Forged 273 crank in a 340 block

[rmchrgr]

I have both pieces sitting in my garage right now. Just curious as to who if anyone has done this. I know there are balancing issues between the two since the 340 crank had more meat on it. However, my little brain has surmised that since the mains and stroke are the same, it would 'only' require lighter pistons which is a mostly good thing. This would clearly be for a stock stroke 340. Lighter reciprocating assembly = faster revving and more rpms. Have not considered rods yet, just researching the crank idea.

Thanks,

- Greg

 

[urdustd]

It'll work just fine. Both internally balanced. Just have the new rods and pistons balanced to the crank.

 

[rmchrgr]

Thanks for the reply. Anyone else add anything?

 

[demon sizzler]

It will be absolutely necessary to re-balance the whole bottom end. This is because all 340 cranks including the cast cranks, had a hole drilled all the way through the front and rear counterbalance/main journal to lighten the heavier 340 piston/rod combo. The 273 crank tends to weigh more but the pistons/rods were lighter.

 

[perfacar]

hi, both cranks weigh the same. the 340 has the rod throws drilled and the 273 is solid. the 340 throws are lighter. also, the register at the rear is smaller
on the 273 crank where the convertor goes. other than that, it fits.

 

[rmchrgr]

Hmmmm, guess I was incorrect thinking that the 273 crank was lighter though I don't have a 340 on hand to compare to.

The smaller register - are you referring to where the pilot bearing would be if it were a four speed? If so that's fine since the car it will be for would be an auto.

 

[RedFish]

Beware the counter bore in the back of 273 cranks too. It probably will need altering to match trans or tq.
oops , yeah what he said

 

[rmchrgr]

Beware the counter bore in the back of 273 cranks too. It probably will need altering to match trans or tq.
oops , yeah what he said

Wouldn't the smaller register affect a manual trans only since the bore is for the pilot bushing? How does it affect the torque converter? I'm stupid...

 

[mikedevore]

Beware the counter bore in the back of 273 cranks too. It probably will need altering to match trans or tq.
oops , yeah what he said

DITTO

 

[1wild&crazyguy]

Wouldn't the smaller register affect a manual trans only since the bore is for the pilot bushing? How does it affect the torque converter? I'm stupid...

the convertor snout sits in the register on the back of the crank and 67 & earlier have a small convertor [fewer splines also] snout and small crank registers while 68 & up are big.
You can grind it out bigger even by hand you mic the amount you remove as you go, or machine shop time.

The outter's are the same with v8's, so no flywheel issue.
The input pilot's are all the same.

 

[perfacar]

hi, you cannot grind out the register!!!! it has to be machined in a lathe. it has to be dead on in relationship to the center of the crank, or it will vibrate and will destroy the rear main bearing and the front pump in the trans. I have machined a 273 to fit the larger converter snout. it's not for the average guy to fix. a machine shop is your best bet, for an accurate job.