Fuel tank assembly question

I'm in the process of doing the fuel tank.. fun fun.

The instructions seem to say as part of step 6 on of section 18-4 that you machine counter sink the row of holes that attach the skin to the baffle.

Why?

Why are these not dimpled? I'm sure there is a good reason, but I'm not sure what it is..

Or am I way off?

You got it. Some builders do dimple, but the advantage of countersinking is that it is easier to fit the parts together without causing voids in the sealant during final assembly.

I believe the reason is that once the tank is assembled, the aft edges of the tank skins are very stiff, and having them dimpled, it's very difficult to spread them far enough to fit around the spar flanges without scratching them. If they're counter sunk, the tank should just slide into place.

Okay that makes sense.. Thanks for the replies (and all the PMs)

Dimple

I dimpled them and had absolutely no trouble with final assembly. If you dimple you'll want to use a small diameter female die for the baffle, as I recall. Because I dimpled, alignment was trivial. I had no leaks in either tank.

small diameter die:
http://www.cleavelandtool.com/Small-Dia-Female-Die-3_32-Rivet/productinfo/DIE4263S/#.VyYY9DArLDc

Apart from all the other correct answers, I think it also has to do with the thickness of the skin as well as the underlying rear baffle flange.
It has been a while but I remember the tanks skin being .040 aluminum
and the baffle perhaps .063 (not sure about that one)
An .040 skin can be dimpled but not an .063 without creating distortion.
Dimpling the tank skin and countersinking the baffle would be a bad idea as well.

When the baffle is set in final assembly it will push/squeegee a bead of sealant ahead of itself in a smooth manner; dimpling will make that action more difficult to achieve; but many people here dimpled it and had no problems. The every 10th hole for later is so that the alignment is preserved until after the baffle is pretty well set. If every hole was countersunk the alignment would be ever so slightly shifting as the first dozen or two rivets were set.

When I set my rivets on this I did as the plans say and one side of my rivets are a thousandth or so proud. Better shallow than deep it said. Thought maybe I had not done it right then checked a friend of mine's quick build tanks - identical. So it is factory correct if they stand up a hair on one side. Sanding for paint will bring it all true.