cut in a rib. To get the maximum vertical height, this hole ends up needing to be right on the reinforcement ring, so a doubler needs to be made and rivetted to the rib. To get as much range as possible, one end needs to be near the bottom of the tank (ie near the drain flange) and the other needs to be near the top of the tank (ie near the vent holes). This means that the sender tube isn't mounted at right angles to anything! Awkward, fiddly brackets need to be made. Also, the sender mounts themselves need to be modified to fit.

On the R tank, I cut a too-large hole in rib 2, then had to rivet a large .063" doubler to it. A couple of brackets, made from 3/4x3/4x.032" al angle were made and rivetted to the doubler, and the sliding ring ears on the sender ground to fit. A bracket is attached to the rearmost stiffener near rib 1, and another to the stiffener ring on the inside of rib 1. The fixed attachment ears will be attached to these brackets. The fuel feed line fits underneath the fuel sender. I'll make a plate to cover the "fuel sender" hole in the inspection plate. The fuel sender plug will be attached to this plate.

My L tank is aerobatic, so it will be slightly different. The sender will be mounted in bays 2 and 3, with a hole in rib 3. I'll make brackets similar to what I did on the R tank. I'm contemplating sealing the sender in with no access, in which case the wire could be brought through rib 2 and the plug attached to the inspection plate as for the R tank. However, the flop tube may hit the back of the plug, which might involve a change to the guides on the inside of rib 1. Or perhaps I'll mount the plug on the back of the tank. But is there enough room between the tank rear baffle and the doubler? Maybe it's worth making an inspection hole in the rear baffle, as suggested for the regular sender?



If I'd read this RV-list post by Dick Martin <martin@gbonline.com> before I bought the senders, I would have done this instead: I have with the help of the engineers at Electronics International (They advertise in every issue of SPORT AVIATION, usually in the back of the magizine.  Phone number, address etc. is there.) developed an excellent, simple  and economical capacitance fuel tank sensor for the RV8. I have it installed in my RV8 and in testing it works perfect. It must be installed in the fuel tanks before you close them.

Following is a description of how to make and install these sensors in your RV. Please keep in mind that I can talk much better than I can write.


You will need the following materials:

Start by making the flat plate sensors. You will need 2 plates for each gas tank. Each plate needs to be approximately 35 sq. inches in area for a total of approximately 70 inches per tank. Make from 025 aluminum sheet. Make the flat plates in the approximate configuration of the T804 tank baffle. It should be 5/8" less than the height of the T804 tank baffle. Length is about 6 to 7 inches, enough that you end up with approximately 35 sq inches of plate area. I would suggest that you place them just forward of the rear flow through hole in the T804. Vertical spacing should be as follows: place one plate on the inner most T804 space up from the bottom of the fuel tank 1/8". spacing from the top is not important but should be at least 1/2". Locate it on the inboard side of the T804 if you have a standard fuel pick up, and locate it on the outboard side if you have and aerobatic flop tube pick up. Place the second plate on the outermost T804 and locate it the same for and aft, but change the vertical location so that the plate is spaced down 1/8" from the top of the tank. Locate plate on the inner side of this outer baffle so that when fueling the airplane you will not disturb the plate with a fuel nozzle or inrushing fuel. It will be necessary to trim the ends of the tank bottom stiffeners in order to maintain at least 3/8" clearance from plate sensor. I simply trimmed the end on a 45 degree angle.

