Tank to spar problem

My tank sits high off spar due to dimple-countersink. I was not the one who countersunk the spar. I have built the tanks. Dimpled the tanks for #8 screws, ATS dimpler. The tank skin bulges .024 off the spar between the screws and .011 at the screw. The tanks fit, but does not leave a clean line at the tank and main wing skin. In other words, the spar is not countersunk deep enough. Is there anything I can do now to correct this? Thank you for your time and thoughts.

Machine countersink the spar deeper. If there is insufficient pilot to keep the machine countersink centered you may have to make a temporary aluminum piece that will keep the countersink centered. Best to make a mockup and try this out top prove the method.

pappa said:

My tank sits high off spar due to dimple-countersink. I was not the one who countersunk the spar. I have built the tanks. Dimpled the tanks for #8 screws, ATS dimpler. The tank skin bulges .024 off the spar between the screws and .011 at the screw. The tanks fit, but does not leave a clean line at the tank and main wing skin. In other words, the spar is not countersunk deep enough. Is there anything I can do now to correct this? Thank you for your time and thoughts.

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If by ATS dimpler you mean ATS hand rivet squeezer, then that is likely at least part of the problem.

The skin thickness on the fuel tanks is .032". A dimple for a #8 screw in that thickness does not form easily.
At the very least, I use a pneumatic squeezer to make screw dimples in skins (Actually I never use a hand squeezer to form a dimple if it will be visible unless there is absolutely no other way), but my preference is a C frame tool and a large hammer.

One of the most common issues effecting the finish quality (not airworthiness) of RV's is under formed dimple countersinks. The thicker the skin, and/or larger the dimple size, the bigger the problem.....

BTW, I caution just "enlarging the countersinks in the spars" without a thorough analysis of whether they are incorrect. Over-sizing the countersinks beyond what is needed is an airworthiness/structural issue.

I'd be interested in some more details about oversized countersinks here. I don't have my plans in front of me, but I think they say it's ok if these countersinks knife-edge. (If not in the plans, I may have read it here). If it does knife edge, it will start oversizing the hole. This seems to be oneof the common hang ups in the wing kit. I know I would appreciate any further discussion on it.

Scott

srt said:

I'd be interested in some more details about oversized countersinks here. I don't have my plans in front of me, but I think they say it's ok if these countersinks knife-edge. (If not in the plans, I may have read it here). If it does knife edge, it will start oversizing the hole. This seems to be oneof the common hang ups in the wing kit. I know I would appreciate any further discussion on it.

Scott

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The hole is expected to be knife edged and become slightly over sized but the countersink is still expected to be a specific size for a close fit of the dimple.