Tanks

by Frank Justice

Even if you are doing only one wing at a time work on both tanks at the same time up through the point of drilling of the ribs to the tank baffle. Also do both sets of things like spacers, fuel pickup tubes, backup plates, etc. If the tank ribs have not already been prepared, go back and perform the steps in the rib preparation section.

Items not in the kit you will need and can get from Van's: Fuel level sender for each tank; Pro-Seal (no more than 1 quart, and if you are careful there will be enough left over for a friend to do his tanks.) for sealing the tank.

There is also sloshing compound (no more than 1 quart); this is put in the tank after it is finished and sloshed around to form a solid film all over as added insurance against leaks. Some professional builders and others do not use sloshing compound and they have no trouble with leaks. You can always put it in later if you do have a problem. Some sloshing compouds do not hold up well against autogas and have been known to peel off in sheets and clog the fuel intake. Randolph 912 is recomended. Also, you MUST clean and roughen every surface inside the tank to ensure good adhesion.

1. Instead of making the curved-tube fuel pickup unit described in the plans, a good alternate is a strainer screen attached to a fitting that screws into a threaded plate attached at the lower rear corner of the inboard rib. This was described recently in the RV-ator. It allows you to remove the fuel pickup strainer for cleaning without removing the access plate and possibly without removing the tank. The strainer is available from Aircraft Spruce.
2. Instead of making the large access plate for the tank the sender unit could be attached directly to the rib if a suitable backup plate were included to properly hold it. This would be less work and reduce the chance of leaks, but would make it very difficult to clean out the tank later. You would almost have to use the fuel pickup described above, since it can be cleaned by unscrewing it from the outside of the tank.
3. You can install a low fuel level sensor, available from Aircraft Spruce, which is just a float switch. Fit it after you have put in the fuel pickup to avoid interference.
4. You may also find it useful to install a fuel level indicator tab at the filler hole so you can easily tell that there is a certain amount of fuel in the tank.

1. Make sure you understand how the reinforcing ring, rib, and access plate go together. The reinforcing ring is on the inside of the tank on the side of the rib away from its flanges; the reinforcing ring is held to the rib by the rivets that hold on the nutplates that the access plate screws go into.
2. Cut out the T-607 reinforcing ring and T-608 tank access plate for each tank out of 0.063 (1/16") plate as shown in drawing 18. Before cutting ake sure that the inner diameter of the reinforcing ring is as large as the cutout in the rib; all the dimensions for the ring and access plate may need to be increased from what is shown in the drawing. Be sure to turn the circle cutter blade to the proper orientation for outer edge versus inner edge cuts. Don't worry about the guide bit hole in the access plate, you will be cutting that part out for the fuel sender unless you are doing the inverted fuel system pickup. To cut the reinforcing ring, cut out the inner diameter first, then without moving anything securely tape over the cut you just made and cut the outer diameter. This will keep the ring from beating itself to death when the outer cut finishes. Since the metal is so thick it is helpful to use cutting oil to keep the cutter cool.
3. Mark the hole pattern on one of the access plates, making sure one of the holes will not come too close to the tooling hole in the rib; the fuel sender hole center can be moved from the plans location to where it takes out the circle pilot hole. Tape both reinforcing rings and access plates together securely and pilot drill all the holes #30. Enlarge the fuel intake elbow hole to 9/16" and the fuel sender hole to 1 1/2" diameter (or whatever matches the sender you plan to use).
4. Disassemble the stack and tape a ring to each access plate in the same orientation as drilled. Take one set and determine which side would be on the outside of the left tank. Put one light reference mark with a center punch on each near the bottom hole. Do the same for the other set except use two marks.
5. Place a ring and access plate in the proper orientation on the proper rib with the ring next to the rib; this way you can center the plate accurately by eye on the rib cutout. Orient it as shown in drawing 18 but rotated slightly as necessary to allow the rivets that will hold the nutplates to be clear of the rib tooling hole. Drill #30 through the plate-ring into the rib. Then enlarge the holes to #19 or 11/64". (Drilling this way and using lots of clecos and then #8 screws is necessary to keep the holes aligned; the ring tends to move otherwise.)
6. Disassemble and put the reinforcing ring only back on the rib using short #8 screws and K1000-08 platenuts. Drill #40 through the platenut mounting holes into the reinforcing ring and rib or use a platenut hole jig.
7. Disassemble. Enlarge all the screw holes to 3/16". Dimple the #40 holes in the rib and countersink the #40 holes in the reinforcing ring.
8. Rivet the nutplates, ring, and rib together. Insert a screw into each hole to insure that it will thread freely into the platenut.
9. Make four spacers for proper fit of the bulkhead elbows out of .032 or thicker material. Drill two pieces with a 7/16" hole and two with a 9/16" hole, then cut away the rest of the material leaving about a 1" square piece with the hole in the middle. (This quantity for both sides.) Verify that what you have made is thick enough that when the elbows are attached to a rib with a spacer and mounting nut the elbows do not turn.

