Good Morning,

I went to order a new crank for a 1972 340 and Summit lists two, one with a 3.31" stroke and one with a 4" stroke. Not sure which one to order or how I tell the difference?

Also, did the 318 and 340 use the same crank?

Thanks,
Jim

3.31 is the stock stroke.
318 and 340 will interchange however 340's were steel and 318's were cast. Your 72 should be last year for the steel crank. In 73 they went with an externally balanced cast version. There are steel cranks found in truck 318's and older 273's and 318 poly's. Sometimes they're drilled differently for oiling and the crank registers will not be the correct diameter on the early 273's and 318 poly cranks.
Unless your wanting to build a stroker motor stay with the 3.31.

340 stock stroke is 3.31". The 4" is for building a stroker.

318 and 340 share the same stroke as well as main and rod journal sizes, but most 318 cranks were cast (some early years were forged) and most 340s were forged steel (later were cast).

Thanks everyone, huge help!!

OK, now they list a "Cast Steel" and a "Forged Steel" crank. Difference is almost $350, ouch.

Can I use the "Cast Steel" as long as it says internally balanced?

Thanks again!!
Jim

Simple answer: Yes, you can.

As many 340s were built with cast iron cranks and the cranks didn't frag, there's no reason why a quality cast steel crank wouldn't serve at least as well.

Your engine was built with a forged steel crank and while it is technically superior, it isn't worth a single horsepower over a cast crank.

I would much rather see you spend the extra money on lighter pistons and balancing. Stock 340 pistons weigh 720 grams plus pin, which is big-block territory. Use a 500 gram piston and the engine will rev more quickly and the reciprocating parts won't be as highly stressed.

R.

The fast 340 stockers use the 318 cast crank, lighter.

Thanks everyone!! Should be ok to use the original dampner for the forged crank?

On a cast crank? Only if you have it internally balanced.

Eagle cast 4" cranks are known for breaking without much abuse. I don't know about their 3.31" stroke cranks.