Drill size for new L2 Taylorcraft wing spars

[everell]

I am replacing damaged wing spars with new wing spar wood. The holes in the old wing spars are oversize compared to the holes in the front spar butt fitting assembly, rear spar butt fitting assembly, front spar strut fitting assembly, and rear spar strut fitting assembly. I assume this is because a bushing is required in each of the four holes for each assembly. I have a copy of the 1944 parts catalog AN 01-135DA-4 which does not show any bushings for these fittings. Can someone enlighten me as to what is going on? If it is just a matter of drilling oversize holes and installing bushings, I need to know drill size and bushing size. Is it to be a tight press fit or is some slop permitted? If not a tight fit, can glue be used as a filler?

[Robert Lees]

Make sure you use close-tolerance bolts in the metal spar fitting to fuselage holes

[Dick Smith]

This isn't a direct answer to your original question, but this attachment has some interesting considerations. I've saved this in my Taylorcraft idea file for quite a while.

Bill Berle's Gilberti jig system

I had the new front spar bushings made up from the 3/4" phenolic type L rod, available through Aircraft Spruce. I am certain it is an acceptable material for this purpose. McMaster-Carr also carries a material called Garolite in several grades. They wanted two weeks to get me a piece, so I drove down to Aircraft spruce here in California just to get the material and keep going.

I took the phenolic rod to one of my EAA chapter members with a lathe and had him turn the pieces down and drill them out. Crispy Critter offered to provide some bushings to me as well, (Thanks Kevin), but my guy came back in town fast enough that he could get them done faster.

Any half-talented machine shop or home lathe hobbyist can do it.

The BIG SECRET for me was to have the bushings made to .007" smaller diameter than the holes in the spar. The hole is .6875 (11/16) and the bushings were 6868 or 6869. This is good for two separate reasons:

First, it allows the bushings to "float" in the hole a little bit. this means that when you bolt the fittings in place you don't have to hammer the bolts in or bend the bolts etc. The bolts should just slide right through the fitting straps "suspending" the bushing in the hole wherever it needs to be.

This in turn means that you can have a reasonably snug fit of the bolts, the bushings, and the steel strap holes. You can have actual .250 true holes in the bushings and the steel straps and get the bolts in with no slop. A good idea for a wing attach, don't you think? If your steel straps are loose on the bolts, use NAS close tolerance bolts that are a couple of thousandths bigger than regular AN bolts.

Second, the smaller bushings allow you to put in a thin film of epoxy and NOT push all the spoxy out when you put in the bushing. The epoxy manufacturer (System Three T-88 in my case) recommends that you do not "starve" the joint of glue.

So, I used T-88 epoxy mixed with about 10 or 20% "cotton flox" as a structural adhesive. The small fibers give the glue some body and prevents the glue mass from cracking under load. This mixture has been used successfully for decades by the composite airplane builders.

I carefully test fit the bushings, bolts, and steel fittings twice before gluing so I had no surprises when the glue was on. An important note was that the bolts holding the fittings in place were too short of a grip length. I had to get new bolts and measure out the needed washers and nuts etc. for the right fit.

I applied a coat of pure T-88 epoxy to the inside of the holes to get the maximum penetration into the wood, and let it soak in for five minutes. Then I cleaned the bushings with lacquer thinner and put them into a small clean cup to prevent any dirt or oil from fouling them. Then I applied the epoxy/flox mixture to the inside of the holes and the outside of the bushings. I pushed the bushings in place and scraped off the excess epoxy from the wood.Then I bolted the fittings in place immediately so the bushings would float into the right position with NO STRESS ON THE BOLTS.

Getting epoxy on the bolt shanks is actually good instead of bad. This is an area where you do not want any looseness in anything.

Although this was more work than simply hammering the bolts in or not having a tight fit, I am confident that the fitting is as secure as it can be. Another important note is that many of the existing small bushings were TOO easy to tap out with a drift punch. The few that would not punch out easily came loose when a drill bit grabbed them and they twisted right out.

The engineers here can flame me to death if they disagree... but in my strong opinion the bushings in a spar root should be glued in place very securely. Any play, looseness, or wiggle room creates a very unsafe situation. I'd rather be forced to drill and ream them out later than have them removable now.

Bill Berle

[everell]

Thank you very much, Dick Smith, for the information on the wing spar bushings. You mentioned that the hole size in the wing spar is 11/16 inch. The holes in my old splintered spar appear to be 1/2 inch. I am wondering if there is any size difference needed when using either A65-8 or C85-12 engine? Do you know of any vendor selling ready made bushings?