Make the bracket shown on drawing 18 that mounts at the tip of each inboard rib. When drilling the 6 #30 rivet holes be sure none of them will come too close to the tooling hole in the rib that the bracket will mount on. Position the brackets on their ribs and drill the corresponding holes in the ribs.

1. Mark the positions of the rib webs on the flange side of one of the tank baffles T-602 as shown in drawing 18. Then mark the positions of the rivet holes 5/16" away. Make sure the end holes will not be too close to the ends of the flanges on the ribs. Try 15/16" rivet spacing for the four internal ribs and 11/16" spacing for the end ribs.
2. Secure this baffle to the back of the other one. Drill the end rib holes #41 and the internal rib holes #30.
3. Separate the two baffles and deburr them.
4. Mark a line on the end flange of all the ribs 5/16" from the web. Clamp the ribs in their proper positions with the centerlines showing in the baffle holes. Drill the ribs through the baffle. Remove the ribs, debur.Put back on with the clecos on the rib side.

1. Assemble the ribs to the baffle with the clecos put in from the rib side. Secure the tip ends of the ribs with threaded rods or your choice of materials as with the leading edge assembly. At this point spacing need not be precise; you mainly just have to secure the tiedown method at the outboard end because later the leading edge skin will cover it up..
2. Make eight spacer blocks, approximately 2 1/2" long, 15/16" wide out of 1/2" or 3/4" wood. Place these blocks on the main spar doubler to hold the tank in the proper position. Place the tank skeleton on these blocks; modify the height of the blocks if necessary so that the rib next to the leading edge skin lines up with the flange piece on the leading edge.
3. Put this assembly on the spar about 1/16" away from touching the leading edge assembly at the closest point.. Secure the outboard rib to the leading edge flange strip in the vertical position with tape. Tape the baffle to the spar on the bottom side. Mark the rib flange centerlines with a dark marker. Extend these lines down onto the main spar flanges and top skin On the leading edge skin mark the position of the leading edge of the main spar doubler (this will be used later to locate the centerline of the skin to baffle holes).

1. Make a rib spacing template as was done with the leading edge assembly. Use the edge of the leading edge skin as the reference point. Mark the position of the centerlines of all the ribs on it. Do not be concerned if the template marks for the top and bottom side are not the same distance from the leading edge skin,
2. Use the template to get the ribs straight and the noses secured the proper distance apart. Secure the rib spacers using the bottom side template, then use the top side template to twist the top sides of the ribs into the proper spacing if necessary.
3. Make an internal (the tank end ribs are different) rib rivet spacing template for the top side of the tank skin on a strip of aluminum at least 1" , preferably 2" wide. At the one end mark it "SKIN EDGE" ; hold this end against the leading edge of the top skin and mark the approximate position of the first, last, and a few other rivet locations using the flats on an internal rib as a guide. Line these marks up for best fit on the internal tank rib fluting guide and mark the locations of all the rivets. Hold the template up to the ribs to insure that the rivets all fall in a flat spot and not too close to a flange gap. Mark this template "TOP, INSIDE RIBS".
4. Make a similar template for the top of the end ribs.
5. Make similar templates for the bottom side, using the trailing edge of the main spar flange as the reference point.

