Need help with numerous cracks on dimpled tank skin!

[AX-O]

My second mistake so far, but my first major one.

I dimpled my tank skins with #8 dimple dies per Vans instructions. The #8 screws were not sitting completely flush on the skin so after all the holes were dimpled, I squeezed them again with the dies a little closer to each other. The first few holes looked good so I continued and re-squeezed the rest. After completing both tank skins I went back to check my work. To my disgust, I saw cracks on many of the dimples. The cracks start at the drilled hole and stop before reaching the flat part of the skins. My stomach has been turning nonstop. I can?t stop thinking about it. My questions are the following. Can I drill the holes wide enough to get rid off the cracks and then use #8 dimpled washers? Would that be strong enough? Would that give me enough edge distance? Should I just bite the bullet and buy 2 more skins (approx $100 each, plus S&H, pull fees and redo all the completed work)? Would that add a lot of drag and just look stupid?

I will call Vans later today.

[vlittle]

I had this happen in a different (non-visible) area at the wing roots.

The only explanation that I could come up with was the low temperature (40F), but that seems weak.

Since they were non-visible, I stop-drilled the cracks and declared victory.

Vern Little

[AX-O]

Originally Posted by vlittle

The only explanation that I could come up with was the low temperature (40F), but that seems weak.

It was around 50F in my garage.

[Kevin Horton]

I think you should order new tank skins. I would expect the cracks to lengthen with time in service. Using dimpled washers would be quite hideous.

What size did you drill the holes before dimpling? Were the holes deburred?

[AX-O]

Originally Posted by Kevin Horton

What size did you drill the holes before dimpling? Were the holes deburred?

The holes were deburred and I followd Vans directions. I can't remember the drill bit size (don't have the directions infront of me).

[szicree]

I can't imagine shop temp being the problem. My first thought upon seeing these pics was that the drilled hole was too small or perhaps not deburred. Too small a hole will definitely cause cracking. As for the fix, I'd say you gotta do it over. The screw is going to put that crack in tension and just rip it right open.

[SportAvServ]

Just thought I would ask, did you use rivit dimple dies or screw dimple dies? The skin was just streched too far, I agree with one of your replys that the maybe the hole was too small, also coming back later and dimpling them again may have work hardened the skin. But to get to it, the only fix is new skins, with the pre punched it won't take very long and a lesson learned. I too have made things over, nothing new to anyone.

Happy riviting.

RT

[AX-O]

Originally Posted by SportAvServ

Just thought I would ask, did you use rivit dimple dies or screw dimple dies?
RT

I used Avery #8 dimple dies and a pneumatic squeezer.

[Mel]

Are you sure that your dimple dies are 100 degree? I have found that you cannot dimple 2024 to 82 degrees, which is a standard non-aviation countersink.

[AX-O]

Just called Vans. The gentleman I spoke with had actually looked at the picture on this forum. He told me to leave it alone and keep working. No need to replace the skins or stop drill it. He said that as far as safety goes, it is fine. The tank is supported in many other places. I told him my concerns about the crack making it all the way to the edge of the skin. He said the worst that can happen is the crack runs to the edge of the skin and it would not be appealing to the eye, but no safety issues. He said he has never heard of a builder using dimpled washers to fix the problem but he knows of cracks on tank skin dimples. He said that the #8 dimple stretches the aluminum a lot.

He also told me that he personally knows of an RV-6 that has been flying like that for the past 12 years and the holes have not cracked yet

All that said. I may still replace the skins. $200 may equal peace of mind.