AN Flush Rivet in 120 Degree Countersink?

Can you successfully set an AN flush rivet (100 degree head) in a hole that is accidentally countersunk with a 120 degree bit? This is in a non-critical structure part - attaching a spacer to the bottom of a seat pan in a Zenith 750 STOL. I was countersinking the spacer and THEN noticed the bit was 120 degrees and not 100 degrees! Yikes!

Just wondered if anyone has ever done this. Will the rivet set flush and form a proper head anyway? I guess joint strength is compromised but in this application about all the rivets have to do is keep the spacer located, there's no shear or tension forces.

If it's a really bad idea, I can recover by using pulled rivets with 120 degree heads, but would prefer the flat shop heads of the AN rivets to the "tails" of pulled rivets - won't poke the fabric on the bottom of the seat cushion as much.

Out of curiosity why don't you just make up a test "coupon" with the conditions you have, set the rivet and see what happens?

HFS

I absolutely intend to make a test coupon, but I won't have access to a rivet squeezer until next week and wasn't going to bother to make the trip to go get it from my friend (a true friend - he loan's aircraft tools! LOL!) if told by others it just wouldn't work.

jaustinmd said:

Can you successfully set an AN flush rivet (100 degree head) in a hole that is accidentally countersunk with a 120 degree bit? ..

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Just thinking out loud here, a 120 degree countersink is shallower than a 100 degree of the same diameter. So unless you've countersunk it for a rivet with a substantially larger head, I'd think that you could just hit it with a 100 degree countersink and make it good.

That said, for something non-critical I'd probably shoot a couple test pieces, and if it wasn't exactly ugly I'd just do it and build on.

Of course, my official advice as an EAA Technical Counselor is to follow the plans to the letter and where practical confine innovations to the envelope described by AC43.13...

--Bob K.

I checked with my friend and he's got a 100 degree cs bit, so when we both get back from holiday trips next week, I'll try your suggestion on a coupon - do a 120 cs, then use the 100 cs bit, and then squeeze the rivet and see how it looks and I'll report how it worked out. BTW - built the plane 10 yrs/725 hrs ago and the spacer will allow just a bit more seat adjustment, but is non-structural/non-critical so no problem there.

Or go up a size or two and re coutersink at the proper angle for the larger head.

Bob Kuykendall said:

Just thinking out loud here, a 120 degree countersink is shallower than a 100 degree of the same diameter. So unless you've countersunk it for a rivet with a substantially larger head, I'd think that you could just hit it with a 100 degree countersink and make it good. ...

--Bob K.

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What you suggested worked just fine! I just went back over the 120 degree countersunk holes with the 100 degree bit and the rivets fit flush and squeezed tight with a normal shop head. It probably helped that I had not fully countersunk the holes with the 120 degree bit as I was going to make another pass at them to get the rivets truly flush - instead, I made the final pass with the 100 degree bit and I think there wasn't much difference in the final result vs if I had initially used a 100 degree bit.