1. Secure the tank skin onto the ribs with the straps as was done for the leading edge assembly. Note how the edge of the tank skin matches to the edge of the leading edge skin. If there is a gap anywhere greater than 1/16 inch mark the tank skin for trimming, remove it, trim it, and try again. Also check for gaps between the tank and top skins. Clearly mark the top and bottom and the tip end root edges of the skin.
2. Mark the positions of the centerlines of the ribs back onto the trailing edges of the tank skin. Use the rib spacing template to mark the rib centerlines near the nose of the skin. Mark the positions of the bottom side rivets using the rivet spacing templates. Remove the tank skin from the wing.
3. Use the templates to mark the top side rib rivet positions.
4. Drill a few guide holes #41 in the skin for each rib; one at the trailing edge, two at the leading edge, and two more in between on top and bottom.
5. Place the skin on the ribs in such a way that you can see most or all of the rib centerlines through the guide holes. Secure it in at least four places with the straps and spacer blocks as was done with the leading edge. The skin should be right against the leading edge skin.
6. Drill the rivet holes #41, starting on the top side at the trailing edge and working around to the bottom. Use the guide holes to reposition the ribs if necessary; an ice pick or something similar can be used for this.

1. Remove the clecos holding the top side of the skin, then loosen the straps. Mark on the inside of the bottom the outlines of all the ribs. Also mark the baffle location at the root end only. Remove the skin from the ribs.
2. On the inside of the bottom side of the skin mark two lines the full width of the skin at 6 7/8" and 11 7/8" from its trailing edge. Mark each position one through ten where a reinforcing rib will go.
3. Cut the reinforcing ribs from the 0.032" angle using the marks on the skin as a cutting guide and mark each rib with its position number.
4. Mark the skin-to-angle rivet positions on the inside of each angle, 1 1/4" or less apart. Tape each rib in position on the skin and drill #41 (or #40) through it and the skin. Remove the angles; cut off the ends of the upright sides (the ones without the holes) at a 45 degree angle to allow more room for bucking rib rivets later. Deburr the angles, the ribs, and the skin.

1. Drill six holes #40 around the drain flange and ten holes #40 around the filler cap flange (do flanges for both tanks at the same time). Countersink the holes in the drain flange from the side with the raised rim for the valve (the drain flange goes on the outside of the skin). If you will be dimpling the tank skin countersink the filler cap flange from the side away from the turned-up edge (the filler cap flange goes on the inside of the skin).
2. Method Details: To accurately locate the hole circles and the large center holes, use this procedure. Measure the diameter of the flange, then set a compass to half that value and draw a circle on a heavy piece of paper. Decide how far in from the outside you want the center of the hole circle to be, decrease the setting of the compass by that much, and draw another circle. Mark the hole locations. For six holes the hole spacing is exactly the same as the compass setting, so walk the compass around the circle. For ten holes use a protractor and make the holes 36 degrees apart using the center as the reference point. Place the flange on the pattern and use it as a guide to trim the pattern to the exact size of the flange. Place the pattern on the flange and center punch through it to place the holes. Put the pattern on the skin and center punch the center hole for making the drain and filler holes.

   Note: The plans call for using AN426AD4-7 rivets in the drain flange and AN426AD3-4 in the filler cap flange. I have chosen to use the 3-4 rivets in the drain flange because this is not a structural joint and it would be difficult to work with the larger rivets.

