in the usual VW car engine the harmonic charactersitics of the
crankshaft are such that the destructive vibration occurs a few
thousand revs above the max possible rpm so the engine never
experiences a problem.

in aircraft installations with large heavy propellors attached to the
shaft the harmonic range occurs at a lower RPM.

particularly with large 3 bladed ground adjustable composite props the
destructive harmonic range comes down to about 3,200rpm. most VW's
would run at around those revs so the problem becomes an unavoidable
one.

the destructive harmonics are indetectable in the cockpit above the
normal engine vibrations but in the engine's interior those harmonics
deliver hammer like blows to the crank web. the hollow area above the
centre of the internal web gets a fair shaking and develops a fine
hairline fatigue crack. this crack leaks copious amounts of oil and of
course there is no way of welding oily electron so the case becomes
junk.

It certainly should be possible to degrease the case by vapor
condensation. Tricloroethane was the solvent of choice before the
enviro-nazis got hold of things.


the problem can be avoided completely by using lightweight fixed two
bladed wooden props.

Better idea is to put a flywheel on the engine and use a redrive that
provides some isolation to the engine from its load. The marine
industry figured this out in the 1950s and the inboard/outdrive is
nearly universal now.