Originally Posted by bmwrench
Those look like Black Diamond valves to me. The black coating often wears on the stem, particularly at the bottom and to a lesser degree at the top-which is where all valve stems wear. It doesn't seem to hurt anything as the stem itself is supposed be nitrided. The thing to watch for on these valves is transfer of guide material to the stem. If this is the case, both valve and guide should be replaced. If no metal has transferred and you measure clearance, be aware that this assembly needs at least .003" clearance to survive.
The right piston crown indicates that it has had water in it-probably from the crankcase vent. I'd bet that the top ring started to rust to the bore at some point and that the ring broke when the motor was started.
Have you seen Black Diamond valves with the stems so perfectly polished before? I haven't. I don't think I have ever seen any that didn't flare the keeper grooves quite a bit as well. Were the keepers ground short so the valves didn't rotate? That's the trick to getting crappy wearing keeper grooves to last longer.
I have set up MANY a BMW airhead guide with WAY less than .003 stem/guide clearance. They worked perfectly AND let a LOT less oil by for a lot longer time. Hellfar, I have set up MANY a BMW airhead piston/cylinder with less clearance than that! Don't get me wrong, I am not big on real tight guides on most engines but airheads don't have seals and it makes a big diff! The ONLY way to run tight guides, or even looser guides IMO, is to size them yourself with a Sunnen valve guide hone. IMO, all other methods are comparatively a like using a chisel. Most people don't have access to a cylinder dial bore gauge let alone a guide dial bore gauge. IMO, measuring both with anything else (of course it takes micrometers to use the bore gauges) is a joke.
I bet you're right on about that right piston. That isn't wear. It's corrosion.