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I bought a harbor freight pneumatic riveter and modified the heads myself. They were originally hardened and couldn't be drilled so I hit them with a pencil torch until they were starting to get red, then let them cool off slowly. Once cool, i clamped them in the mill vice (a drill press would work fine too), and used a drill bit that was just a little larger than the unformed rivet head to dimple the top. I went slowly and measured the dimple with calipers until it was the same diameter as the rivet head. This formed a nice radius in the tip and it forms really nice rivet heads.
Not an extremely precise set of measurements but I hope it helps.
This is taken directly from the "Construction Standards" which can be found in the "Builder Resources" section of the Zenith website. There is also a sketch. Here's the text:
To modify the riveter head: The objective is to bring the outer edge of the rivet down on the skin; the
most effective is when the machined diameter of the nose piece is equal to the diameter of the rivet
head. This will require two different size riveter heads, one for the A4 and another for the A5.
The machine depth is the distance from the sheet to the top of the rivet head; check the depth by
pulling some rivets: if the nose piece marks and leaves a ring on the sheet then it is too deep; simply
file the nose piece down. If there is a gap between the edge of the rivet head and the sheet then the
nose piece is not deep enough.
Expect the standard (flat) riveter head (nose piece) to have been tempered by the manufacturer.
Before it can be machined, de-temper (soften it) by heating it up with a blow torch until it glows. Let it
cool off and proceed to machine the head. With aluminum rivets, it is not necessary to do any
additional heat treatment to the nose piece.
The rivet heads require heating due to hardening during the forming process... does not take much heat! I just used a 3/8' drill bit in my press to form the rivet head required... using an actual rivet to set the depth! No big deal, very easy to do... aluminum is a very forgiving material!
Thanks everyone. I appreciate the advice greatly!