67' 318 crank in a 69' 340

-

Vince Gillespie

Mopars Rock !!!
Joined
Dec 30, 2010
Messages
22
Reaction score
2
Location
Kingston Ontario Canada
Hey folks , I have a question and I would like to hear your thoughts on what I could do or should do .
A few years back I bought a 69' 340 short block and buddy threw in a complete low mileage 67' 318 ( never been apart ) . I tucked the two engines under the shop bench as I was busy working on the body of my 70' Dart Swinger . I dragged out the 340 short block today and realized that the pistons have the four valve relief cuts , pistons are recessed in the cylinder at TDC and the crank has the 1/8" parting line . Everything is pointing to a low compression rotating assembly in the 69' block . The flange on the crank is damaged and I knew it was damaged but thinking it was a 69' forged steel crank , I thought it could be repaired and remachined like new . Being a cast crank I think that is now out of the question .

I have searched the net looking to find out if the 67' 318 would have a forged crank or a cast crank . Some say forged and some say cast , nothing definative . The casting date is 8 / 10 / 67 , interestingly 50 years and 3 days old as of today .

This car is just going to be an old school fun driver , four speed , frame connectors , mini tubbed , springs relocated , Cragars , 325/50/15s , motor has refreshed 308 heads , LD340 intake , 340 manifolds and will have a small cam ( not sure what to use just yet ) all Taylored for cruising and with just a little additude to boot .

Is there any benefit to staying with the lower compression assembly in the 69' block ? Would you use the 67' 318 crank with the 340 rods and pistons ( still low compression ) have it all balanced or use 318 crank and 318 rods with 340 pistons ( higher compression ) and have it balanced ? This motor is going to be a low budget build and will rarely see much more than 4500-5000 rpm . I am a little ways away from tearing into the motor but I would like to know what I could do or what I should do . I'm looking forward to hearing your positive comments !

I appreciate your thoughts/comments and I thank you in advance !!!
Take care !!
 
I would start by posting a close-up picture of the parting line on that 340 crank, as well as a picture of the damaged flange you mentioned, so that crank can be evaluated. The "parting line" on a cast crank usually looks like a sharp-ish rough edge and forged crank is wider and smoother. Usually easy to tell by seeing it. As for the 318 cranks, I personally never saw an LA 318 with a forged crank, but some of the older A 318 polysphere headed engines did have it forged. If you have a forged A 318 crank, it's just as good/strong as the 340 forging.
 
I would start by posting a close-up picture of the parting line on that 340 crank, as well as a picture of the damaged flange you mentioned, so that crank can be evaluated. The "parting line" on a cast crank usually looks like a sharp-ish rough edge and forged crank is wider and smoother. Usually easy to tell by seeing it. As for the 318 cranks, I personally never saw an LA 318 with a forged crank, but some of the older A 318 polysphere headed engines did have it forged. If you have a forged A 318 crank, it's just as good/strong as the 340 forging.
about 50 years ago, I had a LA318 truck. I think it had a forged crank in it. I don't have it any more, it's in TF Paddy's yard, now, so I can't verify that.
I think the balancing is different, so you'd have to address that, if you use the 318 crank
 
Back when Sears used to sell crankshaft kits, I spun a bearing in the 340 that was in my Duster. I was still in high school and didn't have much money to spend so I pulled the engine out, turned it upside down and removed the crankshaft. Turns out it had two 318 rods and six 340 rods on 180 out throws. I think number 2 and number 5. I put the cheapo Sears crank kit which was cast btw in it and ran the fool out of it. I never noticed a vibration. I turned it 6 grand regularly. I don't think using the 318 crank will be a problem regardless of whether you balance it or not. IMO
 
I thought all the 318 LA cranks were forged until 71 or 72. I know the one from my 68 Fury Wagon is definitely forged. I also lnow the 67 year 318's were higher compression, believe they were 9.2:1.
 
I was looking through my old Direct Connection racing manual and found some information that confirms what 540slant6 said about forged 318 cranks.
IMG_20171012_082354.jpg
pm
 
Swap in the crank and rods from the 318, and don't concern yourself with balancing the thing.
Yes, there is a weight difference with the pistons, but because it is located so far from the rotating assembly it will not effect anything
 
Thanks for your replys ! I did a little more searching last night and learned that the differance in compression between the low and high compression 340s was the compression height and not connecting rod length like I thought . If I get a chance to I will post a few pics today of the crank and pistons .
Thanks again for your help guys !!!
 
Stroke it out to the max with a forged crank. Cast cranks are for mail box posts. :rolleyes:
This could very well be the dumbest thing I've read all year.
I've been building engines from Top Fuel down for 40 years.
Under 600 or so horsepower, there is zero benefit to running forged over cast in a Mopar.
Junk cast cranks are a Ford/Chevy thing.
 
