Hi all,

I’m restoring a barn-find R100S (engine number dates it to 1978). No service history other than that the cylinder heads were dual plugged at some point and it had the carbs upgraded to del'lortos. The odometer reads about 46k, but the engine and frame numbers don’t match, so engine mileage is unknown. Before I started disassembly I got the engine running after a carb rebuild and routine maintenance.

With the engine out, I blocked the crank and removed the clutch and flywheel to replace the rear main seal and the oil pump cover O-ring. 

What I found: There’s a polished ring on the rear face of the engine case, in the flywheel cavity, about 2" out from the main seal bore and worse on the left side (photo attached). It does catch a fingernail. There’s old gunk on top of it. There are no metal shavings in the case or on the transmission shelf, and no visible cracks. The main seal looks intact, and the rear thrust washer looks new and is seated on its pegs. The back of the flywheel shows no wear (photo attached in my comment). I didn't measure crank shaft end play before I removed the flywheel.

What I think happened: The clutch, the oil pump cover (it has the later hex bolts) and possibly the flywheel look newer than the rest of the engine. My guess is the flywheel rubbed at some point, from poor seating or excess end play, and a shop later fixed it and cleaned out the cavity. I’ve also wondered whether it’s just clutch dust and oil slurry slung around by the flywheel.

Questions:

1. Has anyone seen this pattern, and which explanation fits better?
2. If it was contact, what else should I inspect before I trust this bottom end?

Thanks,
Allen