Originally Posted by Surfdoc
I am sure this has been posted before but couldn't find anything. Where the tank skin attaches to the spar with the #8 screw. Did you dimple the skin or countersink? I dimpled one tank and it looks bad so I was told to countersink the #8 hole instead. I did one but seems like it removed way to much material and there is a lot of play with the screw. Now I do realize that the plate-nut carries the load but I would like to see what others have done.
Thanks in advance.
Let me guess.... the poor dimples on the first tank were probably done with a hand squeezer?
The fuel tank skin is pretty thick by RV standards, but still no where thick enough to machine countersinking for a screw. The thickness of the skin also takes quite a bit of force to form a nice dimple. Only a hammer with a c-frame tool should be used. If that is what you used, then you just didn’t hit hard enough. If not, you can form the dimples again ( on a finished tank it is useful to have a helper).
BTW, it is not the nutplate that takes the load (In fact in most fastener joints on an RV, the nutplate is never the main load path). The primary load path is the shear load of the dimple within the counter sink of the spar flange. If you machine countersunk all of the holes, the shear strength of all the screw joints would be severely compromised.
It might be good to review section 5 of the construction manual regarding dimpling and machine countersinking.