<VV> Intermittent Crank Jam?

[N. Joseph Potts]

The subject is a well-worn 1966 110hp 4-speed. When I hit the starter, it
turns a partial revolution, then jams. Something that's supposed to turn is
hitting something hard that won't.
     I've charged up/swapped out the battery, redone all connections from it
to the starter, taken the starter out, overhauled said starter and solenoid
(looks OK), even inspected all the teeth on the gear on the pressure plate
(they all look OK). Starter spins well on the bench.
     Naturally, I'd prefer this problem to be in the starter or upstream of
it, but I'm losing hope. At the same time, I really can't quite think what
this is. I don't THINK there's a thrown rod. Obviously, a compression check
is impossible.
     Thanks for your thoughts.

Joe Potts
Miami Beach, Florida USA