340 build question

[Davecuda]

So….picked up a 340 short block to replace my 273. It came with a cast crank, I have a line on a forged one. My understanding is 340’s with forged units are internally balanced and take a different balancer than the cast units. Question is do I still need to change the torque converter or use one of the B&M flex plates?

Thanks!

 

[PRH]

Cast crank 340’s are externally balanced on the front and rear.
Steel crank 340’s are internal, so you would use a neutral damper & flexplate/converter with one of those.

 

[Professor Fate]

Yes, cast 340s used a different torque convertor/flywheel in addition to the balancer. DON'T mix and match.

 

[RustyRatRod]

Also, the cast crank 340 balancers are not the same as the 360 balancers. Just some FYI. What's your intended power level? I ask because the cast crank properly prepped and balanced is good for 550 HP maybe a tick more. They are a good bit lighter, too. Just a thought.

 

[dano]

Also, the cast crank 340 balancers are not the same as the 360 balancers. Just some FYI. What's your intended power level? I ask because the cast crank properly prepped and balanced is good for 550 HP maybe a tick more. They are a good bit lighter, too. Just a thought.

This. I have a cast 340 crank that is internally balanced. They do not need near as much to neutral balance as 360 cranks do. Unless your using forced induction or laughing gas the cast works just fine.

 

[JDMopar]

The cast crank will be just fine. I have had a few cast crank 340's that I beat the living snot out of back in the day. Not a one of them ever gave trouble. You need the 10235 B&M flex plate. Also, since you are replacing a 273, check the size of the register in the tail end of both cranks to see if the one on the 340 is bigger. It may or may not be, depending on the year of the 273. If the register in the 340 crank is bigger, you will also need an adapter ring to put into the crank register in order to use your existing torque converter. Somebody on here makes and sell the adapter ring, or any machine shop worth its salt could make you one.

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[RustyRatRod]

I'm actually LOOKING for a cast crank slant 6. Chrysler used nodular cast iron and not regular grey cast iron. Nodular iron is much stronger and not as brittle as grey cast iron. It has a little "give" to it, similar to a forged crank, plus, it's lighter. But do what you want. This is your build. If you think you're gonna push the boundaries of a cast crank, then go forged. But Ibet probably 90% of street builds out there with forged cranks could have gone with cast. ....and if you decide cast, don't get one of those new "cast steel" cranks. Stick with the stock Chrysler crank. They're a much better piece, IMO, because they were made with good old American metallurgy.

 

[Oldmanmopar]

Cut the large weights off of the converter. Leave the small neutral balance weights in place.

 

[cudamark]

So….picked up a 340 short block to replace my 273. It came with a cast crank, I have a line on a forged one. My understanding is 340’s with forged units are internally balanced and take a different balancer than the cast units. Question is do I still need to change the torque converter or use one of the B&M flex plates?

Thanks!

If you use the 340 only cast crank, yes, you will need the correct balancer and flex plate or converter to go with it. If you're getting a stock forged 340 crank, your 273 balancer, flex plate, and converter will work fine. As mentioned, if your 904 trans is '67 or earlier, you'll need the reducer bushing for the crank to match the button on the converter. You can also use your 273 crank in the 340 without any other major changes. Just get everything balanced during your engine rebuild.

 

[Davecuda]

Thanks for all of the info, definitely helps. Dude only wants $100 for the crankshaft , so going for it

 

[33IMP]

Thanks for all of the info, definitely helps. Dude only wants $100 for the crankshaft , so going for it

Magnaflux the crank before you spend any money on it (grinding/polishing/bearing purchase).
I'd whack it with a hammer too, before I handed over a c-note.

 

[RustyRatRod]

If you use the 340 only cast crank, yes, you will need the correct balancer and flex plate or converter to go with it. If you're getting a stock forged 340 crank, your 273 balancer, flex plate, and converter will work fine. As mentioned, if your 904 trans is '67 or earlier, you'll need the reducer bushing for the crank to match the button on the converter. You can also use your 273 crank in the 340 without any other major changes. Just get everything balanced during your engine rebuild.

I'm unsure I agree. The 340 crank is neutral balanced, but it gets there differently than a 273. The 340 crank had gundrilled first and I think last crank throws, so the 273 stuff, although neutral balanced, gets there differently. It'll be best if he balances "whatever" he decides to use.

 

[Bewy]

For a mild perf engine, I wouldn't hesitate to use a cast crank. Iron is less dense than steel, so all else being equal, lighter. Less rotating weight, good for acceleration. The lighter iron cranks reqd external balancing. Not the only way, but cheapest for the OEMs.