Ciao Bob,

Not exactly certain, but it appears to be about 1/16" in diameter. When I measured the combined thickness the shutter blades it was 1/16", so it needs a grip length of a bit more than that.

When I searched the web last night, I found quite a few wholesale type sites, but no retail. McMaster had solid copper rivets 1/16" diameter, but that may be difficult for me to install.