vibration at 1500RPM in P or N converter weights??

[crash340]

Ok, so I have just gotten hold of my Torqueflite rebuild book by Munroe, inside that in the converter section it details a variety of converter ballance weights pending year and engine etc. I have a 73 340 with 727. At some point the trans has been changed to a 77 truck unit (360). Now, I have a harmonic vibration in P or N as I run up and down through the revs, was horendous with a Poly trans mount, had  to change it out to simply use the rear view mirror!!. Anyway, the table shows the 360 converter has two 60 gram weights fitted, it then lists the 73 340 as having two 25 gram weights fitted, All pretty much in teh same location, either side of the drain bung. whoud having a 360 converter on my 73 340 with the additional 70 grams of weight be the root cause of my vibration? its not a huge viration, with rubber trans mount the mirrors are clear.

Whats people thoughts?

[734406pk]

Yes it sure will! I had a customer with the same vibration issue on a 340. It turned out that someone installed a Torqueflite 727 from a 360 years prior. The converters were balanced differently, but looked the same. Runs smooth now!

[Chryco Psycho]

 

[crash340]

Ok, cool, im much happier now, but, how to fix it, i wont be able to get my hands on a 340 conv so, I pland to trim and thin the existing weights somewhat, I may not be able to get them down to the 25g mark but im sure the closer I get the less vibration I will get. I'll have a crack at it on the weekend. Once the stroker is build I will go internally balanced and an aftermarket conv to solve the issue. I see the factory weights are pressed onto a stud on the conv is that right? or are they welded etc

[734406pk]

I'm pretty sure they were spot welded on. But I didn't try to remove them.

[Chryco Psycho]

Usually welded , you can take a die grinder & thin them down without touching the welds

[crash340]

Ok, back to square one, i have just checked the converter and I have a '73 340 weighted converter so, the vibration is either in the engine or in the trans Damn it, I thought I was onto something

[crash340]

And, correct me if I'm wrong but you cant bolt the converter up to the flex plate in the wrong place because there is one mounting hole offset a little and you cant bolt the flex plate to the crank in the wrong place for the same reason, one hole is slightly out, both ensure correct assembly, right?

[ShelbyDogg]

The 73 340 could have had a steel or cast crank. Are you saying it did not vibrate till you had your trans rebuilt?
The flex plate only bolts on one way .

[crash340]

Rob, i have not rebuilt anything, its had it since i purchased the car, the trans has been changed out at some point proir, the engine is original. No history on the trans. The motor was build Dec 12, 1972, car was Dec 18, 1972. it has a cast crank balancer. You would swear its a bad uni joint as that is the type of vibration however, sitting in P or N its there, If you are doing 60 down the interstate, knock it into N and let the revs drop, nothing, smooth as, bring the revs up and its back, click it into D and still there. So that tells me its either engine (which I'm doubting a little) or front half of trans. I had the trans pan off it not long ago just to check it and change oil when I purchased the car and all looked good inside, very little muck in the pan and oil was good, soe  , there is a trans mech local who has a trans dyno, I could bolt it up and spin it, at least it would tell me if its engine or trans. But a pain in the back end having to pull a trans

[734406pk]

with a build date of Dec '72 it should be a cast crank 340, but 72 was the change over year from forged to cast. can you verify that you have a cast crank? You might have to remove the oil pan and check the casting number. Its a pain, but easier than dropping the trans. Its a long shot, but it would explain why with all the right parts, there is a vibration.

[ShelbyDogg]

You can see in the photo below that a 340 cast crank barely uses any weight offset for a cast crank 340.



You can always buy one of these weighted flexplates from summit. I found this one in a swap meet for $35. B&M Part number is 10235, then pop off both weights on your converter. Taking off weight on one side is just like adding weight to the other side.



My first Challenger had a 440 with 6 pack rods and a regular flywheel and balancer. It shook so hard when I punched it on an open road, that it cracked my 727 tailshaft open at the rear bearing. Had to get the whole thing balanced. All was well afterwards. Speedshop only charged me $110 to balance back then.

EDIT:  Just read that you say that you have the correct 73 cast crank torque converter.  Does it vibrate when you rev it in neutral, not on the highway?

[crash340]

Rob
I do have a cast harmonic balancer and correct weighted (two 25g weights) converter, it vibrates the whole time, just more noticeable in P or N sitting in the drive way. If I'm on the interstate doing 60, knock it into N and let the revs drop the vibration goes, still coasting at 60 in N, if I bring the revs up the vibration comes back. So that proves its not the back end of the trans of beyond.

[Rich G]

I had the same problem with my 340 manual trans when I built the car. Found out I needed a neutral balanced flywheel. Changed that and now it's smooth as silk!

[ShelbyDogg]

Sounds like you have a steel crank with the wrong cast balancer and converter, or someone changed pistons and didn't have it balanced before assembly. You'll have to drop the pan to have a look at the crank.