Wings: Fuel tanks. The fallacy of "the rivet will fill the hole".

I decided to redo my fuel tanks.  See here for why.  The entries specific to the redone tanks are here.

I've read that an elongated hole (one that exceeds the rivet shank diameter) will be filled by the rivet when swelled by the bucking process.  It made sense to me but I never considered testing the notion and perhaps I misunderstood its extent.  Seems it only holds when the hole is nice and round.  Makes sense now...

I had a AN470AD4-5 rivet on my left tank at the inboard attach zee that I had to remove since it was mis-bucked.  I did a horrible job removing the rivet and really messed up its hole.  I decided to replace it with the same size rivet figuring the rivet would swell into hole.  But since the hole had a bad trajectory (due to the angle of the drill bit), the shop head took on a weird shape and was pushed over. 

Here's what the AN470AD4-5 looked like before being fully bucked.  You can see it is taking on the new and misaligned trajectory of the messed-up hole.

Here's what it looked like after being fully bucked.  Who wants to fly with that?

So I decided to yank the rivet and drill out the hole to accept a AN470AD5-5 (my first such rivet in the build).  Here is what the AD4-5 rivet looked like with its factory head removed but the shank and shop head still in the hole.  Clearly, the rivet didn't swell into the hole even after full bucking.  You can also see where the rivet set came in on the attach zee itself (nowhere near a rivet, embarrassingly!) due to a gross misalignment on my part.  That will be sanded out.

And a super close-up of the hole and the removed rivet shop end and shank.  You can see the two distinct overlapping holes:  1) original and 2) new one from the drill bit during previous rivet removal.  And, notice the shape of the shop head and shank.  Removal of this rivet and upsizing the hole is a necessity.  Finally, you can see how I sanded out the damage to the zee from the rivet set.  That took a good 30 minutes of thumb work with 220 grit.

Here are both sides the AD5-5 bucked into place.  Unfortunately, I hit a smile into the attach zee so I had to spend forever sanding that out (you'd figure with all these smiles that riveting makes me happy).  Very difficult with the factory head so close (I also have to sand out the bucking bar scuff on the inboard rib).  I used 85 psi on the gun and a 2 pound bucking bar to squash this guy down.  He wasn't going down without a fight and I wasn't even up against work-hardening!  Sure hope my low fuel level optical sensors survived all of the violence.

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