Just guessing here, but aren't the O-235 adn O-320 cranks solid rather than
hollow (for constant speed props)?

They are hollow, and some models of the engines have
provision for a governor. That hollow crank has been the subject of an
AD; the front end of the crank gets cold in flight due to the prop's
heat loss and the hollow bore, open to the case, gets condensation and
oil in it. Those mix and form sludge and acids that eat away at the
inside of the bore and weaken the crank. See:

http://www.airweb.faa.gov/Regulatory...5?OpenDocument

Hollow shafts are stiffer than solid shafts. The stresses are
all concentrated in the outside wall, with no central material to act
as a fulcrum to stretch the outside on bends. Try bending a piece of
5/8" bar and one of 5/8" tube sometime (same material, of course) and
see the difference. The bar will bend, but the tube will resist bending
until it suddenly kinks. Cranks need to be stiff, especially where they
are loaded with gyroscopic forces, and need to be light, so they're
mostly hollow.

Dan