Engine/drivetrain vibration?

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DartSportDude

Old school A-body owner
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I hope somebody here can shed some light on a possible problem I'm having with the rebuilt 340 engine and/or 727 transmission in my '73 Dart Sport. The engine was rebuilt about 150 miles ago, as was the transmission. I took the Dart into my local shop last week to address a small transmission fluid leak, and after driving the car, the shop owner asked me if I'd noticed a pretty significant "vibration" in the engine or some other part of the drivetrain. The only thing I'd really noticed since the rebuild was a different "resonance" in the sound of the car at highway speeds. I attributed this sensation to the new exhaust system, which was increased to 2.5 inch pipes and new DynoMax mufflers.

Anyway, the shop thinks I should look into the vibration while the rebuild is still pretty new. They determined that the torque converter (replaced during the tranny rebuild) might be the wrong one for a '73 340 (they claim the one in the car is intended for a 360 instead), so they removed two balancing weights on the converter to see if that could be the problem. The vibration is apparently still there. They also checked the harmonic balancer, but it is the original factory part that was reinstalled after the rebuild.

I guess the next step is to have the owner of the machine shop that performed the engine rebuild come over and drive the car to see what he thinks.

Has anyone here had a similar experience? Does a '73 340 with 727 require a different torque converter than a later model 360? If the torque converter and harmonic balancer are not the problem, could the vibration (which you can sort of feel through the seat of your pants) be due to some kind of internal engine problem associated with the rebuild? We also checked the new motor mounts and transmission mount, and they are okay.

This issue is driving me crazy -- I just want to drive the car!

Thanks in advance for any help.
 
.......The 1973 340 most definately needs the proper balabced tc 4 it....u could also buy a b+m flexplate that is balanced 4 it..........kim........was ur little teans leak at the tc?.........
 
If all of the weight was removed from the converter, you went too far., A cast crank 340 should have a "For Cast Crank 340 Only", or something to that effect, cast into the correct balancer counterweight. The converter should still have a smaller weight than what a 360 takes. If the converter is now neutral and it IS a cast crank, get a B&M flexplate for a cast crank 340 to balance it out.

The forged 340 crank takes a flat balancer/damper, like a 318, and a neutral balanced converter.

Did you change any internal parts during the rebuild? Pistons, rods, etc? That would likely require rebalancing the rotating assembly.
 
Been here, done this.............here we go again................

I feel your pain. If I remember correctly, the 340's are internally balanced. This means the balancer AND the converter must have NO counterweights on them....BUT, different years could be external......not sure.

Get under the car, and check the back side of the balancer as someone bumps your key to make sure you look at the entire balancer........If there's a counterweight sunken into the back, take it off if your 340 checks out to be internal balance...........probably held in by an allen set screw.

If you have no counterweights, then something else is probably out of balance somewhere........could be a yoke, the driveshaft, tailshaft, etc., OR in the motor. If your boy didn't balance the parts before he installed, it could be your culprit.

Some other guys here have great experience with this type of thing too...........Let's see if they chime in. Good luck to ya'......and be sure to post your findings!

Bottom line is, I believe, this: All of the small blocks were internally balanced except for the 360, and cast-crank 340.....yours is probably a cast crank if it's original, and should need counterweights...but you'd better be sure it's the original for 73.
 
I had a similiar problem with my 67 Barracuda for quite awhile. It had a vibration that was only noticeable at 50 mph and above. It was a seat of the pant, as the vibration you described. I always felt it was the driveshaft as my transmission had several tailshafts seals with very little miles. Finally, after many discussions, I took the drive shaft to be balanced, not expensive and as soon as they checked the shaft for runout they found variances of .020 -.060. I opted to have a new shaft built and no more vibration. Just a thought.
 
You could check to make sure u-joints are seated correct.After removal and re-install maybe it,s not seated??Just a thought.
 
