[texasprix]

I have a '64 GP with original Slim Jim trans, driveshaft, rear end and 8 lugs.
At about 65 mph their is a pronounce shake to the driveline. The steering wheel does not shake but I can feels vibration when I put my hand on trans tunnel. If I accelerate to about 75 and beyond it almost goes away. Does it sound like driveshaft or wheels? Tires are new and balanced. Two of the wheels were badly bent so I had them straightened and that improved to shake a lot, but not gone. Any advice is appreciated. Thanks.

[AC445]

Hello,if you had regular wheels they should be "roadforce" balanced.Most large tire shops and G.M. dealers have "roadforce" balancers.The problem is the 8 lugs need an adapter to fit electronic balancers. Maybe you can find one to borrow in your area. You could run up to highway speed with the rear end on jack stands ,then remove the rear tires and install the lug nuts and run it again. This will help determine where the problem is. Good Luck

[Stuart]

You could swap wheels from front to rear to see if the vibration goes with the wheels. Another thing to try is putting it in neutral at 65mph and seeing if the vibration changes.

[66 Wagon]

I've had the same experience you describe, at the same speed, on my '63 Grand Prix. In my case it was the drive shaft,
but I have had it shortened due to a swap to a TH400.
It could be the wheels too, I once lost a balance weight on the right rear wheel and it had similar vibrations but in lower speeds

[Keith Seymore]

The driveshaft in the '63/'64 was pretty crappy. It was a two piece construction with a "damper" - a rubber biscuit - molded between the pieces. Knowing how these were made makes it highly suspect.

Additionally - normally a driveline does not have enough mass to upset the vehicle increasingly as vehicle speed increases. It relys on a "resonance", resulting in a characteristic like you describe: it increases to a point and then gets better with increasing speed.

Tires, on the other hand, have enough mass that typically once they start they just keep getting worse the faster you go.

These two facts would lead me to start by looking that driveshaft over very carefully.

K

[Deadhead]

Look at the joint in the DS. If you see cracks in the rubber, that may provide further evidence of shaft being the culprit.

[texasprix]

All good ideas. I will start with swapping tires front to rear and see if there in any change. I will post results.
If it is the driveshaft, do I replace with a custom solid shaft? I understand the rubber "damper" on the stock shaft because the Slim Jim lacked a torque converter to dampen the driveline.

[Deadhead]

Yes. Custom shifts can be made for under $200. A few old threads have sources. Mine for my '63 came from a place in FL.

[cheese country GP]

Check your brake shoe adjustment. I had the same thing on my '62 GP & it turned out to be the brakes were adjusted too tight. Usually spin the wheels till they drag slightly & then back off a couple clicks. If you have 8 lugs, back off 10-12 clicks & that should do it. I know, seems like a lot but it did the trick for me. The pedal still only goes down about half way so plenty of brake left . I did the wheel straightening thing, rebalanced all the wheels, & then had the driveshaft checked for proper balance. Everything was just fine. Did the brake adjustment the way I've described above & the problem was solved. 65+ & smooth ride. Good luck

[pfilean]

Moving tires around and the other suggestions may help isolate the problem. You have to try something to know what to fix. And the rubber isolation in the drive shaft could go bad. If at some time there was a leaking transmission seal the soaked the shaft in oil that could have deteriorated the rubber some. But the symptoms you described used to be diagnosed as bad U-joints. Especially the front one. Something else to check.

[Keith Seymore]

Originally Posted by texasprix

If it is the driveshaft, do I replace with a custom solid shaft?

Yes.

I was in the garage yesterday so I snapped a pic of that production Slim Jim shaft.

K

[texasprix]

Had driveshaft balanced. It was way out of balance. Reinstalled and the vibration is gone. Easy $80 fix. Thanks everyone.

[Engine-Ear]

So, the rubber-lined D/S is still in play, just re-balanced?

Care to share the name of the shop who did it (for others' reference)?

[texasprix]

Original driveshaft was still in good condition so I had it balanced. The shop I used was great. They knew all about the Pontiac shafts and did not try to just sell a replacement shaft.
Also needed to replace one of the u-joints. I cannot believe how much smoother the ride is now. The shop is on Hurst, Tx, between Dallas and Ft. Worth.
Automotive Drivelne Supply
808 W Hurst Blvd, Hurst, TX 76053
(817) 284-8187