factory 340 forged crank

[davescuda]

I have a question on a factory 340 forged crack.
Here is a picture of the differences. Why is one drilled through the #1 journal and the other is not. Is one better than the other? any help

 

[daddio4048]

Probably for balancing

 

[magnumdart]

They don't even seem to be the same forgings.

 

[d55dave]

The drilled ones are 340 (lightened) the non drilled is a 318 forged crank. Or so I believe.....

 

[davescuda]

all are out of 340's. I have 2 with and 2 without

 

[davescuda]

I have a 318 short block I will take a look

 

[d55dave]

Very likely two of your 340s had a 318 stsel crank swapped in at some point ocer the last 45 plus years. 318 has the same stroke, main and rod journal diameter.

 

[4spdragtop]

I would say different years (different bob weights??)

 

[davescuda]

So the 340 ones are grilled

 

[d55dave]

So the 340 ones are grilled

Yep. One of the reasons they are known to rev so well. Less reciprocating mass.

 

[davescuda]

This makes me want to check all my other 340 motors

 

[d55dave]

I could be mistaken. Others will speak up.

 

[moper]

Might have to do with the crank castings themselves. Or the material. Better iron is heavier. I'd say it's probably balancing issues but that's a total guess.

 

[yellow rose]

Usually the drilled pins are 340 because the Bob weight is so much heavier than the 273/318 cranks. Truth be told, I used 318 poly cranks and 273 cranks in my stock stroke aluminum rod engines. I sell had to turn the counter weights down some.

 

[brian6pac]

Every 340 I have had are drilled, 273 - 318 are not.

 

[davescuda]

Another question. Is it better to use a 318 crank in a 340 if you use the new light weight pistons and I good new I beam rod?

 

[yellow rose]

Another question. Is it better to use a 318 crank in a 340 if you use the new light weight pistons and I good new I beam rod?

Depends on who is doing the balance job. I'd rather have a lighter counterweight than drill it like Swiss cheese. So I'd rather use a 318 crank to start with.

It's also hard to turn the CW down unless you have a real rigid lathe and they still don't like an interrupted cut. Then there is cam grinding the CW. You can drive yourself nuts and not gain anything.

 

[davescuda]

The cranks look Identical besides the hole in the #1 Journal, Is there anything else visual to the cranks that I can see.
thanks

 

[davescuda]

Depends on who is doing the balance job. I'd rather have a lighter counterweight than drill it like Swiss cheese. So I'd rather use a 318 crank to start with.

It's also hard to turn the CW down unless you have a real rigid lathe and they still don't like an interrupted cut. Then there is cam grinding the CW. You can drive yourself nuts and not gain anything.

This make sense.

 

[d55dave]

The cranks look Identical besides the hole in the #1 Journal, Is there anything else visual to the cranks that I can see.
thanks

Forging numbers

 

[davescuda]

I took the center crank out of a 72 340

 

[davescuda]

Forging numbers

You can only read one out of the 4 cranks I have loose.

 

[d55dave]

I took the center crank out of a 72 340

But who put it in there? Where did it start life? Honestly..... after 45 plus years it is impossible to say.

 

[d55dave]

Besides, 72 would have most likely been cast from new.

 

[davescuda]

But who put it in there? Where did it start life? Honestly..... after 45 plus years it is impossible to say.

Most likely it was replace with this crank.