Bingo! Just found this thread. I bought a 1987 XL600RM last week and am in the process of rebuilding the motor due to a knocking noise at idle.
Tear down reveals some play in the little and big end bearings.
I just ordered over $1000 worth of parts to do a sump up rebuild. Going to run a Wiseco 97.5mm 11:1 piston, Kibblewhite valves and springs, new guides, rod kit,
cam chain and guide, all new bearings seal and gaskets.
Cant wait to get this big girl on the road.