1970 340 Balance Problem...Bottom Line

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pastortom1

pastortom1

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Under the previous "Fluid Dampner, How-To" thread, there was quite a bit of great information on balancing of early Mopars, and especially the problem we have been faced with relative to our 1970 date coded 340.

To recap, our 70 340 has a vibration problem at 1400 and 2800 rpm's, whether driving or parked. We thought it was most likely a mismatch between the engine and an aftermarket torque converter......it's quite a coincidence that as I was watching Barrett-Jackson a few minutes ago, Rick from the Speed Channel was commenting on exactly that problem, which was faced by many an unsuspecting buyer as they changed their conveters over to the popular "stall" configurations.....Lots of vibration due to the mismatch of counterweights.

I wanted to draw a bottom line here, and ask all of the guys again about the specifics of this problem........

What we have is a 1970 340, with the stock crank and stock harmonic balancer. I've been told that the stock balancer will have a "counterweight", denoting the engine as externally balanced. A-Body guys...IS THIS CORRECT?

If so, what torque converter must be used on this engine? (and if it's NOT a stock replacement Mopar or equivalent, what will be the proper "name" or distinction that we must ask for when purchasing an aftermarket converter?

Bottom lines here..........Is the 1970 340 an externally or internally balanced engine? (from the factory, that is)......How do we tell from outward observation if it's externally balanced?........and due to this distinction, what type of torque converter MUST be used to avoid adverse vibration?

After researching this, I've found that it is a COMMON problem still today, and seems to be especially relative to the 1970 engines.

What are the bottom lines?

Doc
 
340's were all internally balanced through '71 and a few '72's made it through. Balancer should be flat with no counterweight and converter should be neutral with a neutral flexplate. Balancer should look like any 318 balancer. '72-'73 had the counterweight cast into the balancer that said something like "For 340 cast crank only". This counterweight is smaller than what was found on the earlier 360's. Later 360's had weight removed from the opposite side and the '93 & newer 360 Magnums are different are different too. B&M makes flexplates that have the correct balance built into them for each different externally balanced application so the readily available neutral balanced aftermarket converters can be installed without the hassle of positioning and welding the correct weight on them.

A cast crank will have a small, sharp cast flash line running up and down the crank. Forged cranks have a 1/4" or so machined-like line running the length of it.
 
I will throw in my 2 cents,factory a 70 340 is internally balanced as it should have a forged crank.The harmonic balancer would not have any counter weights and would be similar in design to a 318.If it is a internal balanced engine then you should be able to run a off the shelf convertor but if its balanced externally the best bet is a aftermarket flexplate that will eliminate the need for a external balanced style convertor.What makes this difficult is all the possible scenarios,I mentioned before that Ive had many aftermarket convertors off on the balance and vibrate,Ive also seen engines improperly balanced and cause vibration so its hard to pin down.One test is to lightly powerbrake the car and if the steering wheel shakes it usually indicates a convertor problem-usually.
 
I will just add a few quotes from Larry Shepard's (#1 Mopar guru) book, How to Hot Rod Small BlockMopar Engines below. Mike

Forged Cranks-The forged crank is probably the most common type of crank used in racing. I was the only type used in the A engine family up to the early 70's. pg.15

Cast Crank-The A-engine cast crank was introduced with the '71 360 engine. By 1975, all A engines used a cast crank. The automatic transmission 340 engines got one in '72, however the manual transmission cars still used a forged crank until the end of '73. pg.15

The biggest drawback is that 340 and 360 cast crank engines need to be externally balanced. pg.15

Vibration Dampers-The cast crank 340 and 360 engines use a special offset damper in that a large flange is added to part of the damper. The forged damper is symmetrical. pg20

End quote

Hope this helps, Mike
 
I'll ad that yes a 70 340 from the factory is internal balanced. BUT... Somebody could have swapped out the crank for a newer one that is a cast crank which requires external balancing. As far as I know there really isn't a way to check from the outside what crank you have. Like Locomotion said the forged crank (internal balanced) have a wide parting line on the crank throws and the cast crank (external balanced) have a thin parting line. The only way I know to tell is to open up the engine and see what it has.

