The bike had been re-bored with new pistons but was smoking badly. I got the task of putting it right. Stripping the engine down revealed the pistons and bores to be badly scored despite only a few hundred running-in miles having been ridden. The barrels were measured and found to be very poorly re-bored and honed. There was still evidence of pitting on the centre bore, not removed by the re-bore. All 3 liners had different dimensions and all 3 were outside tolerance for the new piston size. On top of that the liners had been bored at an angle, were tapered and oval. My guess is someone went mental with a hone to try to remove the pitting, after a poor quality re-bore. The poor pistons didn't stand a chance and are now scrap.
So, new rings gapped and fitted to the pistons, and new base gasket fitted.