Learning about the Cast 340

[72DMag]

Purchased a cast crank 340 engine over the weekend. The price was good and always wanted a 340 . I'm not too familiar with the later cast version though. After taking the engine apart last night the cast crank is the same one found in a 318.

So did mopar install the same 318 cast cranks in these 340 engines then added the weighted dampner and special flexplate to account for the heavier 340 piston? Is that it or am I missing something?

 

[JDMopar]

If it's the original crank, it isn't the same as a 318 cast crank. Did the harmonic balancer have a lobe on it, and say "340 cast crank only" on it? If so, that is the real 340 cast crank. The flex plate is the same as any other 340. A torque converter or flywheel for that crank/balancer is different and unique to the 340 cast crank. Not the same as a 360.
The difference in the 340 only cast crank and a 318 cast crank, is that the 318 cranks were internally balanced, where the 340 cranks are externally balanced.

 

[72DMag]

Yes I have the dampner. When I googled the cast # it told me it was from 340 or 318.

Yes 318 are internal balance but that's what my question is about. Did mopar take the internal balance bobweight of the 318 (same crank) toss it in the 340 and add weights to flexplate and dampner to account for heavier pistons. It looks like they did based on this engine I got.

 

[RustyRatRod]

I don't believe the 340 used the 318 crank. I'm pretty sure they had their own externally balanced cast crank. If it's a 318 crank, it's probably a balance mismatch unless it was rebalanced. I would be careful and make sure it's all balanced together before I assembled it. JMO.

 

[72DMag]

@RustyRatRod
I thought that too but it looks all original to me. According to website below this crank came in 340 and later 318s. It's strange that 3 cranks are listed for the cast 340 though. The crank cast # is 3751841. Scroll towards the bottom.

mopar casting numbers

 

[Lefty71]

Is that number actually on the crank you have?? If you can possibly date it, you'll notice it was according to lists, never used in a 340 or 318 in the same production years (early for 340, later for 318).

 

[72DMag]

Yeah that's what's here. I do know the block is a 73 not a early cast 72. At least that's what I got off of it.

Good point about the years listed on that website. That crank doesn't start in 318s until 76. At least according to the site. Taking that for what it is..... Again this is my first rodeo with a cast 340

 

[Professor Fate]

Yes I have the dampner. When I googled the cast # it told me it was from 340 or 318.

Yes 318 are internal balance but that's what my question is about. Did mopar take the internal balance bobweight of the 318 (same crank) toss it in the 340 and add weights to flexplate and dampner to account for heavier pistons. It looks like they did based on this engine I got.

Don't believe everything you read on the internet.
THIS is a cast crank 340 balancer:

Notice it says "340 cast crank only". VERY easy to ID. If it doesn't say that, it's not a 340 cast crank balancer. Period.
If it doesn't say that, you either have a mismatched crank and balancer, or some previous owner swapped out the entire rotating assembly to get away from the one and a half year only external balance so they could use more common internal balance parts.
And quit basing your assumptions on crank casting numbers. A 318 and a 340 crank could very well have come out of the same mold, but the finish machining and balancing of the cranks make them totally incompatible with each other, which resulted in two different Chrysler PART numbers.

 

[Lefty71]

After taking the engine apart last night the cast crank is the same one found in a 318.

This may help you on your journey.... Did you look at the crank and main bearing shells for dates and info?? It may help you figure out if this motor has ever been apart before, which should go a long way towards answering if this motor is properly balanced, or if you really need a machinist to start from scratch in answering for you.