1955 mystery drivetrain vibration

[kkarrer]

I wonder if anyone out there has encountered a vibration in the drivetrain of a a 55/331.  I had the engine rebuilt and the trans was recently rebuilt.  We can feel a quick vibration on the 2/3 shift and in 4th it get's bad enough to feel it in the seat starting at about 55mph.  It's not the drive shaft (it was balanced), u joints, or rear wheels or axles as we're in neutral and holding the brake as we rev it up on the lift.  I've tried shifting washers around on the torus/flywheel and that was a little better but not much.  This feels like it's coming from the bell housing area and not up front at the harmonic balancer which looks ok.  The owner says it was not doing this before the restoration.  I had the engine and trans out of the car for the engine rebuild and put them together on the shop floor prior to installation and saw no problems and this is far from my first rodeo with a 55.
Thanks,
Ken Karrer
1955 Cadillacs
512-626-7268
kkarrer@austin.rr.com

[Dan LeBlanc]

Potential starting point . . .

[dplotkin]

Good job Dan, that is a helpful piece that applies to all old cars.

Dan

[James Landi]

Dan,

Your extraordinary download of the service manual is invaluable.  After reading it with some care, I am so impressed by the thoroughness and concern for details in the piece.  (The quiz at the end takes tech accountability to a high level.)  I wonder to what extent modern "advanced engineering" has  taken care of some of "gray areas" that required attention 50 years ago. Do you think "CV" joints would attend to some of these "prop shaft" alignment issues" that occurred with universal joints?

[Dan LeBlanc]

I downloaded it from the 61/62 Cadillac site when I was trying to diagnose the driveline vibration in the 61.  It turned out the rubber in the rear transmission mount has settled causing propshaft misalignment.  I have a NOS mount to install but haven't found the time to do so.  In the interim, I shimmed it up and vibration is gone.  It is quite an informative piece.

[Jon S]

Dan -

Yes, very good information!  Thanks for sharing!

[Dave Shepherd]

 Also the trans mount can become rock hard which can cause it to no longer isolate drive train harmonics from the chassis.

[kkarrer]

Guys,
      Thanks for looking into this one, but I've eliminated all of the usual suspects...u joint, drive shaft, trans and engine mounts, alignment, harmonic balancer, pulleys and belts which pretty much leaves torus and flywheel.  The flywheel can only attach one way to the crank and there are two different pins on the flywheel to align it with the torus.  I'm wondering if those pins were initially 180 degrees out when the flywheel was first mated and if torque was applied and then the mistake was realized, if the flywheel  or torus could have been warped ?  They were fine before the engine was rebuilt.
Ken Karrer
1955 Cadillacs