Quote:
Originally Posted by 25stevem
Yes, Pontiac engineering seem’s to have gone way to the extreme for safety once they went from the 370 motor with its steel crank over to cast iron ( Armasteel) for the then new 389 motors.
Both the 370 and the 389 had the same stroke of 3.750” yet they enlarged the crank mains from 2.623” to 3.00”.
That’s a gain of .377” or a crazy 14.5% increase in diameter!!!
In comparison when the 421 came along with its 4.00” stroke they only increased the main size by 8.5% over the 389.
I think the fact that Pontiac stuck with the 3.250” mains when they took the stroke up to 4.210” for the 455 is because they realized they had plenty of reserve strength with that 3.250” main size regardless of the fact that it was a cast crank.
If Pontiac would have been able to afford to produce enough steel cranks for 4.00” stroke motors back then I don’t think the main size would have ever grown bigger then 3.00”.
One things for sure no post 1959 motor with a cast crank that had no casting flaw’s in it ever broke when used as intended in passenger car format!
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The only forged cranks during the early 421 era were the legendary 990 cranks of the 421SDs. Smokey tried to warn PMD engineers that the 3.25” main was too large for continuous high RPM service. The 1961 NASCAR season proved him correct, when the new 421SDs were failing at alarming rates. Smokey took the bull by the horns and created his own 421SD based on the 389SD NASCAR block and a welded and cut down 990 forged crank. It worked, but PMD dictated that ALL Pontiac sponsored teams would use the 3.25” main engines for the 1962 season. In reality, a proper prepared, machined and nitrided OE Pontiac nodular or Arma-Steel crank is more than strong enough for street service