Is this vibration RPM generated or speed generated, take the car down the highway till ya feel the vibration and put the tranny in neutral and let engine idle down.
If it's RPM I'd start at the beginning by first determining whether the crank is forged or cast. There is no guarantee that the original balancer was correct before you had the engine rebuilt. Once you've determined the crank for sure then work your way outwards to the balancer, flex plate and converter.
If it's speed related I'd check that both u joints are installed correctly and not worn out, as previously mentioned, I've had u joints starting to go and first place I felt them was when you're sitting in the drivers seat.

Terry
 
had weights on my stock 360 727, since went to a internal balanced stroker kit. the weights on the converter were taken off and had it neutral balanced. weights had to be added to balance the converter. i wouldnt just take off weights without having it balanced.
 
Thanks for all the replies. Let me see if I have this straight. My shop is about 99% sure that the balancer is original and correct for the cast crank '73 340 that's in the car. They also think the new TC that was installed during the tranny rebuild is the wrong one, and have subsequently removed all the balancing weights from the TC. I guess that means it's now a "neutral" unit? Are you saying that I can keep the current TC, without balancing weights, by simply installing the correct B&W flexplate for the cast crank engine? If so, do you know of a good source for this flexplate?

From everything I've read, it sound like the correct TC for this application should have weights. Shouldn't I also be purchasing a correct TC (either in addition to, or instead of, the B&M flexplate)?

This problem sounds like a total snake pit! If the TC problem is not the source of the vibration, then what? Geez, I hate to pull the rebuilt engine out of the car (that's about 100% show ready now) and starting tearing into the internals. :pale:
 
A cast crank 340 that was not internally balanced by the machine shop needs a properly located small weight on the converter or a B&M flexplate - 10235 for a 727. Jegs and Summit sell them for $70-$80 each.
Those have a section cut out 180* opposite of the weight location on the OEM 340 cast crank converter. (Accomplishes the same thing and probably saves a few cents in mfg. costs.)

If all of the weights were removed from the converter in use, it has become "neutral" balanced.

Double-check to make sure the balancer/damper has a raised ring with a counterweight and "For 340 cast crank only" cast into it. If that's the original crank and balancer combo, it's a cast crank. Refer to the link posted by Convertriple 73.

Did you use the OEM pistons & rods for the rebuild?
 
Locomotion:

Thanks for all your help. Sounds like you know your stuff. I've owned this car almost its entire life, so I'm pretty darn sure it has the original cast crank and balancer. However, I will follow your advice and double check these parts just to be sure. My current repair guy seems more and more convinced that the problem goes back to the machine shop that handled the rebuild. In his view, the vibration exists when the TC has the weights -- and it's still with the weights taken off -- so that eliminates TC as the culprit. He's already ruled out engine mounts, tranny mounts, and exhaust system mounting. So now he's wondering if the machine shop resurfaced the original crankshaft during the rebuild and somehow threw everything out of balance. Is that even possible?

The invoice from the machine shop indicates they did balance the engine as part of the work. How could they have returned the engine out of balance?

FYI: This was a pretty standard rebuild; the machine shop installed the obvious new parts but maintained the factory specs (with the exception of a slightly hotter Comp cam).

The owner of the machine shop came over today, listened to the engine, and confirmed it's got a pretty significant vibration in the area of 2,500-3,500 RPMs. He going back to the original guy who installed the engine for us after the rebuild (different shop than the one now questioning the vibration) to see if they have any answers.

I can see where this is going...my current repair guy will say he's checked everything and can't find the problem. That means the only option will be to pull the engine out of my freshly repainted and correctly restored engine bay :sad8: and send it back to the machine shop for some kind of re-do. That should set me back another 2 or 3 grand. But what happens if the engine goes back in the car, after all that hassle and expense, and still vibrates??

What a pain in the *** this is turning into!
 
Something doesn't sound right with the balance job. If the invoice says it was balanced, but the converter with weights was for a different application, it couldn't be right. You can make the rods and pistons all weigh the same. But the crank still needs to be put on a machine & spun to get its counterweights right.

