What the hell, I'll chime in...

I put an expensive underdrive pulley on my SC300 when my balancer seperated. 10,000 miles later it SHEARED 2 of the bolts off my flexplate to crankshaft, and rattled the others out. This was on an untouched 150,000 mile engine. Am I saying this will happen to you? No, not at all, just make sure you're not replacing a harmonic damper with a solid unit.

 

The vibration required to back all 8 bolts out to to the point of breaking the final two would have to be so immense you could not stand to drive the car. Have you ever

driven a car with an incorrect balancer? I drove a customer car that had a newly assembled 347 in a 68 mustang coupe. At 2500 rpm it would rattle the dash bezel loose.

It was crazy how bad it vibrated. This particular car had a E7 block and stroker kit designed for 50 oz 302. Since he put it in a 68 he assumed it required the old 28 oz

balancer. The problem was fixed by putting a late model damper on it. I understand you say the engine was "untouched" but there is no way that you could have ignored

or even managed to drive the car with such a imbalance. It would have rattled your teeth out and wore the soles of your shoes through the floor board. An automatic is

much safer to run a set of pulleys on and will "absorb" much more harmonics from an incorrectly balanced bottom end. The fluid filled torque converter bolted to the flex

plate is the biggest harmonic damper ever, but the amount of vibration it would require to back out 8 correctly torqued bolts would feel like you were riding in a helicopter

with one blade. I can see from the pictures without a doubt you had a MAJOR failure, but to blame it on the "Expensive" crank pulley is not really feasible.

Not to mention you drove it for how long with a balancer that was failing? The OE balancer didn't just separate over night, it was failing for a while.

 

There were no changes to the engine aside from the timing belt. It was my fault the balancer seperated as I had to use a torch to heat the bolt to remove the original damper to replace the timing belt. It only ran for about 1000 miles seperated before I put the solid underdrive pulley on. Keep in mind, this was on a 2JZ inline 6, so the resonance is totally different than a V8. I could definitely feel a vibration at 3000 RPM or so, but nothing teeth jarring.

 

I agree with the amount of imbalance in addition the flex plate center would begin to develop cracks.
The converter has a lot of weight with the thin flex plate as the only link with the crank.
The crank flexes/squirms all the way from the front to the rear.
In my sprint car at 500 + hp the flex was so bad once it was moveing the main bearing caps but this motor normally ran as high as 7800 rpm lap after lap.
Had no flywheel and depended on the special fluid front damper while being direct drive to the in/out box.
If it was not for an all 'forged' crank and parts and a race block the motor would have blown in a hundred pieces.
Good luck

 

A inline 6 is a far "smoother" operating engine than a V configuration. There is a main cap for every cylinder and only one rod per journal. I'm just saying the harmonics

that would be required to do that much damage would show up on the Richter Scale.

 

Well I have no other explanation. I run that car daily at 3x the stock horsepower and had been before the underdrive pulley. That's my experience, take it as you like.