It has Otto head gaskets which had their final tighten down about 100 miles ago. The head doesn’t have any problems, touch wood.
I took out the push rods from cylinders 1 and 2 and looked down the push rod tunnels with a torch. I’ve looked all around the rocker box, in the corners, and within the springs. I haven’t found the metal fragment from the collar. I may buy a small magnet on a stick and have a poke around with that.
The bike is currently sitting in the workshop while I decide whether to take the head and barrels off.
Square 4 mk1, Square 4 mk2, Ariel Colt, Matchless G12, BSA C12.