Hi Chas,
A rough guide for solid rivets is:
• The rivet diameter, D, should be equal to or greater than the thickness of the thickest part in the joint and less than 3 x the thickness of the thinnest part.
• If the stack-up (assembly thickness) exceeds 2,5 x D, the rivet diameter should be increased, or another type of fastener used, such as a Hi-Lok or lock bolt.
• Where fluid tightness is essential the stack-up should not exceed 2D. This is because a bigger stack-up will require a larger force in the joint to ensure sealing, which may not be readily attainable.
The info comes from a well-known, large US Aerospace Corporation
Andries