Swap in the crank and rods from the 318, and don't concern yourself with balancing the thing.
Yes, there is a weight difference with the pistons, but because it is located so far from the rotating assembly it will not effect anything
^^^^^^^^^^^ THIS^^^^^^^^^^^^

And..... Cast cranks are really tough. I know a guy that spun one 7200 rpm's in a stock eliminator (3.31 stroke factory cast).
 
- The bobweight using the 340 pistons and rods is around 2326 grams.
- The earlier light rod 318 bobweight is around 2147 grams, and that is what the 318 crank is balanced for; that is a difference of -179 grams. So 340 rods+pistons with a 318 crank will be waaaay off-balance.
- Put the heavy 340 pistons with the lighter (early) 318 rods and the bobweight will be around 2271 grams. That is still 124 grams heavier than the stock 318 bobweight... No way will it be balanced, not even close.

As you can see from the above, the 318 cranks are counterweighted for a significantly lighter bobweight; that is why the idea of not need rebalancing is incorrect.

OP, if you want to rebalance the 318 crank for the heavier 340 rod and piston package, you will have to:
- drill out some of the 2 end journal pin material (like in a 340 crank)
- and/or add weight to the counterweights (And even if you do the above, you will still have to add weight.)
Either way it will be pricey to balance this combination of 340 pistons with the 318 crank.

I would just bite the bullet and drop in some lighter KB 340 hypereutectic pistons and balance the 318 cast crank to that. If you need help on numbers for the bobweights of those pistons with the 318 or 340 rods, let me know; I'll be happy to run some numbers for you. A final bobweight the same or LOWER than the stock 318 bobweight is where you want to be; a lower bobweight will make sure you take off weight from the crank, not add it on, and that is the low cost way to go. And with the KB flat tops, all of a sudden, you will have a much better compression ratio, which will make this engine's low RPM operation a lot torquier.... which is good for a cruiser with attitude.....

FWIW, OP, the general concensus is that the cast cranks are fine for your level of engine. So don't fret too much over that IMHO... though it would be nice to stumble across a forged 318 truck crank. My son's 340 ended up with a 273 cast crank and it is (supposedly) built to breath to the upper 300's HP range.
 
This is very good news ! Thank you whitepunkonnitro , I appreciate your help !!!

So I checked out the crank this morning and it is a cast crank #3462387 which is late 72' -73' 340 . I did take some pictures but will try to upload them later . The crank flange looks as thou someone had a difficult time removing a flex plate or flywheel . They used a cutting torch ,cutting part way throu the flange between two bolt holes .

The pistons are #3671155 and using a vernier height gage the pistons look to be approx. 65 thousands of an inch below the deck at TDC .
 
Do you have any pictures of the un-repairable damage?
And keep in mind the 67 crank is going to have a smaller hub for the flywheel or converter than the 69 crank.
 
340 is forged crank up to 73. I have heard of a few 72 with cast crank but never actually seen one to verify. 73 cast crank 340 was one year only. Also requires a unique damper with an offset balance. Its clearly marked "use with cast crank only".
The difference in the 340 compression was a piston change. The piston with 4 valve reliefs are the later low comp ones. If my memory serves me right, the change was made in 72.
 
340 is forged crank up to 73. I have heard of a few 72 with cast crank but never actually seen one to verify. 73 cast crank 340 was one year only. Also requires a unique damper with an offset balance. Its clearly marked "use with cast crank only".
The difference in the 340 compression was a piston change. The piston with 4 valve reliefs are the later low comp ones. If my memory serves me right, the change was made in 72.
I read somewhere that in '71, the 340 compression dropped from 10.5 to 10.25.
 
Stroke it out to the max with a forged crank. Cast cranks are for mail box posts. :rolleyes:

You think so? Tell that to anyone whose run a 351 Cleveland in stock eliminator, or early NASCAR. The stone stock CAST IRON cranks routinely saw 9500 RPM.
 
340 is forged crank up to 73. I have heard of a few 72 with cast crank but never actually seen one to verify. 73 cast crank 340 was one year only. Also requires a unique damper with an offset balance. Its clearly marked "use with cast crank only".
The difference in the 340 compression was a piston change. The piston with 4 valve reliefs are the later low comp ones. If my memory serves me right, the change was made in 72.
I had a '72 340 motor that had a cast...
 
I built a 72 340 years ago with a 67 318 steel crank. out of a Plymouth Fury. The difference in the crank was it didn't have the holes through the rod journals, and I had to have the back of the crank milled for the hub of the 727 torque converter to fit in deeper.
 
Ya , some people really need to just step away from the tools and let someone who has the necessary skills in their hands do the work for them !
This crank is now nothing but a boat anchor !!
 
-
Back
Top