I've found an M20, freshly rebuilt by a guy that does this in his spare time. Says he leaves the side covers unbolted so you can look inside. I have spoken with him and he appears to know more than I do, which is pretty easy to do. ou: Here's the ad. Is it worth the money? He also said he'd swap my M21 to an M20 for around $500 if it didn't need anything other than bearings and gaskets. http://nashville.craigslist.org/pts/4928036379.html
He rebuilds them as his second job, autos, manuals, has lots of cases. I may go look at it in a week or two. Didn't know if $900 is a fair price for what he has if it meets expectations.
ad states 26 spline input, and large output (32 spline) that makes them more desirable in most peoples' books. Remember picking up used M-20's for $100 at albuquerque swap meets, but they were all 10 input and 27 output (early castings) and that was fifteen years ago.
Well, it shows a pic and he said he had others but this one was a fine spline M20. I'm going to go take a look at it regardless if I get down to Nashville in the next week.
Dad usually tries to get $600-$800. That includes new bearings,brass,gaskets and whatever else is worn. If any gears have been replaced his price is definitely in line.