by Brian Moentenich

My area for construction (half of a 2-car garage) does not permit easy assembly of the wings to the fuselage while its still in the jig. Waiting until most of the skins have been installed before installing the gear mounts looked like access would be extremely limited. I decided to remove frame F-604 (before it was riveted in) and temporarily assemble the wings to it in someone else's 2-car garage. I bolted the wings together with F-604 upside down. They take up an area 4' x 21'. Eight 3/8" hardware store bolts were used in the inboard & outboard holes of the steel splice plates of the spars. I also installed two 1/4" bolts on each side of the spar which also go through F-604B and F-604C plus a 3/16" bolt beneath (top side) the 1/4" bolts for alignment. The gear mounts do not receive these fasteners.

During initial assembly of the fuselage frame, I used two 5/8" thick pieces of particle board glued together for a spacer (where the spar will go). The actual thickness of these pieces was measured to be 1.240". The spars were measured to be 1.210" thick. Because of the difference in thickness, I made two shims out of 0.025" material and used them during assembly of the spars to F-604.

The 1-1/4" x 1" angle (between the fire wall and F-604) had been trimmed for clearance of the gear leg tubes. It, the F-6101 (gear attach web), and the 3/4" x 3/4" x 1/8" angle had already been fitted, drilled & clecoed together before I removed F-604 from the jig. These were clecoed to the F-604 assembly in my friend's garage. Fortunately, my kit already had the gear legs drilled to the mount. I assembled the legs to the mounts & used 5/16" hardware store bolts in the hole to align the legs to the mounts. Using clamps, I positioned the gear mount/leg assemblies on F-604 and kept adjusting them until they were properly located (21-3/4" from the bottom surface of the skin) as shown on drawing # 59. I paid particular attention to making sure when I drilled the holes, there would be sufficient room to install the nuts. I also checked to make sure the angled steel reinforcing plate on the mount didn't cross directly over any bolt holes (it didn't).

Drilling the 3/16" Holes in the Gear Mount

The objective is to drill all of the holes in the gear mount in exact (or within a few thousandths of an inch) alignment with the holes in the spar. Once bolts are installed, the load will be carried by friction between F-604 and the spar plus a few bolts. The first hard landing will probably overcome the friction and the holes surrounding the bolts holding the load will deform slightly. Once this happens, the remaining bolts will pick up the load if their holes were accurately drilled..

My original plan was to use a 3/16" diameter transfer punch to mark the location of the spar holes on the mount. I would then remove it, center punch them deeper and use a drill press to form the holes. I bought a stub 3/16" drill bit. Starting with a center drill (which I already had), I would use a pilot drill first and finish with the stud drill. My backup plan (if the holes weren't located accurately) was to drill them 1/32" oversize and use Devcon plastic steel filler in the voids between the bolts and oversized holes. I did not do either of these things. I was afraid I could not drill the holes on the bench to the necessary accuracy. It would be quite difficult to hold & locate the mount under the drill press. I also didn't know how well the Devcon would stick to the steel mount and its yield strength was quite a ways below that of the steel mount and the bolts.

I did use the transfer punch to mark the steel mount. Leaving everything assembled, I inserted a 1-3/4" long 3/16" OD brass tube (from a hobby shop) in the hole to keep the drill bit from contacting the aluminum. The ID of the tube was 0.003"-0.004" larger than 5/32". Using a 12" long 5/32" diameter drill bit, I started the hole. I switched to a long 1/8" bit to drill through, back to the long 5/32" to enlarge it, and then removed the brass tube and used a long 3/16" bit to finish up. Before using the 3/16" bit, I dulled the cutting edges on the flutes starting about 3/16" back from the business end. This would reduce the risk of enlarging the 3/16" hole in the aluminum. When I was done, the long 3/16" bolts needed light tapping to get through the spar portion and fairly heavy tapping to go through the steel portion. The holes in the steel should probably be drilled with a #12 drill - but I did not want to use it in the spar holes because the bolts fitted so snugly. I may need to use the #12 drill at final assembly - this will not be a problem.

Drilling the 1/4" Holes in the Gear Mounts

There are 4 holes in each mount which requires enlarging the existing 3/16" diameter holes in the spar. For these, I started by drilling them out to 3/16". I did not take particular care to avoid reaming out the spar holes since they would be drilled larger. I made a drill guide by putting a 1/4" hole in a piece of 3/4" x 3/4" x 1/16" angle and clamped it about 9" from the mount. I used my long 3/16" bit to align the 1/4" hole in the angle to the hole through the spar & mount. This would serve to keep the bit lined up with the hole during drilling. I then used a long 12" bit (again with the cutting edges of the flutes dulled) to drill it out to 1/4". Three of the 4 holes can be drilled from the steel side which is recommended. One has to be drilled from the aluminum side because the tube for the gear leg is in the way.

There are also a total of six 1/4" holes to be drilled through the 1-1/4" x 1" x 1/8" angle into each mount. Four take hex head bolts and two take flat head bolts (they are countersunk with a 100o tool). Matching up to existing holes is not required on these as none of these holes have been previously drilled. I drilled only the four holes for the hex bolts (from the steel side whenever possible) using my temporary drill guide for the long 1/4" bit. I waited until I reinstalled F-604 back into the fuselage (together with the 1-1/4" angle) to drill the holes for the flat head bolts. I had a 100o countersink for 5/32" rivets. Its body diameter is just large enough for the 1/4" flat head bolts. A shim needs to be installed between the steel mount bracket and the 1" x 1-1/4" angle. Assembling the mount back in the fuselage is the only way of determining what thickness is needed. I first drilled these holes to 5/32", and then used the countersink with all pieces still clamped together. After countersinking, I used my long 1/4" bit to finish the hole. These should be final drilled starting from the aluminum side to assure concentricity of the countersink to the hole. When the skin is installed, one of the bolts can be used to dimple it. This countersinking operation could also have been put off until the skin were clecoed on.

Where did I Get the Tools?

I bought the long 3/16" drill bit from Parkrose Hardware. It cost about $8.50. The 1/4" one was $10.50 or so. I found a tool supply store which sells quality stuff for about half that. It is Palm Abrasive & Tool Inc. on 14th & SE Belmont in Portland. They sold me the transfer punch & 3/16" stub bit for a total of $2.45. The long 1/4" bit was about $6. I also bought a very nice 6" dial caliper with graduations of 0.001" for $25. I think I'll be back there a few times.

How Could I Have Done a Better Job?

Mackinnon Tool Co. in Portland (235-8555) sells drill guide bushings. They said they could get one 3/16" OD which would work - but it would take a week or so. Its ID would probably be a closer fit than the Brass tube I used. I didn't order one because I was in a friend's garage & didn't want to impose for that long.

I could have used the drill guide for the 3/16" holes like I did for the 1/4" holes.

Am I Satisfied?

Yes. All the bolts fit quite snugly (maybe a bit too snug) and I feel they were drilled within a few thousandths of where they should be. Considering the fact that each mount is attached with approximately 20 bolts, I think it will work fine. Besides, I don't plan to make any hard landings!

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