3. Place the drain flange on the outside bottom of the skin as close as possible to the corner of the baffle and the root rib but not so close that the shop heads on the flange rivets would hit either the rib or the baffle. To do this place the flange on the inside of the skin using the rib/baffle locations marks as a guide; drill a small hole through the center of the flange; then place the flange on the outside using the hole to locate it. Mark an arrow on the flange to aid in aligning it properly later. Drill through the drain flange mounting holes into the skin. Enlarge the drain hole to 7/16".
4. Place the filler cap flange on the inside top of the skin near the tip edge as shown in drawing 18. This position is optimum for an RV-6; some builders of RV-6A's place the filler hole 7 to 8" from the rear skin edge instead because the -6A sits at a different angle. Note that the flange has a slight bow to it so it will follow the curve of the skin; orient the flange so this bow is in the proper direction. Drill through the two flange holes into the skin which fall along a line between the tip and the root. Cleco the top side of the skin to the ribs and cleco the flange in place. Drill the rest of the mounting holes through the flange. Mark the outline of the flange on the skin, then remove it; remove the skin from the ribs.
5. Locate the center of the flange on the skin using the paper template. With that as a center point make a hole in the skin just large enough to allow the top of the filler cap to pass through. Cleco the flange onto the skin and enlarge the hole as necessary to allow the filler cap to go in properly.
6. Make two of the clamps that will hold the vent tube to the filler cap flange as follows: Cut a strip of 0.032 aluminum about 3/4" long and 1/2" wide. Drill a hole #40 1/4" from one end. Put one with each filler cap flange.
7. Drilling to the Baffle and the Spar
8. If you plan to countersink the skin, cleco the angles and the filler cap flange onto the skin.
9. Cleco the skin back onto the ribs and baffle and put in position on the spar. If there are gaps between the leading edge skin and the tank skin, trim the tank skin until it fits like you want it.
10. Strap the tank assembly down to the spar.
11. Draw lines across the skin at 3/16" above (for rivets) and 1/4" below (for screws) the leading edge of the spar doubler (the mark you previously made on the leading edge skin). Mark another line 1/4" from the trailing edge of the tank skin for screws (on the bottom side use the trailing edge that will exist after you trim the skin). If the trailing edge of the tank skin extends farther down the spar than the leading edge skin, mark a line even with the leading edge skin. Mark another line for screws along the skin edge next to the leading edge skin and 3/8" in from that edge.
12. Mark the rivet and screw positions. The spacing given in the plans will work except for the following: the inboard-most screw hole in the row along the trailing edge will have to be moved back toward the tip by 3/4" or it will run into the end of one of the spar flange strips; the screw hole in the same row at the outboard end needs to be 1/2" instead of 3/8" from the edge so there will be room to countersink the platenut screw holes easily; the next screw in may be so close to a spreader angle that it will be difficult to dimple the hole or squeeze one of the platenut rivets for it; locate the six holes on each side along the skin edge so they are between the big spar rivets so they can be reached with the dimple tool later..
13. Drill the screw holes #30, then drill the rivet holes #41 (or #40).

