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 1957 347 and the 1958 370 both used a 3.5625”’ stroke crank with a 2.623” main journal.
The 1959 389 was the first to use the 3.75” stroke crank and 3.00” main journals, however the 370 and 389 did share the same 4.0625” bore.
Pretty cool that the 1955 287 grew by 102 cubic inches to a 389 by 1959.
