Announcement

**Yellow Duck** · 08-14-2005, 03:48

Re: Drill Jig Available for Spar Upgrade

Bill, not ready yet, but would certainly like to install the 85 on the Duck or find a project to rebuild and install one on it.

Would be intersted in your observations on the up-grade. Please keep us posted.

**Jim Herpst** · 08-14-2005, 06:21

Re: Drill Jig Available for Spar Upgrade

Bill lets get in contact offline I suppose. I would like to borrow as soon as I get the bushings. Where did you or does one find the bushings?

**VictorBravo** · 08-14-2005, 11:22

Re: Drill Jig Available for Spar Upgrade

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.

**birdlegs** · 08-19-2005, 13:11

Re: Drill Jig Available for Spar Upgrade

Bill; Very nice technical inputs regarding the STC's details, this is in language that's user friendly,and its work's like this one that remind me what a asset this tribe can be, albeit the politics..... with recognition for keeping it real.
Greatful reader, Brad

**VictorBravo** · 08-19-2005, 23:49

Re: Drill Jig Available for Spar Upgrade

Thanks for the kind words Birdy. I made some minor corrections and spellchecks to this post and included it in my big document noted elsewhere.

Bill

**VictorBravo** · 08-27-2005, 11:31

Re: Drill Jig Available for Spar Upgrade

The spar drill jig, bushing, pins and drill bit have been shipped out to the first guy on the list, Tom Taylor, sent yesterday. As soon as it is sent back to me, it will be available for the next person who is interested.

Bill Berle

**Robert Lees** · 08-27-2005, 15:44

Re: Drill Jig Available for Spar Upgrade

Good-Oh, Bill, this kind of exchange is good for the group. All My jigs, fixtures are of course welcome to all, on a return-postage-paid-basis as well...

**VictorBravo** · 09-02-2005, 01:35

Re: Drill Jig Available for Spar Upgrade

Tom has finished using my jig and is returning it. I'm glad to report that he said it worked well for him. Whoever else isinterested in using it, please send me an e-mail to make arrangements.

**chrisvanosdol** · 09-09-2005, 12:16

Re: Drill Jig Available for Spar Upgrade

How about posting a picture of it.

Originally posted by VictorBravo

I have just finished installing the new 11/16" bushings in my spars for the 85 horse upgrade. Based on the original idea iof a drill jig made by Rob Lees, and the ideas of two of my local airport "good old boys", I asked one of my EAA chapter members to make up a simplified version for me.

Along with a little bit of cleverness on my part (related to the instructions on the back of the epoxy bottles), it worked very well.

The "tool" set consists of a machined block of aluminum, two double-ended alignment pins, a drill bushing, and a hard-to-find 11/16" Forstner drill bit. You will need only a few blocks of scrap wood, a couple of C-clamps, and a good drill motor.

It was my original intent to permanently donate this tool to the Taylorcraft Foundation so they could loan it out to members in return for a small contribution to the Foundation. Recent events have changed that situation; however I am still very willing to loan this tool out to any Taylorcraft owner who is doing the STC upgrade (or preferably their own 337 / field approval for the power / weight upgrade). Use of this tool is free of charge for any legitimate Taylorcraft owner.

I will need to get this tool set back after I loan it out, both so that I can use it again myself if needed or for others who may be doing the upgrade.

Anyone who is interested is welcome to have a copy of my notes and experiences using this tool and my analysis of the strong and weak points of the STC.

Bill Berle
N29544
1940 BC-65

**VictorBravo** · 09-10-2005, 09:44

Re: Drill Jig Available for Spar Upgrade

Here are a couple of photos. Thanks to EAA Chapter 40's Ralph Snyder for making the jig, and thanks to Rob Lees for the original concept and design of the jig.

Attached Files

**Jim Herpst** · 09-10-2005, 10:38

Re: Drill Jig Available for Spar Upgrade

Bill, I did not yet thank you for your reply earlier on this thread regarding the bushings. I will send you a private e-mail when I am ready to borrow the jig.
Thank you for being ever so helpful like Mr Lees of this site and Mr Bruce Bixler of the TAYLORCRAFT ... CLUB.

**VictorBravo** · 10-01-2005, 09:49

Re: Drill Jig Available for Spar Upgrade

Dan Smuda of Minnesota has kindly returned the spar drill jig kit and it is now available for the next person who needs to use it.

I am very glad that this tool is being used and seems to work well for others.

Bill Berle

Announcement

Drill Jig Available for Spar Upgrade

Drill Jig Available for Spar Upgrade

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