Someone should have a part # for the new converter to confirm what the original application was. If it was for a 360, it had too much weight for a cast crank 340. If all of the weight was taken off, that's not correct either because it needs a little, but less than a 360.

Ask the machine shop what they did to the crank to balance it. The only ways I know how they could have correctly balanced it would be if they added mallory metal to the rear crank counterweight, used a B&M plate or used the old, original converter. Not sure if they can figure the extra weight needed to simulate the converter or flexplate counterweight when they spin the crank without actually having one bolted to the back.

I'm not up on the details of actual machine shop work, but I believe a balance job basically works like this:

* All rods & pistons are machined to weigh the same. Rings, rod bearings, pins & locks are added into a formula.

* Weights representing the formula totals are attached to the crank to simulate the rotating assembly and the crank is spun.

* The resulting balance machine readings tell the operator where and how much the assembly; crank (balancer and flexplate/converter if installed), are off from optimum.

* Weight is then added or removed from the crankshaft counterweights to balance the assembly. It can take several "spins" and machining to get any unbalance minimized.

It is a good practice to have the correct balancer and flexplate/converter, that are to be used, bolted on the ends for greater accuracy. They MUST be bolted on, or weight compensated somehow, if you will be using counterweighted units, like in a cast crank 340 or 360 application.
 
oldkimmer said:
.......The 1973 340 most definately needs the proper balabced tc 4 it....u could also buy a b+m flexplate that is balanced 4 it..........kim........was ur little teans leak at the tc?.........
Click to expand...

If your balancer is correct, from the pics- problem is plate/converter
 
Thanks again for all of the feedback. This situation keeps changing; my repair shop now thinks the new TC installed after the tranny rebuild might, indeed, be the correct one since it DID have weights. I think they were initially confused, since they probably didn't realize that earlier 340s had a forged crank (internally balanced) and thus should have had a different TC than the '73 version with a cast crank. This weighted TC was on the vehicle when the rebuild was done.

However, I suspect Locomotion may be right -- the problem may well lie with the balancing job done by the machine shop. If so, I guess the engine will have to be pulled and sent back to the machine shop for rebalancing -- correct? Is there any way to compensate for the balance problem with the engine still in the car?

Also, what's the long term implication of "living with" the vibration? Beside being a little annoying, will this condition shorten the engine life or cause other problems?

Sorry for all the uninformed questions, but I'm just a cosmetic restorer when it comes to these cars -- mechanics was never my strong suit!
 
Did the shop have the vibration damper when they balanced it? Was it internally balanced or balanced taking the '73 340 damper and torque converter weights into account? You need to find out what the engine shop did to balance it.

If it was balanced "externally" like a stock '73 340 then get the B&M flex plate for you application and run your torque converter with no weights. That way you'll never have to worry about having weights on your torque converter.
 
I have a 340 flexplate from B+M if you need it. NIB.

OOPs saw you have a 727.
I do have a balancer though if You want to try that.
Tom

carstuff005kj5.jpg
 
My thoughts....
First...REPLACE THAT BALANCER. Its a good rule of thumb to replace the balancer when doing a rebuild.....the rubber could have deteriorated and the outer ring slipped on the inner thus making it so timing is off and balance is off.
Secondly, take the driveshaft i to make sure its balanced correctly.
Thirdly, defintely get the B&M flexplate for your application.

Eliminate the variables BEFORE you pull the engine.
 
Another update on my situation with the rebuilt 340:

- It appears that the torque converter installed during my tranny rebuild (prior to the engine rebuild) did have the correct balancing weights, and was likely correct for the externally balanced '73 340. Unfortunately, those weights have now been removed from the TC in an attempt to correct the engine vibration (it didn't work).

