273 crank in a 340

[Confusedcuda]

Can a 273 crank or even 318 be used in a 340, Thanks

 

[70aarcuda]

yes....but it need to be balanced for the heavier pistons for the 340....weight will be needed to be added to the crank.....mallory metal

 

[Plymouth 65]

Technically yes, rotating and reciprocating inertia's are different though. 65'

 

[Max1196]

Technically yes, but how the factory did it was have weight removed from the "pin end" to compensate for the heavier "bob weight" that would exist on the 340, a bored hole in position 1/2 and 7/8 throw of the crank.

 

[pishta]

Even a poly crank would work. But as said, it would need a serious balance job for heavy 340 slugs. Now if your going SCAT rods and light pistons, you may be closer than you think. The 273 ran a huge wrist pin, the larger bore LA 318 didnt to be roughly the same reciprocating weight. 340 rods and pistons were heavier so they took weight out of their bearing journals instead of adding weight to the counterweights.

 

[dibbons]

Technically yes, but how the factory did it was have weight removed from the "pin end" to compensate for the heavier "bob weight" that would exist on the 340, a bored hole in position 1/2 and 7/8 throw of the crank.

In other words, the forged crankshafts for the 273, 318, and 340 are all the same basic forging and strength, just the manner of balancing differentiates them?

 

[pishta]

yes, add the poly 318 to that too. PAW used to sell poly 318 forged cranks drilled and balanced to "340 forged" specs. just be careful on the hub register size.

 

[Max1196]

Same crank but weight removed from the appropriate location to account for the needed "bob weight" compensation.

 

[nm9stheham]

In other words, the forged crankshafts for the 273, 318, and 340 are all the same basic forging and strength, just the manner of balancing differentiates them?

Correct. We have a 273 cast crank in my son's 340. As long as your 340's new bobweight is below the 273/318 bobweight of 2160-2170 grams, then the 273/318 crank's can be balanced by just removing weight. We used KB pistons and Scat rods, which dropped the bobweight over 400 grams from the stock 340's 2326 gr down to 1893 gr. Easy-peasy balance job.... simply remove weight. The shop took off 81-82 grams from each end counterweight for these pistons and rods..... about $100 for the work... cheap.

As correctly said, if you put the stock 340 pistons & rods on the 273 crank then you either have to drill out the end crankpins or add weight to the counterweights to get it to balance. Even this may not be perfect, as the minor counterweights on the 340 crank may be different a bit than on 273/318 cranks; I've never had them side by side to see. Edit: I am studying Max1196's pix right now; tnx!

Here is a pix of those holes. (These are actually on a SCAT stroker crank, but it is the same idea.):

 

[Locomotion]

yes, add the poly 318 to that too. PAW used to sell poly 318 forged cranks drilled and balanced to "340 forged" specs. just be careful on the hub register size.

This has bit several people, including me a few decades ago. I can never remember the change-over year from small to big - '66 or '67?

Isn't the crank on the right shown in MAX1196's photo a forged 318 truck crank? I thought they didn't have a bored journal, and were said to be stronger, but 340 cranks did.

 

[nm9stheham]

You are correct.. no bore inside the journal on the 273/318 cranks.

 

[RustyRatRod]

Even a poly crank would work. But as said, it would need a serious balance job for heavy 340 slugs. Now if your going SCAT rods and light pistons, you may be closer than you think. The 273 ran a huge wrist pin, the larger bore LA 318 didnt to be roughly the same reciprocating weight. 340 rods and pistons were heavier so they took weight out of their bearing journals instead of adding weight to the counterweights.

Depends on which one. The early Polys won't work. The later ones into the 60s will. There's a year split on them somewhere where a difference is but I cannot remember where it is. The Poly guys use the 4" 360 cranks in the later Polys for buildin Poly monsters. I always kinda wanted to do one, but I'll never get around to it now.