I am rebuilding a top end of dirt bike for the first time. It is a 2001 RM 250, and the manual says to insert the ring into the cylinder and just check the ring end gap. Do I have to do that with the new rings that I just bought or is that just to check if the old rings are bad? Also, it says the service limit is 0.80 mm is that the maximum gap? Is there a minimum gap that it should be? I have heard that sometimes the new rings need to be filed? That doesn't make sense to me. Can you please clarify? Do you have any other areas that I should look out for while replacing the top end? I really appreciate that Dirt Rider keeps the readers informed on used bike market and tips for keeping old bikes running well. Please keep writing about two-strokes!
Scot Zundel
Pittsburgh, PA
Scott, The manual does have a minimum end gap measurement, but I'm not sure what it for an RM250. Generally you don't want less than 10 to 12 thousandths of an inch or the ring ends can weld themselves together. You should check your new rings, but this isn't usually an issue with modern parts, and especially with a used cylinder. In the past tuners bought rings for the next size piston and filed the gap for the best possible fit, but that was in the steel cylinder era. -Karel Kramer