1. If you are countersinking the skin do all the rivet holes now except the ones for the drain flange.
2. Disassemble the structure and then put the end ribs back on the skin. Check the amount of gap between the leading edge of the skin and the nose of the outside ribs. If the gap is greater than about 1/16", make plates out of thin scrap that fit the skin contour. Put the sealing plates in place on the inside and drill them through the rib.
3. Make four small, flat sealing plates out of thin scrap for each tank to approximately fill the gap between the rib corners and the baffle plate. Drill one hole #40 through each into the ribs. Dimple the rib holes and the plates.
4. Disassemble the tank and deburr all the holes.
5. If you are dimpling the skin, dimple all its rivet holes except the ones for the drain flange. Also do the holes in the ribs where the skin goes as well as the stiffener angles. Countersink the baffle rivet holes where the skin goes.
6. Countersink the #40 holes in the baffle that connect it to the outside ribs.
7. Trim the bottom side trailing edge of the skin if it extended below the leading edge skin.
8. Bevel the skin that is adjacent to the leading edge skin down to the same thickness as he leading edge skin (.025).
9. Drive a #6 rivet into the tooling holes in the outboard end rib to close them (not the root rib).
10. Initial Assembling and Sealing the Tank
11. Since the Pro-Seal sets up fairly quickly, have everything ready before you mix it. The cooler the temperature of the room, the longer working time you have. At around normal room temperature you will have about one hour before it begins to get stiff. If you want it to set up faster for any reason you can use up to twice as much catalyst without affecting the cured strength. Every surface must be completely clean so that the Pro-Seal will stick to it; Pro-Seal is not solvent-based so any contaminant at all will make it come right off. If the Pro-Seal fails to stick the tank will leak. If you do well enough with the Pro-Seal you will not have to use sloshing compound. Pro-Seal will not come off your hands; if you don't want black for a week use gloves.
12. Roughen the mating surfaces of the rib flanges, reinforcing angles, sealing plates and the inside of the skin with a stainless steel brush (find in welding supplies) or a Scotchbrite pad. Clean the surfaces well; methyl ethyl ketone is preferred. Acetone has been known to leave a film that Pro-Seal will not stick to. Set out the rivets you will need and clean them in solvent. Do not touch any surface with your fingers after cleaning. You should actually clean and roughen every surface inside the tank in case you have to slosh it later.
13. Set out palettes (squares of cardboard or wood) for mixing Pro-Seal on and several strips of various thicknesses of skin scrap or wooden popsicle sticks for applying and spreading it. You can measure the proper amounts of the two compunds either by weight or by volume. By weight the ratio is 10 parts of the resin (white) to one part of catalyst (black); by volume it is about 7:1.
14. You can make a simple balance scale to do this as follows: drill three holes in a piece of wood about 1/4" x 1/2" x 12"; one hole is 10" from the center hole and the other is 1" away in the opposite direction. Hang this by its middle hole and make some light-weight pans to hang from the other two holes. You now have an accurate 10:1 balance. You can also measure the material accurately enough by volume by building one mound of catalyst and seven equal-sized mounds of resin.
15. Put rivets through the holes for the stiffener angles and tape them in place. Spread a thin layer of Pro-Seal where the angles go, especially around the rivets, and on the angles. Back rivet the angles to the skin. Smooth down any excess Pro-Seal that came out from under the stiffeners.
16. Put Pro-Seal on the drain flange, the outside skin where it will go, and the rivets. Assemble and rivet together.
17. Put Pro-Seal on the filler cap flange, the rivets, and the inside of the skin where it goes. Assemble the flange to the skin, clean any excess Pro-Seal out of the countersink/dimples, and drive only the two rivets on the outboard and inboard sides (note that the skin isn't tight against the flange elsewhere). Attach the outboard rib and the one next to it to the skin with just enough clecos to get the skin into the proper curvature at the filler cap flange. Drive the rest of the rivets except one toward the leading edge. At that position attach the vent tube clamp strip also. See the picture in the construction manual.

