I'd assume they are .50mm over size. My questions is what's wrong with the pistons? Why not just re-ring them if they are clean, heck of alot cheaper. Also does SBT serialize their engines with a number? Call SBT with that number and ask them for the correct pistons. Just not sure why the tear down of a 25 hour engine? If it was complete and the guy said it ran I would have tried to fire it or at the minimum lay it in and try to drank it over for a compression test. Before I spent a dime I'd call SBT and VERIFY what they put into it.
I had to strip down the motor because the rotary gear went, leaving brass shavings in the case. The cylinders and sleeves are great, but one of the pistons has a ding at the top edge above the rings from ripping the motor apart. The piston kit I was looking at was a brand new pair with gaskets for $81. I will call SBT about their piston sizes. Thanks