[Walt Fricke]

The crank bolt issue does not seem to have anything much to do with fatigue failure. The bolts themselves look more or less OK (if you discount where the bolt heads were slightly machined by the center of the clutch disk during shifts and other times when they could move relative to the disk). Threads sometimes looked a bit worn, but basically the bolts are intact and about normal diameter. Not like the tree ring fractures from a stress riser (because not fractured), nor like the necked down before breaking (or if caught just before that point) of many rod bolt failures.

You engineers would have a better idea if the preload was removed (allowing the bolts to rotate and loosen) due to elastic stretch allowing the flywheel face to be periodically unclamped from the flywheel just enough that it could rotate the small amount loose bolts allow, over and over. With the bolts backing out a little each time. Or something else. The other characteristic of this problem is metal transfer from the flywheel to the crank, and crank to flywheel. I have one flywheel which looks lunar in that area. The crank, which looked equally bad, was tossed long ago.

Porsche tried to deal with this with a crank pulley damper on the 2.8 RSR.

But perhaps this was torsional? The harmonic added just enough extra what - angular accelleration? - that the clamping force was overcome, allowing the small back and forth motions, which tended to loosen the bolts when moving one way, but not to retighten them when moving the other way?

As to 4th or other orders, I am just repeating what was said in an impressive technical article in Pano long ago (memorialized in one of the Upfixins - collections of Porsche Club of America Panorama magazine technical articles).