How far oversize a hole?

[jlfernan]

I'm building an RV-9A. While drilling a 3/16" hole to mount the horizontal stabilizer to the vertical attach bars, the drill wobbled and the hole ended up
.014" oversize. Through the vertical bar it starts out fine but kind of coned out, ending up at .200" at the rear HS spar. The AN-3 bolts I have measure 186". My question is, at this location, how oversize can you go before drilling it to the next size? Sorry about the blurry photo.

 

[Kevin Horton]

I'm building an RV-9A. While drilling a 3/16" hole to mount the horizontal stabilizer to the vertical attach bars, the drill wobbled and the hole ended up
.014" oversize. Through the vertical bar it starts out fine but kind of coned out, ending up at .200" at the rear HS spar. The AN-3 bolts I have measure 186". My question is, at this location, how oversize can you go before drilling it to the next size? Sorry about the blurry photo.

The aerodynamic loads from the elevators are reacted through these bolts, so if it was my aircraft, I would like to have a nice close fit at those holes. In principle, the purpose of the bolts is to clamp the parts together, and it is the friction between the HS rear spar and the vertical attach bars that provides the load path, but it would still be better to have a bolt with a good fit.

Is there enough edge distance on all pieces to use a 1/4" bolt here? If so, that is the simplest solution. If not, there are expensive oversize close tolerance bolts available. Or, you could ask Van's for an opinion from their engineers. Advice from their engineers is much more credible than advice from us.

 

[Walt]

Get with our local fastener guru here on VAF and get a first or second over bolt and reamer. I would really avoid going all the way up to a 1/4 on this hole. In the future you should try using reamers to do the important holes whenever possible, they make much nicer holes than drills. I personally use NAS close tolerance bolts in many of these locations, just makes for a quality job. First or second over is routinely approved for all bolts in every situation I have ever run into and I'm sure van's engineers would agree.

 

[jlfernan]

I did use a reamer. I started drilling with a #30, then a #20, to #13, then a 3/16" reamer. The #13 unfortunately is the one that wobbled and the reamer could not fix it. The other three are perfect.I know it's impossible to determine something exactly without seeing it in person or laying your hands on it. I went out and took some measurements to see if they'll help. The aft side of the vertical bar the hole is
.188" high by .189" wide. Where it exits the rear HS spar the hole is .188" high by
.200" wide. The AN3 bolt measures out to .186". I don't know if this will make a difference whether to leave it or go to 1/4".

 

[jlfernan]

Solution decided

After speaking with vans I'm going to order and oversize bolt and the appropriate reamer from our friends at GAH. The questions I have are, I'm assuming NAS bolts are as strong as AN bolts and what size should the starter hole be for a .201" reamer?

 

[G-force]

Just ream it. Typical chucking reamer can easy remove 1/32" of material in one pass. Turn it slow (electric drill) with tapping fluid (I like Relton A9 for aluminum, kerosene or WD-40 will work fine as well for one hole.)