RD Nice one for the great mechanical disaster album.
Owain. Are your new pistons another oversize? The machinist you used may have bored and honed to published figures, and assumed the pistons were smack on for dimension. They may have been not quite right and it is always best to double check by measuring the piston, allowing a clearance and boring to that. In effect the freshly bored cylinder is like a big nut, only at the correct bore at the bottom of the " thread ". Honing attempts to make the sharp peaks of the thread into a plateau, and is done with various methods, not all ideal, such as...fixed stones in a rotating carrier, adjustable for fine increases in cut, stones on a three legged carrier, pressed onto the bore by centrifugal force, and abrasive balls bonded onto what looks like a sweep's flue brush. Some of these are more suited to breaking the glaze, rather than accurately honing to a finished size.
Ask to see the hone....you need them to use one that has some form of incremental adjustment if you are going to use your new pistons in that old bore. The existing bore needs accurately sizing to the new pistons give an adequate running clearance.
Don't just dump the old pistons...They have some value.
Swarfy.