The plates must be spaced off from the T804 3/8". Fabricate spacers from the Delrin to accomplish this. I have been told that Delrin spacers are available commercially, however, I couldn't find any when I needed them, so I made them myself. It is very important that you completely insulate the flat plate sensor from all the rest of the fuel tank baffles , skin etc. The reason for the 2 sensors is so that you can get a more accurate reading of the fuel capacity in both full as well as low modes. My installation will measure to l quart increments.   The 2 connectors must be connected electrically to each other. I did this by installing a piece of 24 ga teflon wire between each sensor and then of to the BNC connector installed in the T803 root end rib.  I would suggest soldering and eyelet connector rather than crimping.  Attach the wire to the flat plate with a 6-32 x 3/8 mach screw and a Hi temp friction nut (no plastic insert to deteriorate from gas). Anywhere on the sensor plate is ok, I installed mine near the center. Route the wire up to the fuel tank vent line and twist it around the vent line and route it through SB437-4 plastic bushings on the tank baffles. ( I cut a 1/16 x 1/16 notch on the ID of the bushing). 1 twist every 2 inches is sufficient. Route the wire from the outer sensor to the inner sensor to the BNC connector. I also put a few dabs of Proseal on the wire and vent tube to prevent vibration of the wire. Locate the BNC connector l " aft of the fuel tank vent fitting in the T803 root end rib. Per DWG 18 "Inverted Fuel Alteration section". When you have soldered the stripped end of the 24 ga Teflon wire to the BNC it is very important to thoroughly insulate the wire from the BNC back over the wire insulation at least 1" with Proseal. If you don't do this it is possible for moisture to follow the outside of the wire to ground. Completely cover the BNC connection with Proseal on the inside of the tank.  That completes the sensor installation.

When installing the instrument, you need to place the interface module as close as possible to the BNC connector outside of the fuel tank. I installed mine 3 inches downline from the fuel tank. Technical questions about calibrating the instrument can be answered by Ron Roberts the engineer at Electronics International in Oregon who helped me engineer this installation. I bought my instrument from Vans, he had the best price. Some of you that I talked to, indicated that you were going to build your own instruments. I think that's a good idea if you are an electronics expert, however, I don't understand electronics well enough for that, so I will save money on some other part of the plane.

I will be happy to talk to anybody about it on the telephone if you call me at 920 432 4845 cst mornings.



Other information relevant to fuel senders:

http://www.chiefaircraft.com/Aircraft/EngineInstruments/Westach.html lists WES 395-5S-1 Capacitative Fuel Level sender 24", for US$74.50 each.

http://www.rst-engr.com/rst/articles/KP89SEP.pdf -- Jim Weir's article on a DIY capacitance-type sender.

http://www.bucks.net/avelec/ -- Avelec FMS Fuel Monitor looks like a capacitance-type sender.

I was worried about the fuel sender installation and read up on it extensively. Bottom line: it wasn't worth it. When was the last time you were in a plane where you trusted the gauges? One tip: do the right one first since the innards face toward you and use some soft metal like coat hangar to set the bends. Then just make two from that out of the supplied material.

KHarrill@aol.com wrote to the RV-list: "After sacrificing a half dozen coat hangars *and* the float pivot wires on my SW fuel gauge senders, I finally found a combination of lengths that produced the 33 - 240 OHM range as suggested by someone here on the RV-list.

The magic numbers are: 3 3/8" from the pivot to the bend 4 1/2" from the bend to the float

I am a little concerned that these lengths are so different from the specifications Van includes with the senders, but they work. After destroying the wires that came with the senders, I fabricated new ones from piano hinge pin.

Someone else: My sender goes from 30 to 200 ohms (not per drawing) and has a dead zone at the empty end of the range. I'm using 5 inches for the second bend (much more than the drawing).

Laird Owens: I'll be going with Mitchells for most the engine instruments, so I wanted to stay with that style.  I used the 24" bendable probes that ended up being cut down a little. I mounted them in the third bay out, off the rear baffle near the top of the bay. They bend in the 1st 5 inches, going from the probe, up near the top of the 3rd bay, then pointing down (and thru the 2nd rib) towards the bottom of the inboard side of the 2nd bay. They end up at the bottom of the 2nd bay. I'm using a flop tube in the inboard left wing, so I couldn't go all the way to the root rib. Calibation was easy and the installation was very clean.

Ernst <ernst.totland@saab.se>: I have Westach's capacitive (bendable) senders and dual fuel level instrument. I have been flying my -6 for 2 1/2 years now and never had a problem with it. I am very happy with the system now that it is installed and calibrated, but it was a pain to calibrate. I have mounted my senders on the rear baffle, in the innermost tank section. I did this to cover as much of the fuel volume as possible and to avoid interference with the flop tube. I have mounted the sender with the tube placed diagonally from the upper/outer corner of the baffle, to the lower/inner corner. In this way I have very little non-measurable fuel. I am reading full scale when the tank is about 75% full, which I think is better than with a float system.

