318 v 340 Crankshaft
Are the cranks the same? I was under the impression that both have the same stroke but different bores. Is that true or is the stroke on a 340 different? Thanks
No, the cranks are different. Both the 340 and 318 have a 3.31 stroke and the same rod and main journal sizes, but that's where the commonality ends. You are correct, however, the stock 318 bore is 3.91", and the 340 is 4.04".
All 67 & > 318s in cars used cast cranks that were balanced for light duty rods.
All 68-72 340s used forged cranks that were balanced for heavy duty rods.
The 73 340 used a shot peened cast crank that was balanced for heavy duty rods.
Alternative sources for LA 318 forged cranks are all balanced for light duty rods and include some LA 318 trucks, 64-67 273 LA, and 57-66 318 poly V-8.
The confusion may arise from the 360 crank. This cast crank has a 3.48 stroke and is balanced for heavy duty rods. The rod journals are the same as the 318 & 340, but the mains are larger. For this reason, the 360 crank can not be used in the other LA blocks without turning the main journals and re-hardening them.