273 crankshaft/340

nm9stheham said
The others are right: putting stock weight 340 pistons Like the SpeedPro's) will indeed not balance with a 273/318 crank.

My son and I put a '68 273 cast crank in his 340 with SCAT I-beam rods (602 gr IIRC) and KB pistons (595 gr piston and 131 gr pin). That took the bobweight down from 2326 grams for the stock 340 rods/pins/pistons down to 1897 grams for our combo. That is also way under the stock (early) 273/318 bobweight of approximately 2147 grams.

So that 273 crank will balance fine in a 340 with light pistons/pins/rods by taking weight OFF. Our setup balanced up with around 82 grams drilled out of each of the large end counterweights. I just weighed our new parts as a double-check (they were to specification), made the computation and handed the number and the crank to the machinist and told him to balance it to that bobweight. (He was skeptical until I handed him a copy of the spreadsheet LOL.)

So you are on a solid track, OP, assuming your pistons are decently lighter. If you get me your new piston and pin weights, I can get you a close number for your new bobweight in a few minutes.

Get the later 273 cranks with the larger register and you will be fine, or turn it out to the larger diameter. I can't recall the pilot bushing that you may have for options with the smaller register; you might read up on this in the Transmission subforum. Or get a 318 crank.

FWIW, the 273 and 318 bobweights are identical for all years up to at least '74 (and I think beyond '74 but not 100% sure on that); Mopar put in some super heavy pins in the 273 to make up the weight. And the 283's and 318s up to around '74 used a lighter rod at 726 grams.

67 steel crank is what i can get. From what i read it has the smaller register. an auto has to be drilled or turn down the converter. I was wondering if it was the same on 4 speed?? I'm not sure on piston choice there is not really much available for the 340 the block is already 4.070. the cyl walls look bad but measure ok.. I think it will clean up to 4.080 if not 4.100 may be what i have to work with. but i wont know until i have them sonic the block and mag it. My plan was running my x heads but they have large combustion chambers. i need to check them when i get them back from machine shop i'm thinking they were about 74cc. I really didn't even plan on rebuilding it it actually ran ok slow but ok. it smoked a little on start up intake leaked & valve covers leaked it had an old purple cam in it. I planed on reseal upper end and swap in smaller xe268h cam. when i pulled valve springs i noticed couple very loose guides so I pulled the heads sent them to machine shop. oil pan looked kinda dirty so i pulled the short block to clean it throw new set of bearings in and put new gaskets in it that snow balled into the bearings are bad crank needs turned but its already .020. Now im rebuilding the whole thing.... actually thinking now if i got to but crank rods and pistons buy a stroker kit... But how well will a 422 run on factory head or exhaust manifolds.......