Thanks for the replies. The prechamber relief in the pistons that are now in the engine are shaped like a short paper clip. They look as if they were simply cut with an end mill that was move about 3/8" or so. They have sharp corners, not a smooth transition down into the recess.

The prechambers are rounded on the bottom. From what Steve said it sounds like the pistons are the early type and the prechambers are the late type.

The bores were honed to fit this set of pistons. I then brought the bare block home and scrubbed away all the honing grit, and began building a short block. My parts store can get the correct pistons on Tuesday. If the new pistons are exactly the same OD then it will be relatively easy for me to change the pistons. But I have always seen the machine shops bore the cylinders a thou or two small, then hone the rest of the way to fit the pistons that will go in the bore. If I have to take the block back to the machine shop, rehone to fit the new pistons, then clean the block all over again, then it will be easier but more expensive to replace the prechambers.

Can I use the early OR late prechambers in this head with no trouble. This is a pin type glow plug engine, will there be any difference when using early prechambers which were meant for loop type glow plugs?

Thanks for all the information,