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A little bit late to the discussion, but I just rebuilt a lower on an MS-20, but the parts are the same. Mine had no seal in it and as you can imagine, the engine was a mess with extra lube getting out.
I made a tapered brass gauge that I was able to get to the seal area to get a ring on and then measure. Mine has a seal bore diameter of .555″ diameter. I wound up giving it a quick hit with a 9/16″ hand reamer and with that, CR lip seals 3621 pressed in with a nice snug fit. Easy work to get around an NLA part……
The interesting thing is that I could see no evidence of my engine every having been apart. Could that seal have just been missed during original assembly? No messed up screw heads, no torn gaskets… Hard to say since it is 80 years old but it makes you wonder.
Nice “work -around”!
Prepare to be boarded!