The piston was actually a +.030” second hand Polson piston that had negligible wear. I checked the ring grooves for wear which was negligible then I took the piston to a local machine shop that has a piston grinding machine to finish the piston to +.020”. The cylinder was then bored to suit. There appears to be ample wall thickness behind the oil ring but the decision to use an old piston was clearly a mistake.
The piston was not a thermal slot piston so that is not a cause of this failure. I will be doing a complete engine strip to look for ring parts.
I am one who does not like thermal slot pistons. Thermal slot pistons were a disaster in the Norton Commando with most suffering the same failure as the piston above. In fact all the prior piston failures that I have seen were thermal slot pistons. There is a thread on this site regarding a similar failure to a VB thermal slot piston. Most of the conduction of heat away from the piston is by the rings. This article provides good ring information.https://www.lsxmag.com/tech-stories/pow ... trous-kit/