Thorp Air Command - T18.net

I'm getting ready to drill the H beam to the spar angles and web of the outer wing panel.
The plans call for drilling these holes with a #12 drill and installing NAS 178-6-8 "Hi-Shear" rivets with NAS 179-6 Collar.
'Feisty' was built with AN-3 bolts in these locations.
A #12 drill is too big to fit in the original holes of the H beam. A standard 3/16" (.1875") drill will not fit either, but it's close.
The used AN-3 bolts mic'd @ .185" give or take, a #12 drill is .189"
What say the experts? Should I drill #13 (.185") or drill #14 (.182") and ream ream up to .185 or other value? The #14 bit is a little loose in the hole and centering would be in doubt?
I understand a drill will not make a round hole, more like a hole with lobes...or is that just when drilling sheet-metal?
I'll be drilling down thru the original holes in the H beam, through the new parts using a drill press and support blocks to minimize damage to original holes in the H beam.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

I would drill small and ream. Although that is hard to do accurately with that many holes. Get good, American made split point drills and they don't make the triangular hole you mentioned.
I use a mill with a DRO, but not too many folks have access to those.
Cubes

If you accidently have a oversize hole, a close tolerance bolt (cup head) is .001 - .003 larger in diameter for a tight fit.
Phil
118BC

Agree with Cubes. AN bolts are always undersize by 3 to 4 thou. Mic all your AN 3 bolts and replace any that are not within a thou of all the others. Select a drill (it may be metric) that gives the closest size to bolts. A light tap through with soft faced hammer is the aim. You will need to do the same exercise withe the wing joint fittings.
Cheers
Tony Schischka
ZK-VMS

Thanks All;
Plus phone calls with Bruce F & Gary J.
I have a 3/16" split point drill that mic'd the same as my bolts. By carefully setting up my drill press and installing AN3 bolts as I drilled, I got a very snug fit on all and it turned out very well.
Maybe I got my self all worked up over a small issue, but the original H beam fit would be hard to duplicate to the center section clevis, Serious Pressure to get it right!
I then took the assembly to the airframe and installed it, the original bolts were a nice snug fit. I'm satisfied that I can continue with the clean-prime-rivet & bolt together part.
Then to drill/celco on the ribs and rear spar...
I'll be flying to Sedona in Brand X again this year, but I'll bring some progress pics to share around the wine table at the Sky Ranch.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Excellent. Looking forward to pictures
Cubes