My piston in my 87 CR250 has a big ol' score in it centered right over the exhaust port bridge. It also flat spotted the rings a little. I pulled the jug recently, there was no damage. 4 ride later, there was a giant score. 2 things happened differently, I switched to Amsoil, but I also went 4 rides without changing the tranny oil. I read in Gorr's book that a leaking right side crankcase seal could cause that kind of damage. Maybe, because I left the oil in there so long, the oil that did leak by had a lot of those abrasive particles? I'm just trying to figure out why the bridge scored the piston. The plating is fine, however, it was somewhat more polished over the bridge and the polishing extended in a line the width of the bridge to the top of the cylinder. I'm going to be putting a new piston in here in the next week or so and I don't want to score the new piston as well. I have ordered a new left side seal and replace it as a cautionary measure, but I haven't seen the bike consuming much gear oil (it does suck some through, though). I ask for Mr. Gorr's expertise as he is the one who stated wear over bridges may be attributed to tranny oil blow-by. Oh yes, there were no holes drilled in the piston over the bridge, but the piston had at least 30 hours with no damage and then scored within 10 hours.