The parts store gave me the wrong pistons, but the cylinders were bored and the pistons fitted to the bores before the problem was discovered. Changing the prechambers was the only practical fix at that point. This has cost me a BUNCH of extra time and about $300 extra for the prechambers to replace perfectly good prechambers. It's just one of those things that you have to take care of yourself when you do the work yourself.
Have a great day,