I'm rebuilding the top end of my '66 R50/2. I've taken care of the "butterhead" issue ok (at least for the time being) but ran across a weird condition with one of the pistons. First, I noticed that the oil scraper ring groove had collapsed to the point that it trapped the oil scraper ring rendering it immovable. I "fixed" that by gently prying on the slots located in the grooves to the point that a new ring fit in the groove without interference. Then I noticed that one of the wrist pin circlip grooves had deformed outwards. This had deformed the piston material outboard of the groove to the point that the wrist pin could not be inserted or removed from that side. Oddly, this piston (right hand side) was marked "1-66" while the left side piston (in good condition) was marked "2-67." Does anyone know if the bad metallurgy of the heads in '66 also affected some pistons? The different markings would indicate that the left side was replaced with a new piston some time after the bike was sold new.