- There may be some question as to whether the harmonic balancer that's been on the engine for years is the correct one. This is still TBD

- After talking with everyone involved in the engine rebuild, it appears the machine shop did NOT include the TC for the cast crank engine when they balanced the motor. Thus, it appears the balancing might have been done to the specs of an earlier (internally balanced) 340 -- not a externally balanced '73 motor

- Short of pulling the engine, sending it back to the machine shop and rebalancing everything with the correct TC included, my repair guy thinks it may be possible to have his tranny shop "spin" the current converter, compare that reading with the balancing done on the engine as part of the rebuild, and then add weights to the converter to compensate for the difference (i.e., the out of balance condition).

Obviously, you'd have to have same balancing configuration if you ever replaced the converter in the future.

Does this scenario sound do-able? If so, what should I be on the look-out for, and what should I be wary of?

This approach, if it works, would certainly be less painful than pulling the engine and starting all over again.

Thanks for all your help.
 
DartSportDude said:
- After talking with everyone involved in the engine rebuild, it appears the machine shop did NOT include the TC for the cast crank engine when they balanced the motor. Thus, it appears the balancing might have been done to the specs of an earlier (internally balanced) 340 -- not a externally balanced '73 motor
Click to expand...

Was it internally balanced like an early 340? The early 340s are internally balanced and have neutral balance vibration dampers and neutral balance torque converters (no weights). It sounds like they aren't sure or maybe I am reading it wrong? If it was internally balanced like an early 340 then a neutral balance 340 vibration damper would fix the problem. No weights are on the converter now so nothing would need to be done there. This is all assuming it was internally balanced like an early 340 which I am not sure they did?

Here is an internal balance vibration damper:

http://store.summitracing.com/partdetail.asp?autofilter=1&part=DCC-4452816&N=700+115&autoview=sku
 
You could do as you are talking about with taking the transmission to the shop, but if the balancer did slip and that is causing your vibration as redfastback gas suggested just balancing the transmission to the balancer will not prevent the balancer from slipping again.

If you don't want any more problems with your motor replace the balancer and instead of putting weights on the torque converter install the B&M Flexplate in the link below. The bolts only line up one way so the weight will always be installed on the correct side of the torque converter.

If the vibration is still in the car have your driveshaft balanced.

http://store.summitracing.com/partdetail.asp?part=BMM-10235&autoview=sku

Hope this helps.
 
At this point, we believe the machine shop balanced the engine like they would have done on an early (internally balanced) 340. We suspect they did not know the difference between the forged and cast crank motors (it would have been nice if they'd checked the crank before doing the balancing, huh?), and thus they did not factor in the torque converter weights as part of the balancing job. It appears they did have the damper on the engine when it was balanced, so now we just need to confirm whether the existing damper was for an internally or externally balanced engine.

As a side note, this damper has been on the engine for years (before the rebuild), and the car always ran fine -- so I have to believe the damper is correct for the '73 motor.

Assuming our assumption about the balancing job is correct (I say assumption, because the machine shop kept no records of the work they performed), then the use of a neutrally balanced damper with the B&M flexplate would make sense. I'll talk to my repair guy to see what he thinks.

Thanks guys, for all of your help!
 
I would talk to the machine shop and ask for compensation with having the engine r&r'd, torn apart, and balanced CORRECTLY, rebuilt and repainted. They should have seen this, noticed the crank, and spun it correctly. That is their job.
 
I'm no expert on 340's, but I thought that I read somewhere 72 340's were steel (forged) cranks up to April 72, and vibration dampers changed on 04-11-72 from forged to cast type... On big blocks a cast crank meant a different vibration damper (says on it for cast cranks only and usually larger in size) and the automatics came with balancer weights on the torque converter and were matched to the engine in that position, only (the torque converter has 4 possible positions with the flex plate)... Also, on the big blocks an E was stamped on the engine ID pad to mean a cast crank... So, if you bought a 440 with an E stamp, you better get the torque converter with it... Hope this helps, good luck...
 
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