vibration issue

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David Dickerson

David Dickerson

Stickler
Joined
Feb 26, 2018
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New York
I have a vibration issue that I will need to use the process of elimination to diagnose. I thought that this issue would have fixed itself after my most recent rebuild of this car. A little history: I acquired a 1970 340 TA motor back in 1980. It was in parts, but complete. I had it balanced and built and installed in my 74' Duster. It was backed to a 904 Tranni. This is where my memory is a little sketchy. I can't recall a vibration problem with the 904, but it didn't last very long with the 340, so I found a 727, bought a TCI coverter, installed a 8 3/4 rear and put in 410 gears. I recall vibration issues at around 2800 to 3000 rpm after that and remember changing to 323 gears to make things more streetable and it also reduced the vibration at highway speeds. Fast forward to my most recent build of the car. The engine was disassembled and balanced again. I added a GV overdrive and went back to 410's. Also urethane engine mounts, a new Summit balancer (internal balance). I still have a vibration that is very annoying and makes the car uncomfortable to drive. I am getting ready to drop the tranni and switch out the TCI converter with a new Hughes. I have read some previous forum questions regarding this issue. One specifically mentioned some of the early aftermarket converters causing vibration issues. Before I do this, I wanted to reach out for any other thoughts regarding this issue. Also wondered if anyone thought I should replace the flexplate while I have the tranni out. Doubting that the flexplate could be the issue.
 
Things I think I remember...

The hole in the end of the crank on some engines is different diameter then others. The torque converter front nose must match.

Some engines are externally balanced and the balance is on the torque converter

Some engines use a externally balanced front dampener.

Vibration in neutral at 3000 rpm or only while in gear?

Vibration at 3000 rpm in park?
 
So it does it parked in the driveway as you bring the rpm up? If so, drop your fan and alternator belt and test it to make sure it’s not an accessory, I had a very bad vibration I was chasing and it turned out to be the alternator
 
Dana67Dart said:
Things I think I remember...

The hole in the end of the crank on some engines is different diameter then others. The torque converter front nose must match.

Some engines are externally balanced and the balance is on the torque converter

Some engines use a externally balanced front dampener.

Vibration in neutral at 3000 rpm or only while in gear?

Vibration at 3000 rpm in park?
Click to expand...
All 340's up to 72 were internally balanced. That is my understanding. Won't know about the hole and torque converter nose until I drop the tranni. Flex plate bolts up easily. I believe it is the original. The vibration is present whether in neutral or park. Mostly noticeable while driving under load at specific rpm. You can feel it in the seat of your pants and see it in the rear view mirror.
 
crash520 said:
So it does it parked in the driveway as you bring the rpm up? If so, drop your fan and alternator belt and test it to make sure it’s not an accessory, I had a very bad vibration I was chasing and it turned out to be the alternator
Click to expand...
The alternator is new and all the pulleys are March performance. Since this issue was present after the first build, and everything but the torque converter and flexplate has been replaced, I find it hard to believe it is something else.
 
It couldn't hurt to pull the fan belts. It would suck if you went to all that trouble to pull the trans, only to find it was something simple. Also, look for a good sized weight welded to the converter. It could be you were sold one for an external balance.
 
The crank pilot (front nose) sizes are as follows.
TF904 to 1967: 1.550"
TF904 1968 on: 1.810"
TF727 1962 on: 1.810"
If the TA 340 came with a standard transmission though, I don't have the size of the crank hole. I know when I worked at a GM dealer back in the day, the crankshaft for a standard was different than an automatic. Perhaps someone on the board who has a standard transmission in his car can give you the size of that hole in the crank.
Also, a 1969-1973 340 had a cast crank, so the converter will have two small weights on it(flywheel side of the converter) that measure .875" x .625" x 1.80".
The attached PDF shows you (rear of crank) the difference between a cast and a steel crank.
Hope this helps.
 

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