What kind of 2 stroke street bikes? The cr134 I half built had similar issues. Fresh it went 10 hours,pulled the top end and wtf! Could have been to long on the ring,also suggested it could have been the mix. Xamax & 537@32:1,that ended that. Fresh top end,bridge relief at .0015,15 minutes with similar results. Sent back for professional analysis,with the pistons and rings. The exhaust bridge has issues,its wide and tight. Well I will see the bore tonight with about an hour on it. I am ready to strangle the owner,from day one I insisted on a new bottom end. Old,used and abused,same engine I discovered the: swirl taper bored carb,seemingly modified intake,at least 1.5 lbs of aluminum removed from cylinder,squish clearance removed. Gm went shooting off on a thread about the right crankseal causing this type of abnormal wear. I would think that relieving the bridge .002 from the bore would be sufficient. What else causes the bridge to get hotter than its mating parts?