Hard time getting tight fit for AN8

[dtibbo]

I'm having a hard time getting a tight-fit for the AN-8 wing attach bolts, and right now just playing with scrap for the AN-8 bolts. The instructions are asking me to drill a tight-fit for the bolt, which is 1/2" diameter.

On a scrap wing attach block, using a drill press:

Drilling to 1/2, the bolt drops in with wobble.
Drilling to 31/64, the bolt doesn't fit even with tapping.
Reaming the 31/64 hole to 1/2, the bolt drops in but without wobble.

First glance I'd need a size increment of 1/128, but it seems nobody sells them, and every forum has success doing the above, drilling undersized and reaming.

Any tips?

[gammaxy]

I'm not looking at my plans right now, but I thought those were supposed to be 3/8" bolts.

Here's a post I made about the bit I used: viewtopic.php?f=34&t=752#p6072 Looks like the price has about doubled since I bought one.

I think my fit is about like your reamed hole--maybe a little tighter. It won't quite "drop in" the hole, but a little thumb pressure gets it in. When I put the bolt through the spar tunnel and both spars, all the pressure requires taps from a mallet.

[Bryan Cotton]

The classic A models use 3/8" bolts. I believe the upstart B models use 1/2".

My Hummelbird has 1/2" wing attach bolts. I drilled undersize and reamed to something like .4975. I'll check the actual size next time I'm over there and remember. Also I polished my bolts with scotch brite to get the final fit. They go in with grease anyway so I am not concerned about corrosion. The polishing takes off the high spots that interfere with the fit. Got that tip from a machinist buddy.

[racaldwell]

Hi David,

I have been doing the same as you for my AN8-45 bolts for my Xenos spar. Same results as you. I have now just bought two piloted reamers from Pan Am Tools. The small one is piloted about 15/32" to 0.4795" and the other is piloted 0.4795 to 0.4950" I plan to use my 3/8" piloted 15/32" counterbore as the 1st step from the existing 3/8" hole.

The AN8 bolt measured 0.4970" so I now need an expansion reamer to go another 0.003" diameter. I tried a 1/2" expansion reamer with the screw backed all the way out but it made a good 0.5000" hole. I hope to find a machinist I can borrow one from. But so far, that hole was the best fit so far. The 3/8" piloted 1/2" counterbore made the sloppiest hole, the 1/2" drill bit was next sloppiest. Maybe I should go with the undersized 0.4990" reamer; which I will if I cannot find an expansion reamer that will go 0.4980" undersized.

Maybe crocus cloth on the cad plating to remove it like Brian suggested and greasing the bolt will get it down to 0.494" and then it will fit the reamer I have. I am also going to turn down the shank of the reamers to 3/8" for an inch or so to allow the use of my small variable speed pneumatic drill. I know space will be tight when I actually do the spar job.

Let me know if you find the perfect combination.

Rick Caldwell
Xenos 0057

[dtibbo]

Thanks all, I think I'll check the micrometer for the bolt and go from there with the EAA chapter. Ten thousandths narrower should do the trick, right?

They've got a "build night" every Tuesday, so I'm thinking I'll show up with the chunks and see if I can snag some assistance.

I'm doing the Waiex B-Model, so the two wing attach bolts are a half inch.

[Bryan Cotton]

By the way, playing with scrap is a great idea. I drilled about a zillion homes in a stack up of scrap before I drilled the 4 real holes for the spar. They came out good.