[Walt Fricke]

Chris

Just to be clear: the flywheel issues the 2.8s had do not involve failure of the six flywheel bolts. They don't shear off, nor obviously elongate or in any way break. I've even reused a few in other applications, so the threads are not distorted.

So either a loss of clamping force due to harmonic vibrational axial loads leads to the bolts unscrewing and the mating faces to fret. Or torsional effects from harmonic vibrations exceed the clamping force, leading to bolts unscrewing and mating faces fretting. But I don't think fatigue is at work.

Increasing the clamping force would seem to mitigate either of these possible causes of bolts backing out, I'd think. In any event it solves the problem.

Perhaps the 9 bolt cranks don't have this problem, even with much smaller bolts (M10 instead of M12) at lower fastening torque because the increase in the diameter of the mating faces produces a sufficient increase in whatever one calls the ability of a joint to resist slipping under torque to overcome the forces trying to loosen things or otherwise induce the bolts to back out.

You'd think a flywheel with loose bolts would show up pretty clearly whilst driving on a track, but having driven perhaps as much as 20 minutes in this condition I can tell you that isn't necessarily the case. Shifting on track does not really require a complete release of the clutch if timing is decent. Where the issue shows up unmistakably is in the paddock. Can't engage first gear, because of the bolt heads rubbing on the disk's steel hub.

The first time this happened to me I assumed I'm goofed up torquing the bolts. I replaced the crank and flywheel. Next time I had the car out I recognized the problem more quickly. And I was able to poke at the flywheel through the drain hole on the bottom of the bell housing. I could move it. Time for another rebuild, though this time flywheel and crank end were salvageable.