318 crank into 340? pitfalls

Since 318 cranks started in '68, then the pilot bushing ought to be drilled out. But check, of course. Forged versus cast shouldn't make a difference as, for the 318, both type were designed and balanced to the same bobweight.

Damper and flywheel will be the same. Make sure your damper does not have a cracked rubber ring; it probably will, and, if so, should be replaced. A cracked ring prevents it from doing its job of damping, and the outer ring might let loose and fly off with a lot of damage as the result.

What pistons are you using? Or, have those not been selected yet? And what is the 7 digit casting number on the side of the rods? Those are going to be critical for getting to the balance that you want without adding weight to the 318 crank. (If you can answer these questions, I can run some numbers for you.)

As noted, the 318 crank is balanced for a much lighter 'bobweight' (which is the sum of the weights around 2 of the big ends of the rod, added to 50% of the weight of the 2 pistons, pins, rings, locks, and small end of the rods). Your rods look to be well lightened at the small end on the beam but the big end is the more critical part of the rod weight. Stock 340 bobweight is around 2310 grams, and stock early 318 bobweight is around 2150 grams, + or -. We have used a 273 cast crank in a 340 with light rods and pistons; the 273 uses the same bobweight as the early 318's so it can be done IF the pistons and rods are right.

If you are using pistons like the L2316F forged replacement, it will be impossible to reach the bobweight needed to balance with the 318 crank without adding weight. Something like KB243's is needed. Even then, it will be close.

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