There are two potentiometers on the sender used for adjusting the "empty" and "full" positions on the instrument. On my senders these were interactive, so it was a process of several iterations to set these properly. This would mean filling up and empty the tank several times. To avoid this I made a small calibration tank which was easy to fill and empty, and I did a calibration prior to mounting the senders in the tanks. It did not work out well. When I was ready for the system check on the finished plane, the calibration was way off. I had to dismount the tanks and go through the calibration procedure again, this time with the "real" tanks (much larger fuel quantities). Having set the "empty" and "full" positions on the scale, I made a scale in between by filling up each tank 5 liters at a time, marking the needle position. This calibration have been very stable during the 2.5 years I have been flying. I have checked the calibration several times (5 liter increments) without finding any deviation at all.

The problem with my installation is that it is quite awkward to reach the senders (and the potentiometers) on the finished plane. The tanks must be dismounted.


Leak Checking

R tank leak test (water) (26KB)Pressure testing R tank (30KB)
Pat Kirkpatrick <PKIRKPATRICK@FAB9.INTEL.COM> said to the RV-list:
  1. Tape off the vent lines with duct tape.
  2. Tape over the fuel filler holes with duct tape.
  3. Get a balloon ( stretch it out or blow it up first ) and put it over the fuel pickup fitting. If you use a smaller ballon it fits tight enough to seal ok.
  4. Set the regulator on your air compressor to about 10 PSI (don't panic hear me out). If the 10 PSI isn't enough for the next step, increase it a little and try again.
  5. Using one of the blow off guns with the vent holes on the sides, push it against the drain valve so it opens and pull the trigger.The vent holes on the sides of the nozzle will make sure the pressure doesn't get too high in the tank. Most of the air escapes through the holes. Watch the balloon! As soon as it reaches the point where it starts to swell quickly pull the air off. The balloon will continue to fill for a second on the pressure in the tank.
    * Before the flames start from the above step think about it. Only a slight amount of pressure is getting into the tank to begin with because of the nozzle holes of the blow off gun and the ballon acts as a safety device. It will blow off way before the pressure in tank even starts to flex the skins enough to cause a problem (remember the G forces these tanks are made to take with much heavier fuel in them).
  6. Let it sit a minute and listen for gross leaks. If the balloon is holding its size go to the next step. If not, fix the big leaks.
  7. Carry the whole contraption outside, or if the weather is cold to the shower.
  8. Start spraying down the tank with soapy water mixed in one of those spray bottles you get at K-Mart or steal your daughters. Just look for the bubbles. To make sure everything is working right, you will have some bubbles around the ballon, and duct taped parts. Spray down all of the rivets and end plates (even my permatex #2 around the gaskets and screws didn't leak).
  9. If you find a leak, rinse and dry the place then mark it with a sharpie and continue on. When your done rinse the whole thing off with the shower sprayer and dry it. Take off the duct tape.

As for the fuel caps, good luck they are not made to be air tight and I will cross that problem when I put fuel into the tanks and see if it is really a problem. Don't fear the water and soap on the tanks, the pressure from the balloon will keep it out if there is a small leak.

The whole procedure takes less than an hour assuming no leaks.

Bill Boyd said to the RV-list:

Seal up the inspaction ports with Permatex Aviation Form-A-Gasket or other suitable goo on both sides of rubberized cork gasket. Use a squirt under the screw heads for extra security -- those threads extend into the tank and form a possible leak path.

Flare short lengths of 1/4 and 3/8 in. aluminum tube and fit them with AN-818 & -819 nuts and sleeves (you DO save scrap pcs of tubing, don't you?) Plug the 3/8 stub with an AN-4 bolt and some permatex. This attaches to the fuel fitting on the tank.

Attach the 1/4 tube stub to the vent fitting on the tank and attach to it a length of 1/4" ID plastic tubing about six ft long. If you once built a RANS (not Van's) homebuilt, as I did, you will have saved this blue polyurethane tubing from the so-called fuel system and now you can put it to a safe aviation use at last. Otherwise a trip to the corner hardware will net a similar, clear tubing (polyethylene).  Lash the plastic tubing onto the aluminum stub with safety wire or a clamp.  Stand the tank on its end (remove from wing first). Arrange the clear tubing into a U-shape and carefully fill the lower half with water. This is going to seal the air in the tank and give an indication of the pressure applied and act as a blow-off if you try to over-do it.

