Review of fuel tank filler flange rivet choice

I was all set to use 4x11.5mm domed rivets with the revised flange design (changing from 4x13mm rivets for the original flange design) and have them on order.

But I noticed from Pascal Latten's blog (who also used longer rivets than the 4x10mm supplied to avoid countersinking) that he had to use a 4x10mm rivet in one of the flange locations in any case (presumably with the countersinking as prescribed in the plans) in order to avoid a conflict with the breather tube.

Which prompts me to revisit just what the plans prescribe - why no countersink all the holes, and just use the 4x10mm rivets supplied (grip range 3.5-5.5mm for 73 TA 5-3)? There was a reason not stated for doing this in the first place, as revealed by Pascal's blog.

I wanted to design the appropriate degree of countersinking, for this revised flange design. I am using 120degree countersinks. With a total thickness of 6.13mm (3.1mm + 2.4mm + 0.63mm) to rivet together, and a 4.1mm hole diameter I want the diameter of the counter sink to be such that the depth at the 4.1mm boundary is well within the grip range of the 4x10mm rivets (3.5-5.5mm)

Countersink diameter = Hole diameter + 2 * tan(0.5 * 120 degrees) * depth at the hole edge

Choosing a 5.0mm grip, a depth at the boundary of 1.13mm calls for a countersink diameter of 8.0mm

Choosing a 5.5mm grip, the depth at the boundary of 0.63mm calls for a countersink diameter of 6.3mm

I will use 8.0mm countersink diameter.

These two options were trialled on 2 x 3.1mm sheets (total thickness 6.2mm) as well as a 4 x 13mm rivet without countersinking. The height of the latter's tail was ~3mm greater than the tails of both of the 4 x10mm rivets. It would be reasonable to suppose the 4 x 11.5mm rivets (without countersinking) would have tails ~1.5mm greater than the 4x10mm rivets. Pascal used 4 x 16mm rivets, making his tails more than 6mm longer, judging from photos. Hard to say whether 1.5mm excess tail length would have caused a conflict. Why bother finding out?

So I am just going to stop making such a bloody fuss about things, not get so hung up about countersinking (for it is needed for one of the rivet locations) and use 4 x 10mm just like the plans say! I have "wasted" $30 on buying these rivets, including postage, first the 4x13mm then the 4x11.5mm rivets. So much for being too clever!

I just have to decide whether I am committed to the modified flange design. For if I am not I'll need to flip the inner flange in order to be able to use it (i.e. make use of the non-chamfered edge). This will decide which side I should countersink.