340 Forged Crankshaft
Just an FYI to ensure the crank you find is the one you actually want:
There are two basic OE steel cranks for small-blocks. One was used in 273 and early 318 engines--polyspheric and LA--and the other in 340s. The easy way to tell them apart? The 340 crank's 1-2 journal throw is fully counterbored (drilled through), while the 273/318 crank is essentially solid in the same area. It may have a shallow counterbore but it will not pass all the way through the journal throw. Since the 340 rods were considerably heavier, more weight was removed from the crank to achieve "zero" or internal balance. I've seen a ton of steel 273/318 cranks being misrepresented as 340 cranks over the years, usually unintentionally. I've pointed it out to a number of swap vendors and most didn't realize there was a difference, while a few admitted they didn't look closely enough.
Long story short: If you use a 273 steel crank in a 340 and don't rebalance the rotating assembly, you'll have noticeable vibration. That being said, if you're OK with rebalancing, run it. Strength is not an issue.
This is an OE 340 steel crank, showing the full counterbore you want to see. Also evident is the wide parting line that tells you it's a steel forging rather than a casting's thin line, along with some gnarly forging flash around the counterbore:
Just an FYI to ensure the crank you find is the one you actually want:
There are two basic OE steel cranks for small-blocks. One was used in 273 and early 318 engines--polyspheric and LA--and the other in 340s. The easy way to tell them apart? The 340 crank's 1-2 journal throw is fully counterbored (drilled through), while the 273/318 crank is essentially solid in the same area. It may have a shallow counterbore but it will not pass all the way through the journal throw. Since the 340 rods were considerably heavier, more weight was removed from the crank to achieve "zero" or internal balance. I've seen a ton of steel 273/318 cranks being misrepresented as 340 cranks over the years, usually unintentionally. I've pointed it out to a number of swap vendors and most didn't realize there was a difference, while a few admitted they didn't look closely enough.
Long story short: If you use a 273 steel crank in a 340 and don't rebalance the rotating assembly, you'll have noticeable vibration. That being said, if you're OK with rebalancing, run it. Strength is not an issue.
This is an OE 340 steel crank, showing the full counterbore you want to see. Also evident is the wide parting line that tells you it's a steel forging rather than a casting's thin line, along with some gnarly forging flash around the counterbore: