Every late model RM250 with the two intake bridges will scuff the back of the piston because of a poor cylinder design. No amount of oil holes in the piston, or relieving of the bridges or polishing will make any difference. The only way to fix it is to heli-arc weld the top inside corner to a larger radius (from .5mm to 10mm). Of course this requires replating. I originally worked on this problem when I ran Max Power and did warranty work for Wiseco. We tried all sorts of piston shapes and port mods all to no avail. We also made Suzuki aware of the problem but they just blew it off. You see, because of this problem they sell lots of $100 pistons and $500 cylinders. Thats the economics of dirt bikes, sell the bikes cheap, design them to wear out parts, and charge a fortune for the replacement parts.
Since 1999 I've fixed lots of RM250 cylinders in this fashion and haven't had 1 come back with the intake bridge scuffing problem. The fact is the old style oval intake was a reliable design and they never should've changed it.
Kawasaki had a minor problem with they're cylinders and quickly changed the casting core, width and radii of the intake bridges on the KX250.