MoparMark91, if you are doing a 340, then the 318 crank has one problem: the counter-weighting is not set up on the 273/318 cranks to balance the heavier stock 340 pistons and rods. To use those heavy pistons with a 318 crank, you will need to have a shop balance it and ADD weight to the 318 crank; adding weight tends to be pricey.
So you either:
- Find a 340 crank to use with those L2316F pistons, which are stock piston weights. To not re-balance the crank assembly, you would need the heavier late-318/340/360 rods, (not the earlier, lighter 318 rods, like came in 1969 318's) to use these pistons with a stock 340 crank.
- If this is not an all out race machine, then the cast SBM cranks have a reputation for being quite durable. That gives you another option. You use lighter pistons/rods that will be balanced with the 318 cast crank at a shop. You spend $$ on new pistons and rods that are already weight matched out-of-the-box, and then save $$ since the shop will only have to weigh a set, compute the bobweight, and then take weight off the crank.
You may actually do yourself a favor for longevity by going to lighter rods and pistons and staying with the 318 crank. My son and I did that on his 340: we used KB pistons, and SCAT rods, and the 273 cast crank balance was simple and cost only $100. That combination reduced the bobweight on the crank by almost 20%, from 2326 to 1893 grams; that is a LOT, and takes stress off of the crank and rods. That engine is set up for around 400 peak HP.
The KB pistons are around the same price as the L2316F's and the SCAT rods are just under $300 for a set of 8, which is not much more than re-conditioning a set of stock rods. Both pistons and rods are already weight matched out of the box, so that cuts out the majority of labor time and cost for the balance work; you end up only balancing the crank.
So there are options.
And here is another source for crankshafts:
Crankshaft Supply, Inc. -- Experts in Crankshafts & Cylinder Heads
