1987 RM 125 top end issue!!!!!

The exhaust bridge is seizing the piston, and ring. Either the bridge was not properly relieved, or you have a blown right crank seal. The cylinder needs to be checked with a bore gauge, and determined if the bore is still good. Out of round or oversize? Its all fixable!

The exhaust bridge is a thin strip of metal that runs through the exaust port. As the engine heats up this thinner metal can expand and cause that scuffing of the piston if it's not ground out a little to allow for expansion. If the crank seal is blown the aluminum from the clutch plates that is in the tranny oil can build up on the bridge and cause clearance issues.

And no, the crank seal has nothing to do with side to side movement of the crank.

The piston is aluminum, it will make a nice desk top item. See if the bridge cleans up, WD40 and a green scotch brite pad, and see if it comes off? Someone put a sleeve in it? Chances are good, the bridge was never relieved properly. The bore should be like .0015"~.002" bigger than the skirt of the piston, clearance. That bridge in the middle, needs to be about .001"~.0015" extra! If you check the bridge with a feeler gauge and the new piston skirt, it needs to be .0025"~.003" at the bridge, the rest of the bore should be .0015"~.002". I think the service limit is like .003"? But that gouge in your piston, paper weight. This does happen a lot, cylinders get bored by guys trained in 4 strokes? All the ports need to be chamfered also.