I'm in the middle of fixing up a'80 KX125 I just bought and yesterday I found a piece of dried-out rubber that's bolted onto the the rear of the frame in the area behind the shock. I looked up an OEM replacement and it says it's one of two "rollers" but I don't know what it does. Do I need to buy a new one or can I just leave it?

 

welcome to KF. sounds like ur talking about the chain rollers that keep the chain off the swing arm. yeah you need them.