1. Make up a wad of Pro-Seal a little less than the size of a golf ball.
2. Apply some to the flange of one of the center ribs as well as the place on the skin where it will go. Put the rib in place with one cleco on the top side and two on the bottom at the tip. Repeat for the rest of the inside ribs. Put clecos in every other hole, working aft.
3. Put the tank in a cradle if you have one or get a helper to hold it. It must be supported securely for these rivets to come out right. Clean any excess Pro-Seal out of the countersink/dimples or the rivet heads will stand out after driving. Put clean rivets into the open holes and wipe off the Pro-Seal that oozes out onto the skin. These rivets may come out too high on the outside if you are not careful; hit them lightly as you start to force out any remaining Pro-Seal under the heads, then use a slight rocking motion with the rivet gun to compensate for the fact that the rivet set surface is slightly curved. Drive these rivets carefully. If any go wrong complete the rest of the rivets before going back to drill any out.
4. Make up another batch of Pro-Seal about the same size. Completely coat the end rib flanges, including filling the flutes and a heavy bead around the tip. Put these into the skin and rivet on.
5. Put a generous filet of Pro-Seal on the inside of the outside rib-to-skin junction and a film over the rivets that seal the tooling holes in the outboard end rib. Clean off all the excess Pro-Seal on the inside of the skin at the outboard end where it sits on the leading edge flange.
6. Pro-Seal and rivet the T-605 attach angle (and sealing plate if necessary) to the inboard rib. You ay have to notch the attach angle to clear some of the skin to rib rivets. If necessary also attach the nose sealing plate to the outboard rib. Pro-Seal and rivet on the rib-to-baffle corner seal plates.

1. Install the bushings in the holes in the inside ribs.
2. Cut a piece of 1/4" tubing 46" long. Put a tubing sleeve on the tube with its flange end toward the near end. Flare this end of the tube
3. Feed the other end of the tube through the root rib hole. Put one of the smaller size tubing nuts on the tube with the threaded end toward the flange. Feed the tube on in through the bushings.
4. Put a gentle bend in the end by the filler cap flange and push the tube in to where the root end is inside the tank.
5. Put the long end of a bulkhead elbow through the root rib hole from the outside with Pro-Seal and put one of the previously-made square spacers and a nut on it to secure it to the rib.
6. Secure the vent tube to the elbow.
7. Bend the tube at the filler cap flange end so that its end is close to the corner of the rib and skin (the highest point in the tank). Secure the vent line with the metal strip on the filler cap flange and Pro-Seal.
8. Fuel Pickup Tube (not applicable to inverted fuel system; see construction manual)
9. Put a sleeve on the end of the piece of 3/6" soft aluminum tubing with its flange toward the near end. Flange this end of the tube. Put a tubing nut on the tube with its threaded end toward the tube flange.
10. Put the tank access plate on the end rib with two screws. Install a large bulkhead elbow in the plate with the elbow inside and pointing down. Bend the tubing so it will curve out from the elbow to the lower rear corner of the tank. The tube can be bent around an aerosol paint can if a tubing bender is not available. Cut off the tube so it will end just before it touches the tank baffle.
11. Pinch the end of the tube closed. Make about six cuts three-fourths of the way through from the top side of the tube just above the crimp with a thin saw blade.
12. Make another tube just like it for the other side and put with the other tank parts.
13. Secure the pickup tube to the elbow and the elbow to the access plate with Pro-Seal. Make sure the tube end is down against the inside corner of the tank.

1. The arm on the fuel level sensor has to be bent and shortened so it will swing from one stop to another and just touch the bottom or top of the tank at the same time. Van's sells a "right" and "left" side sensor which differ only in the hole pattern in the flange; there will be a mark on the box to indicate which one goes on which side.
2. You may also want to calibrate your gauge-sensor pair at this point. Connect the gauge to the sensor and power it with about 14 volts such as from a car battery with the engine running. If the gauge does not read exactly full and empty as you move the arm full up and down, you may be able to bend the stops on the sensor so it will.
3. Hold the fuel level sensor up to the rear edge of the root rib at about the level it will sit in the access plate. Make sure you have it right side up; in this position the arm will swing to a straight-up position before you bend it. Bend the arm down carefully to avoid any stress being placed on the arm bearing. When it is bent to the point where it goes up and down by the same amount before hitting the sensor stops, find the point on the arm that will be a float radius above the bottom and below the top when the arm is moved from stop to stop. At that point bend the arm so the lower end will be at a right angle to the rest of the arm, parallel to the bottom, and parallel to the root rib. Put the float on this part of the arm and cut off the excess, leaving about 3/16" protruding from the float. Put a washer on the end of the arm and then swage (flatten) the end of the arm to hold the washer and float on.
4. Put the sensor body through the access plate hole, making sure it is aligned for vertical movement of the arm. Mark the sensor mounting holes on the access plate. Pull out the sensor and drill the marked locations #19 or 11/64" and deburr. You will also need to notch the access plate to clear the protrusions and rivets which attach the fuel sender body cover to its flange, and to radius the edges of the hole in the access plate so they don't dig into the sender body or prevent the flange from seating fully on the access plate. Mount platenuts for the fuel sender screws on the inside of the access plate (the drawings do not show these; they are necessary if you want to be able to remove the fuel sender without removing the access plate).
5. Make a cork gasket out of the same thin cork used for the access cover gasket; the gasket supplied with the sender is too thick and may loosen up in time. Put the sensor back in with the cork gasket and secure with #8 screws, putting a small amount of Pro-Seal under each screw head. Then put a small amount of Pro-Seal on the inside to fill the gap between the access plate and the sender body.
6. Bend the arm if necessary to insure that the arm travels from stop to stop as well as reaches the top and bottom of the tank.
7. Remove the access plate from the end rib.

