Engine/drive train vibration on 1941 Cadillac

[C Gorgas]

I have a vibration as I go down the highway most noticeable at speeds 35 to 47 mph. The drive shaft has been balanced as well as all four tires. I checked/tightened the engine at the front to be sure nothing was loose. The car runs absolutely smooth at speeds below 35. Anyone have thoughts? Input appreciated. Chet Gorgas 25441 

[marty55cdv]

   Chet, if you haven't checked the rear drums to make sure the large nut is tight , you might want to.

[Steve Passmore]

I had exactly the same problem on my 41 coupe last year. The vibration was unbearable after 35MPH.
Everything had been balanced including the wheels. It turned out to be 'Flats' on the tyres from standing. A great shame as they were like new but a new set of tyres was an instant cure.
Some say if you run it far and fast enough the flats will go but I couldn't stand that vibration for 100s of miles.

[Bob Schuman]

Chet,
The rear motor mount on these cars has a way of collapsing. The rubber mount is inside a steel cage bolted to the transmission extension housing and also bolted to the frame crossmember. If the metal cage is resting on the crossmember, the rubber insert has collapsed. That will very likely cause vibration. A new rubber insert is available from Terrill Machine and probably other vendors, and is not difficult to install.
Has any other recent maintenance been done? New clutch pressure plate assemblies are sometimes found considerably out of balance, which will also cause a bad vibration. The 1941 Cadiilac Shop Manual describes the procedure to balance the clutch.
Bob Schuman

[C Gorgas]

Thank all of you for your input; I will follow all of the above suggestions and report the findings. I rather suspect the clutch and will do as the manual says if the other ideas don't solve the problem. Chet G 25441

[C Gorgas]

I installed a different vibration dampener and most of the vibration went away. My next step is to do the trial and error washers on the clutch to see if I can remove the last of vibration. Thanks to all for your input. Chet G 25441

[Quentin Hall]

When you say " vibration dampener" , do you mean rear transmission mount? Just trying to anticipate any dramas I may have when I finally get on the road.

[C Gorgas]

By vibration dampener I am referring to the front of the engine where it is bolted to the crank shaft. I replaced that and the vibration is minimal. I also replaced the rear motor mount. I guess I will trial and error with washers on the clutch assembly to see if I can smooth things out. The drive shaft has new u-joints and has been balanced. If I can be persistent I will get the problem   

[Jay Friedman]

In balancing the clutch you could also consider buying a set of 6 special "clutch bolts" which are manufactured so that they all weigh exactly the same.  Your '41 should have a Long Manufacturing Co. Clutch and Autozone sells the correct 5/16-18 x 1 inch bolts for it:

http://www.autozone.com/drivetrain-and-differentials/clutch-pressure-plate-bolt

Here is another set on ebay.  (Don't be put off by the fact that this set of bolts is for Ford, since Ford cars also used Long clutches.)  http://www.ebay.com/itm/Dorman-HPX-710-122-Pressure-Plate-Clutch-Cover-Bolts-Ford-Long-Style-5-16-18x1-/381567228497?hash=item58d72dda51:g:LhwAAOSwP~tW4yDq&vxp=mtr