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Originally Posted by Sizzler
Nothing wrong with cast iron cranks. 391 cranks ARE nice, because they're steel and NOT cross-drilled. If you need all that. Most can get along with a cast crank.
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Yes, cast cranks are fine for most purposes. However, when you get into a longer stroke such as 4.125 and larger, the piston has to travel a longer distance in each revolution; therefore even though two engines are turning the same RPM, the piston in the engine with the longer stroke is traveling much faster. When you reach a certain RPM, which varies by the stroke length, "normal" parts aren't good enough to take the abuse. That's when you need a stronger crank, rods, etc.