Double flush rivets, what length to use?

[RV-Ogler]

Should the shop end still be 1.5 the diameter in length? What do you guys like to use ?

 

[Ralph Inkster]

Do not use a shorter rivet thinking it easier to flatten - if the shop head doesn’t fill the dimple/countersunk (looks like a ball sitting in a hole) is not structurally sound.
The shop head should flatten & fill the hole just about as much as the factory head to be proper.

 

[mturnerb]

A shorter rivet might appear easier to flatten - but if the shop head doesn’t fill the dimple/countersunk (looks like a ball sitting in a hole) is not structurally sound.
The shop head should flatten & fill the hole just about as much as the factory head to be proper.

Not my experience at all using the rivets specified - getting a shop head to conform fully to a countersunk/dimpled hole isn't a realistic expectation, and I don't think that's what is required structurally. Per Van's Section 5: "...The factory flush head will set almost perfectly flat. The finished shop head will be flush with the skin, but it will not fill the dimple completely. This has been described as "an acorn sitting in a dimple." Do not fall into the trap of trying
to use a longer rivet to "fill the hole."..'"

Typical appearance of shop heads (bottom rivet in photo, elevator trailing edge):

These on a canopy rail fill more of the countersink:

 

[Ralph Inkster]

Shop heads in your pictures look good. I have been able to get more spread on mine.
The big caution would be where the shop head was smaller in diameter basically looking like a small ball in there. I have seen some trailing edges like that, not good, probably caused by using the called out rivet (3.5) but dealing with a thick layer of adhesive between the skins & wedge, maybe should have used the next longer rivet in those cases.
Note I edited my earlier post for clarity.