Rod lenght?
 

Why do they make cranks for a 6" rod(small block chevy).will a 5.7 rod crank work with 6" rod (a 5.7 rod crank is lighter) rod lenght has more to do with the piston i.e. comp height than the crank? please you pro's tell me if I'm thinking wrong. I do love this site. :shock:

Who's crank are you looking at ?
Some are designed with larger counter weights for balancing, other's are the same crank in another part numbered box.

Exactly. But if you have a choice but the one for the shorter rod, because the crankshaft will be lighter due to the smaller counter weights.

Has to do with extreme rod angles with extremely long strokes.Long stroke +short rod =flat rod angle=rod/piston shoved thru cylinder wall :cry:

my 3.875 stroke crank calls for a 6 inch rod. i was wondering the same why i couldnt use a 5.7

the counterweights on the crankshaft are too tall, would hit the rod..

Correction.....
The taller counterweights would hit the bottom of the piston skirts, not the rod.
You can use a 6inch rod on a 5.7 crank but you can't use a 5.7 rod on a crank designed for a 6inch rod.

You get the prize for the correct answer!!! 8) 8) 8)
The counterweights for a 6" rod crank will hit the skirts if a 5.7 rod is used, however you can run 6" rods on a 5.7 rod counterweight. The reason for two different? Larger counterweights are easier to balance, less heavy metal. Larger strokes make this even worse, limiting the counterweight size. The 3.875 crank would need to have the counterweights turned down, then balanced with more heavy metal to use 5.7 rods.