They will take an M8 allen head bolt. Cap head I think they call it.
I think M8x20mm is the right length, but check the length as that's from memory. It looks crap if its a bit long as you get the spare bolt sticking out through the nut when you tighten it.
The problem I have found it that the diameter of the allen heads varies by a mm or 2, so some will slip right in the counter-bored hole in the clamp and others wont fit.
Same with the nut. The original ones were not a regular M8 nut (which would take a 13mm spanner size), they were made with less material around the threaded hole so although still M8 threaded they would be maybe 11.5 or 12mm measured across the flats.
I ordered some bolts on ebay and the heads were slightly too big. Best to take a seatclamp and go somehere like a local fastening supplier if you have one locally.
I usually file the flats on the nut a little on each face, until it fits in, then a quick spray of satin black.
If anyone knows where to get the smaller nuts still with M8 thread I could use some too.
It could have been that the originals were a close imperial equivalent to M8, hence slightly smaller.