This has nothing to do with PastorToms car but I'd like to say about Mr.Shepards statement that 360 balancers "have a large flange added to the balancer" IS NOT CORRECT! The 340 balancer has a flange added to it but the 360 has a groove cut out of it. I've owned all 3 styles, 340 cast crank, 340 forged crank, and 360 balancer so I've seen them all. He also states that from 72 on 340 auto cars had a cast crank and manual cars had a forged crank. I have a friend that bought a 73 340 4 spd. Cuda brank new and when he tore it apart it had the cast crank in it. I have Mr. Shepards book and it is a good book but either his proof readers did a lousy job with it or he don't know balancers like he thinks. I also found a few other things that were questionable in it. Just because he wrote a book don't mean the's the #1 Mopar guru.
 
With this information, it should be relatively easy to choose our next course of action.........outward observation should suffice.........I have a distinct feeling that the converter has counterweights............

BUT, if not, we have probably covered all of the bases, and gone the most inexpensive routes possible before pulling the motor.

One thing I DO know beyond the shadow of a doubt, I really appreciate the A-Body web site............all you guys are great, and we appreciate the input and expertise from seasoned Mopar enthusiasts.

I'll post a thread on our results in a couple of weeks or so (threw my back out..........gotta go to bed, so I'm told).

Doc :thumbup:
 
With your vibration at 1400 and 2800 u have an engine balance problem, if u were to rev it to 5600 it will be there also, because it doubles with rpm......providing ur neutral balance converter is the problem there would be a continous vibe at all rpms...360s had a weight added till 1978, cast cranks were in 318s for the 1969 model year, BUT were Internally balanced, even up to the end of production....kim........
 
Fishy, first off, original poster states that engine has original crank.
'
"What we have is a 1970 340, with the stock crank and stock harmonic balancer."

Secondly, I think Larry Shepard did a little more than write a book. Let me quote from his book again: Larry Shepard began working for Chrysler as a suspension engineer in 1968. By 1970, he had moved over as staff engineer of Chrysler's drag racing program, where he would guide the winning efforts of drag racing competitors, including Bob Glidden's legendary 340 Plymouth Arrow. He is currently a staff engineer with Mopar Performance.
END QUOTE

So here we have the Chrysler staff engineer that developed the very motors we are speaking of for drag racing, his whole career devoted to Mopar engine performance since 1968, and you don't feel he is a #1 Mopar guru. I find your statement hilarious.
 
Just a couple of thoughts and one fact........ First, I think the term "guru" is probably overused and over emphasized in many things from Mopar knowledge to tiddly winks. Second, I'm sure Mr. Shepard is among the most knowledeable Mopar minds out there, and yes I also have his book. Third, (and this is the one fact that I alluded to) I bought my Duster 340 (manual transmission) new in '73 and it DOES have a cast crank and NOT a forged crank.
 
ValiantMike said:
Fishy, first off, original poster states that engine has original crank.
'
"What we have is a 1970 340, with the stock crank and stock harmonic balancer."

Secondly, I think Larry Shepard did a little more than write a book. Let me quote from his book again: Larry Shepard began working for Chrysler as a suspension engineer in 1968. By 1970, he had moved over as staff engineer of Chrysler's drag racing program, where he would guide the winning efforts of drag racing competitors, including Bob Glidden's legendary 340 Plymouth Arrow. He is currently a staff engineer with Mopar Performance.
END QUOTE

So here we have the Chrysler staff engineer that developed the very motors we are speaking of for drag racing, his whole career devoted to Mopar engine performance since 1968, and you don't feel he is a #1 Mopar guru. I find your statement hilarious.
Click to expand...
U would not be outta line using SHEPS name with the likes of TOM HOOVER and the rest of the RAMCHARGERS...........I f it wasnt for Shep,Where do u think the MOPAR PERFORMANCE programm would be?.....I think u already know........NON EXHISNT.....KIM..........
 
fishy68,

360's did, in fact, start out with a rather large counterweight cast into the balancer through the '70's. It was about 2-3 times as big as the 340 counterweight, but smooth with no writing. I don't know exactly when, but they went to removing metal from the balancer at some later point on the opposite front face.
 
