by lpaaruule » Sat May 30, 2015 7:03 am
I've found that the same AN bolts from different manufacturers vary in size by several thousandths. However, the same AN bolts from the same manufacturer have only varied by a couple thousandths in my limited experience.
Having said that, I used a .2470 reamer for the bolts you are referring to. It worked absolutely perfect for the bolts I have - 3 or 4 light taps with a mallet.
For the AN3 bolts, I use a different method. After I've enlarged the hole to a #30, I use a #14 to get close to the size. Then I use one of my custom ground #12 drills to get the "perfect" fit. While the hole isn't as round as a reamer, I've found it's as goods as I can make it without spending a fortune on reamers.
To make the custom #12 drills, I chuck one in my drill press then lightly sand the spinning drill with some 220grit or finer sand paper. Then I drill a #14 hole in some scrap 3/16" thick aluminum, then enlarge it with the ground #12. I then test the fit with one of my AN3 bolts. The first couple tries the hole is usually too loose, but I usually get it just right after the third or forth iteration of light sanding. It doesn't take much, and I've had to start over a few times because I sanded too much.
I've done this for tight and light drive holes, and all the different bolt sized I've encountered so far including the flush screw/bolts used in the main spars. I now have an assortment of custom ground drills in a bag in my toolbox. I take a yellow paint pen and paint the the non-cutting end, then use a sharpe marker to label what it is used for (AN3 Light Drive, AN3 Tight Fit, AN25, etc)
I'm in the process of joining my front and rear fuselage's together, and it's working great for the AN25 bolts.
Make sure that if you drill bolt holes without a drill press than you use a drill guide. If your like me, without a drill guide for at least the #14 hole, the hole will end up slanted, and the bolt head will never seat flat. Of course a lot of guys don't have this problem because they just use a #11 or #12 drill on all the 3/16" holes and don't give it a second thought. As you've probably heard, it's usually acceptable to use the next numbered drill size bigger than the bolt your using.