[Wendell]

I have to wonder why we use a # 40 bit for an AN426 3/16" rivet when a closer tolerance bit seems more appropriate?

 

[rocketbob]

I use #41's.

 

[vlittle]

Easy answer from AC43.13.

The rivet swells to fill the hole, and the cross-sectional area of the rivet determines its strength.

If you drill a smaller hole, the rivet won't be as strong.

Vern

 

[24Golf]

#41

When we went to the Alexanders Technical RV Builders school in Griffen, GA they taught us to use #41 bits and reamers in place of #40's for that exact reason. We are currently on the finish kit and have used #41's all the way through... I don't think it makes much difference, but we did do the math and it does make sense to use 41's

 

[alpinelakespilot2000]

When we went to the Alexanders Technical RV Builders school in Griffen, GA they taught us to use #41 bits and reamers in place of #40's for that exact reason. We are currently on the finish kit and have used #41's all the way through... I don't think it makes much difference, but we did do the math and it does make sense to use 41's

So what size dimple die are you using? A custom made #41 die?

After you dimple with a #40 dimple die the hole is going to be just as wide as if you had just drilled it with a #40 drill in the first place. Worse, however, is that now you've stretched the hole to #40 with the dimple die rather than drilled it. That's how cracks get started. Maybe it's such a small difference that there will be no ill-effects, but the logic of drilling with one size and then dimpling with a larger size kind of escapes me.

 

[N941WR]

When we went to the Alexanders Technical RV Builders school in Griffen, GA they taught us to use #41 bits and reamers in place of #40's for that exact reason. We are currently on the finish kit and have used #41's all the way through... I don't think it makes much difference, but we did do the math and it does make sense to use 41's

This falls into the, "thousands of airplanes have been built with..." category.

In this case, thousands of planes are flying that were drilled with #40 drills and no reamer so why change what works?

Besides, that means running a tool through every hole a second time, which takes even more time. Drill w/ a #40, deburr, dimple, and move on.

 

[Mel]

KISS!

I gotta go along with Bill here. No need to over complicate things.

 

[flytoboat]

#41 bits/reamers

So what size dimple die are you using? A custom made #41 die?

After you dimple with a #40 dimple die the hole is going to be just as wide as if you had just drilled it with a #40 drill in the first place. Worse, however, is that now you've stretched the hole to #40 with the dimple die rather than drilled it. That's how cracks get started. Maybe it's such a small difference that there will be no ill-effects, but the logic of drilling with one size and then dimpling with a larger size kind of escapes me.

A #41 bit/reamer is .0960". My 3/32 male dimple die measures .0930 so there is still clearance and I don't think I am stretching anything because of a #41 reamer and a 3/32 dimple die. A check of a few holes with a magnifying glass shows no difference of dimpled holes reamed with #40 vs #41.

(added) I only use the #41 reamer for match drilling pre-punched holes so it's not an extra step.

 

[Wendell]

I want to thank everyone for their input. I spent 400 hours with another builder building a 6A airframe in 2000 and the rivets never seemed as loose as they are now so I did the math and started thinking. I am currently building an 8A by myself and I am thinking too much for my own good. Vern?s answer hits the spot for me and I also ordered the book titled Aircraft Inspection Repair and Alteration
Again thanks
Wendell
RV-8A