1. Mix up a batch of Pro-Seal about the size of a golf ball. Spread it in a solid strip on the skin where the baffle will mate with it, but stopping just past the rivet holes; don't put any farther back where the screw holes are. Put a solid strip on the flanges of the outside ribs, but just a patch around the rivet holes on the inside rib flanges. Put an extra blob where the outside ribs and the skin meet. Spread it on the mating surfaces of the baffle. Put the baffle in place and secure with clecos in every other hole.
2. Install the AD-41H closed-end pop rivets in the #30 holes that attach the baffle to the ribs, then install the rest of the rivets.
3. Clean all the excess Pro-Seal off the skin flanges and the baffle flanges; otherwise it will stick to the wing. Either clamp the skin securely to the baffle with enough pressure to squeeze out the excess Pro-Seal or cleco the tank to the wing using the #30 holes. Let it sit for at least a day or two.

1. Many but not all builders slosh the tank to ensure a good seal. If you were very careful with the cleaning and the Pro-Seal you will not have to. You can always go back and do this later if leaks develop. See the procedure below for checking for leaks.
2. After a week (longer if the weather is cold), you are ready to slosh the tank. If you want to continue with other tasks, leave the tank on the wing and go to the next section to do the bottom skins. Then after a week slosh the tanks and cleco them back on the wing.
3. Take the tank off the wing. Seal the access plate hole with a piece of truck inner tube or something similar. Put a 1/8" pipe plug in the drain hoe to seal it. Remove the inverted fuel system flop tube if installed and plug that hole. Pour a quart of slosh into the tank through the filler hole using a funnel. Put the filler cap in and rotate the tank slowly in every direction so the slosh will reach every nook and cranny. Keep the tank sealed.
4. A slight pressure will build inside the tank from the evaporation of the solvent; this will force the slosh into any holes which may be present. After a few minutes loosen the filler cap to relieve the pressure but don't remove it. Drain the slosh through the drain hole. Rotate the tank so all the slosh has a chance to work its way out. Blow air through the vent line to clear out any slosh that got in it. Remove the filler cap and clean it.
5. Check the outside of the tank for evidence of significant leaks. If any are seen, wait a few days and slosh again.
6. Do not put gasoline in the tank for at least several weeks.

Install the access plate with just the cork gasket. Put a little Pro-Seal or fuel line sealer under the head of each screw. Secure a balloon over the fuel pick-up fitting. Put the tank drain valve in the drain hole, sealing the threads with a fuel line sealing material borrowed from your A? mechanic or a small amount of Pro-Seal (don't worry, it doesn't harden up and prevent later removal). Inflate the balloon through the vent fitting. Brush soapy water or leak detecting compound over all the rivets and joints. If there is a leak around the fuel filler cap tighten the nut on it.