Poly- urethane motor mounts"?

dano said
I think extra vibes come from the engine balance, as in was you engine rebuilt and balanced, stock or rbuilt with out balancing. Motors I've rebuilt that were all balanced didn't vibrate much more with Poly than stock with rubber. Again, durometer will dictate the vibration outcome.

I've seen motors with rubber jump around and Shake more than what mine does with Polly but then again the bottom end was done by an Army Aviation mechanic, he worked on helicopters and he had a 10-second Street drivable Dart! He had a Stash of 340s, Boy Lemme tell Ya!