318 crank in 340?

[ramcharger_440]

It appears as though a 318 and 340 crank have the same dimensions.

What are the limits in using a cast 318 crank in a 340?

I would like to know both horsepower and rpm limitations.

Many thanks again, guys! You have been a great help to this newbie.

[Billydelrio]

I have no first hand experience on this subject, but I would say that a cast crank 3.31" stroke with a heavy 340 piston should not be taken above 6700 rpm and a factory 340 block should handle up to 500 horsepower at a similar rpm.

There are differences between the 340 and 318 cranks.

Lets get some more info from the more experinced guys.

Billy

[dave571]

I think it would work, but it will require expensive balancing, to get it right, as the 340 pistons are a lot heavier.

Cast 340 engines are externally ballanced, while cast 318 are internally ballanced. Far from desireable, but certainly do-able.

I think 72 rr tried this and had some problems, but I'm sure he didn't ballance it either.

[turbododge]

That swap is usually done with the forged 318 truck cranks. Most of the high perf, solid pin cranks that were made were really just 318 truck blanks with a bit better grind, nitriding and maybe 8 flywheel bolts. I have not heard of any major balancing issues with them. Many spin the forged ones to 8K.

[rumblefish360]

I've personnaly spun cast cranks to 7000 without a problem. The 318 crank needs to be balanced with it's new pistons, even if it's old 340 pistons. The'll need to shave off some weight. Thats all.
On a HP note, cast cranks are good till approx 600 HP. Check the crank out by a race shop.
For long life , have it heat treated again and keep power reasonable. Not buck crazy wild.

[mr_340]

If you look up the casting numbers on the '72-'73 340s and 318s, you should find they use the same casting. They are just balanced differently. When using a 318 crank (cast or forged), I'd look for some lightweight pistons for a 340 to make the balancing easier. The 340 pistons are some heavy slugs.

[pishta]

Its not all just piston weight! the 340 rods were alot heavier than the 318's too. Mopar drilled a 1 inch hole through the 340 cranks throws to keep the weight down because they used heavier rods on the 340's. Have yours drilled too and itll be alot easier to balance. It should hold up. Find a Poly 318 crank. Itll work too with a little help and it is forged.

[dwc43]

Originally posted by ramcharger_440
It appears as though a 318 and 340 crank have the same dimensions.

What are the limits in using a cast 318 crank in a 340?

I would like to know both horsepower and rpm limitations.

Many thanks again, guys! You have been a great help to this newbie.

A cast 318 crank will work fine. You need to use the '68 or '69 318 rods with the full floating piston pins. THey are considerably lighter than 340 or 360 rods and with arp bolts will turn 6800 all day without any problems. We use the a lot in some of our dirt track engines. Also get some lighter Keith Black pistons and have the whole thing rebalanced and it will work out great.

With lighter rotating masses, hp or tq will never increase. IT cant be seen on the dyno at all, but the amount of time for it to accelerate from idle to max rpm will be greatly reduced so you will see a quicker car in the end.

[FearThe4Doors]

Originally posted by pishta
Mopar drilled a 1 inch hole through the 340 cranks throws to keep the weight down because they used heavier rods on the 340's.

yep, i was about to say that. I believe other than that, they are the same. You CAN run a 318 crank, but like everyone said, the overall chunk will have more mass since you dont have those holes.