Using shop air and your blow-gun, add air through the sump drain until you are showing about 30 inches of water pressure (difference in the column heights in the U-tube.) Then proceed with the soap solution on all the rivets. Do this in the shower if you prefer, with the wife taking compromising pictures. Expect a leak at the fuel cap stem, as mentioned elsewhere. Turn on side to check the far end; set the tank on sawhorses or bench and let the tubing dangle- just don't let the water spill. Works like a charm and won't over-stress the "hull."

If a leak is present, you will see a stream of small bubbles around the offending rivet. Ask me how I know this. Thankfully., it was a pop rivet and in an outside seam (why, I have no idea; I did the tanks so long ago I can't recall why I placed a pop rivet in a line of AN-426's, but I did). A little Permatex under the head and a squeeze with the hand squeezer and no more leak. YES!!!

This is a job I dreaded for years but it was quite painless once I got around to it. Be sure you tightened all the fittings inside the tank before closing up the inspection plates.

RV-List message posted by: Rob Rimbold <rimbold@ntr.net>: When I test our tanks, I'll use this method:

Seal all openings to the tank, and attach a length of clear hose to one of them. Hang the hose such that the end which isn't connected to the tank is pointing upward, and there is a "U" in the hose below both the open end and the tank connection.

Fill the hose with water, until it reaches partway up the hose on the side of the "U" nearest the tank. The water level will be higher on the side of the "U" that is open. Mark this level on the hose.  Note the ambient temperature, and go away. Check on the level periodically.  It will move in relation to the temperature, but the amount of movement can be calculated, or you can take your readings only at identical ambient temperatures.

If the level of water moves more than a temperature change would allow, then you've got a leak. Use a soapy solution to locate it. The more water you use, the more pressure is applied to the tank. You can fit more water by having a deeper "U" or a larger diameter hose.  You might want to add some dye to the water to aid in viewing it.


Fixing Leaks

RV-List message posted by: Scott McDaniels <smcdaniels@juno.com>: Check the archives for more detailed info, but I have fixed a couple of rivet leaks using one of the closed end blind rivets supplied in the kit for the fuel tank rear baffle installation (part #?)
  1. Modify the rivet using a block of steel with a #30 hole drilled in it.
  2. Machine countersink the hole to the depth required for a 3/32 AN426 rivet.
  3. Insert the blind rivet, and slip another steel block (with a small hole in it) over the rivet stem and whack with a hammer to reform the rivet head to a countersunk shape.
  4. file/sand off the edge of the rivet head to reduce it to the proper diameter (same as 3/32 rivet that it is replacing).
  5. Re whack and sand as necessary to get it close to a 3/32 rivet head size.
  6. Remove the leaking rivet and open up the hole too #30. Thick grease on the drill bit helps keep the drill shavings from going inside the tank.
  7. Install the custom made rivet with a small amount of proseal, add a little touch up paint, and you are done.

RV-List message posted by: George Kilishek <aeronut@mci2000.com>: I've had good initial success with Loctite 90 thread sealant in stopping a small leak. You put it on the outside surface.  Capillary action takes it into the leaking site and it cures in about 24 hours @ 72 degrees F.

RV-List message posted by: Scott Gesele <scottg@icsnet.com>
If the leak is in a bay that you can reach through the inspection plate or fuel filler, just dab more pro-seal on the rivet.

Assuming the above isn't true:
a) Create a fixture to hold the tank in your drill press, then use a fly cutter to cut a rather large hole in the rear baffle in the bay that is leaking. You must be able to get your hand in this hole.
b) Replace the rivet that is leaking (install with pro-seal), check all other rivets in this bay (pro-seal anything that looks like it might leak in the future).
c) Clean out all aluminum shavings.
d) Cover the hole created in step (a) with an aluminum plate (.063), pro-sealed in place and riveted with self-sealing blind rivets.