Hello, Auction purchased a c.1950 MM ZAU, hasn't run in maybe 15 years (sat outside). Pulled plugs and soaked a bit. Motor spun freely, several times as I checked compression (pretty good) and valve action (all operate). Went to spin it again and she won't budge (starter works well, just clunks into the ring gear). Ring gear is good, no missing teeth. I placed a wrench on the crank nut - doesn't budge, locked tight. Note that nothing else has been apart and nothing has fallen into any cavities. What would suddenly prevent rotation of the motor, when just prior to this it spun freely?