Ok I didn't mean to knock Mr. Shepard at all but as I look at what I wrote I can see it came out that way. Sometimes I write things that don't come out the way intended. As good as he is (and I know that an engineer would know more than me) I do know that he has some wrong info in his book in a couple of places. I read his book several months back and found a few mistakes and they didn't look like typos. One example is mentioned right here about him saying 340's through 73 had a steel crank when equiped with a manual trans. We have 2 examples posted here that show that. According to Locomotion I stand corrected on the balancer in the early yrs. I don't know when they started using the notched out balancer then because my 74 and my 79 360's both had the notched out balancer. Not one with a large counterweight. So have a good laugh on me guys.

Also ValliantMike your correct about the original poster saying it has the original crank in this post but I remember about 2 other posts of his about this same problem and IIRC that was questioned.
 
I can understand where you (fishy) were coming from now,as good as people are we are all only human so I dont take every piece of info in a book as the gosphel-heck there are how many versions of the bible?.But in saying that there are definitely those (Shepard being one) who shine and are true enthusiasts/professionals and without them the hobby wouldnt be the same.As a side note my brothers numbers matching 73 340 four speed challenger has a cast crank as well as a engine from an original 73 340 four speed duster.Ive also heard of late 72 auto cars with cast cranks but never saw one,my late 72 340 has a steel crank (four speed). :sad:
 
Perhaps I was wrong calling him the #1 Mopar guru (that would be Galen Govier), but we all would have to agree that he is one of the most knowledgeable. The only reason I posted this info was to show Pastortom1 the production time schedule between the two cranks. Going back to my source and checking further I find that Shepard states that forged cranks were phased out of production A engines by the end of 1974 and replaced by cast cranks. With that being said, I'm sure you could of bought a 1973 Cuda or Duster 340 and received a cast crank one day and a forged one the next. I know that my 1973 340 Cuda had a cast one, but Chrysler being Chrysler most likely mixed them in production until inventories were where they wanted. Who knows? I personally think it sounds like that Pastortom1s rebuild was most likely not balanced correctly, but thats just pure speculation on my part. Mike
 
Also I would like to add that 360s in 1 ton duallys were rumoured to have had forged cranks, but Ive never actually see 1, although 400s with 4 gears had forged cranks, as well as 440s in trucks having forged cranks and 6 pac connecting rods. I do not claim to know everything, but I do know alot of things.......older+wiser kimmer......
 
The 360 forged crank is just a rumor. I've worked and researched small blocks since 1973 and never seen any examples or documentation. Just rumors.
I'm sure it's safe to say that they don't exist in the OEM world.

1972 was the transition year for 340 cranks. Some leftover forged units made it through in early '72 model production but they were all cast after that.
Of course, "never say never". Seems Chrysler often had an exception here or there.
 
Uhmm... Remember, with Chryslers, "never say never!" Theres always one of none floating around out there. You can say as a general rule, yes, but never "Always"!
 
Here's a close-up of the 70 340 balancer from the back side......Look correct? Some have said it should be flat and symetrical, others have said the opposite in previous threads.

Maybe I'm looking at the wrong side here........Looks like a counterweight cast right in to me, and a hollow on the other side.......that would mean external balance........but it's a 1970 cast with the original forged crank.......

What say ye' ?

106_0610.JPG
 
I say that the balancer in the photo does appear to have a counterweight on the backside. But, that is NOT an OEM balancer!

An OEM 340 balancer will have a small counterweight cast into the front side and will be a raised area around the pulley. It will also say "For 340 cast crank only" or something like that. It is obvious.

Since the engine has apparently been messed with, you can't be 100% sure that it's the original forged crank. But the vibration would suggest that this counterweighted balancer/damper was installed on a forged crank.
 
I didn't think the thing looked right..........

They changed all of the pulleys to aluminum.......I would not doubt that they changed the balancer too............

I guess to totally eliminate the possibility, I'll have to change the balancer to a neutral or stock configuration for the 70 340..........

Any other input? :lol:
 
I think Myron is right. That balancer looks like an aftermarket piece. The kind that has a removeable weight that you position differently for what ever engine your using it on. If it is you should be able to remove the weight all together since it's just held in by a couple bolts. You'll have to take the balancer off to do this I think.
 
If the weight is not removable, a 318 balancer will work to replace it.

Hopefully there are not any other "inconsistencies" with the engine!
 
Locomotion said:
If the weight is not removable, a 318 balancer will work to replace it.

Hopefully there are not any other "inconsistencies" with the engine!
Click to expand...


what he said..
 
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