"It will at least put the vibration rpm way out of the useful range."

Hmmm. Not sure what you are trying to say, here. Since the mass of the piston is not altered (reduced) by the use of maximum Zoot cranks, and for discussion purposes, neither is the mass of the rod, fundamentally the primary and secondary shaking forces retain the same minima. You can improve the static balance of the crank/rod combo, which is what I assume was done to get some Zoot into it. That's certainly to the good.

Without a modal analysis of the crank, it is hard to be sure, but if the single-cylinder primary dynamic imbalance is say, at 150 Hz (corresponding to 9,000 rpm) I'm not sure you are going to see flexible crank modes below 150Hz with the stock crank, and not sure the amplitude of any of these higher modes would be noticeable vibration-wise when it is swamped by the 150Hz. Ditto for the 2x secondary shaking force at 300 Hz. So I can't make sense of the original statement.

Where the Zoot should really pay off is in regard to crank stresses in torsion, which is what kills Fox Stunt cranks at the induction port. Of course, they also fail with annoying regularity across the crank web from fatigue due to bending stress. If the material is higher strength, based on a better alloy and better heat treating, and if it reduces the stress concentrations inherent in low cost machine work that characterized the factory part, then the alternating stresses relative to the total allowables should be much more favorable--and the cranks much less likely to depart in flight or shed important pieces inside the case. Both of these are admirable qualities resulting from Zootness.

At 9,000 rpm, you should be concerned about the fatigue limit (reversed loading in torsion) at say, 2x10^6 cycles, which is only 230 minutes or so of operation. You'd need to know the exact stress ratios (alternating stress and constant stress) for all of the torsion and moment loads to improve the approximation.

You can reduce the alternating stresses on your stock crank by using a lighter prop (wood) and lighter spinner. Reducing the alternating stresses will push the fatigue cycle count to the right, getting maybe 2 or 3 times the operating minutes to failure.

Dave