Quote:
Originally Posted by Mike S
The block I have is 2 bolt main and not drilled for 4 bolt, I remember years ago when I was using studded 2 bolt mains I cracked the block where one of the studs screwed in. Forgot which main but it was in the middle. Alot of people said is was from using a 2-step and trans brake with solid motor mounts. I since put a mid plate and motor plate in.
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If you used the 2 bolt early style motor mount, it only straddles #3 main, so all the force is concentrated on one bulkhead. The later style 3 bolt mount was moved further towards the front of the block, and straddles #2 , and #3 bulkheads. It was redesigned in 1970 when the longer stroke 455 made it's debut. Evidently Pontiac engineers had the higher torque output of the 455 in mind when they redesigned the motor mount locations for the 455, which later trickled down to all displacements using the improved location.
Of course you have now circumvented the side mounts by fitting the car with motor plates, probably one of the best choices for eliminating side stress on the bulkheads completely, especially if you're using PMD blocks, as opposed to aftermarket blocks.