A (forged) 340 Crank is Not a (forged) 273/318 Crank?

[70Hardtop]

They both weigh the same, but the mass is located differently, Left= 340, right is 318
View attachment 1715166470

Thanks for great pictures! I was wondering about the differences also.

 

[70Hardtop]

Nope.. there both steel. look again

You mean "they are", or they're. Not 'there'

 

[Krooser]

All I know is my 3.79 crank weighs 63 back breaking pounds.

 

[Max1196]

You mean "they are", or they're. Not 'there'

Eye gots me an "F" inn grate 10 inglish 40 yeers ago, anm stiil no goots atit!

 

[A/MP]

The forged 273 cranks were 4130 steel. There are many variations of the materials used. Back in the 50-60's was considered strong but 4130 but not the best for high rev HP motors. Just a few years later the 318-3 crank, was structurally stronger. The machining of the counterweights were rougher compared to 5-10 years later of the 318 and 340 cranks. Street car with 6000 rpm, N/A, 350 and the factory forged crank works.

 

[Max1196]

You can have two identical cranks, that weigh the same and they will take different balance corrections. With the same bobweight.

A few degrees difference in where the counterweights are makes a big difference in balance correction.

It's not always where you'd expect it either, 318 from the factory balance

 

[yellow rose]

The forged 273 cranks were 4130 steel. There are many variations of the materials used. Back in the 50-60's was considered strong but 4130 but not the best for high rev HP motors. Just a few years later the 318-3 crank, was structurally stronger. The machining of the counterweights were rougher compared to 5-10 years later of the 318 and 340 cranks. Street car with 6000 rpm, N/A, 350 and the factory forged crank works.

I don't think so. You'd have to post something that says the 273 crank was 4130 because Chrysler listed ALL their cranks as 1018 steel. Even the 8 bolt cranks by Kellogg were 1018 steel.

I don't believe Chrysler ever used 4130 in a production crank.

 

[pishta]

I culled a 273 crank for use with a 340 setup and I noticed the 273 had THICK wall piston pins, I mean like DIESEL thick, to offset the lighter piston weight, I guess so they could use a 318 crank without Swiss-cheesing the counterweights!!! Couldn't really tell the difference in it and a '68 LA 318 crank, both using the skinny beam rods.

That is correct. Smaller bore 273 used the same bobweight as the poly 318 and it added the weight in the pins.

 

[STRorange]

That is correct. Smaller bore 273 used the same bobweight as the poly 318 and it added the weight in the pins.

Thats a strange thing about Mopar steel cranks, I've seen a few 'busted ones' even in stock hipo engines, but I have never seen a 'busted' cast crank in any Mopar engine lol...

 

[pishta]

read up about forged cranks. Some are forged as 180 cranks and then twisted to create the 90 degree planes! Others are forged as 'non-twist' units with more expensive dies. That was an interesting fact.

 

[Bighead440]

Thats a strange thing about Mopar steel cranks, I've seen a few 'busted ones' even in stock hipo engines, but I have never seen a 'busted' cast crank in any Mopar engine lol...

A friend has a '65 Belvedere II hardtop with a Poly and it broke the forged crank on the #4 main (IIRC). It started knocking and he continued to drive it gently until we could pull the oil pan. We could rotate the flexplate almost 1/4 turn before the balancer would turn, it was broken on enough of an angle to still run. Startling to us, especially when the car was babied it's whole life in 2bbl. form. He also bought a mint low-mileage '72 Newport with a skipping 400 that had slung #7 rod and broken the camshaft. We pulled the valve covers and none of the rear rockers on either bank were turning-LOL. We did some trading and I ended up with the thick-web "230" block, though it needs one bore sleeve.....