I had a similar vibration as John M mentioned but with the differential pinion. Power on = no vibration, power off = no vibration, but cruising it would vibrate.

My experience with driveshaft problems is limited but I have found a couple of things. First, if I put the vehicle in reverse and back up under load and hear a "ping", I know there is a bad U-joint. they seem to bind up in a forward direction and applying power in reverse breaks them loose. At this point if the u-joint is greased it will fail immediately. Done this one twice.As Ted mentioned, the bushing in the tailshaft causes many vibration problems. These can be identified by driving 60-70 MPH and varying the throttle. A bad bushing will not vibrate under load but when you let off the throttle it begins to vibrate.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

Chuck, I have done the same thing, rotated the drive shaft 180° and reinstalled. Vibration gone. For the life of me I can't explain it or anyone else who has done it can. Usually one of the first thing we try. Just one of those FM's, I guess.

I do know the trick has been around awhile because some old Dudes (older than me... ) told me about it!!


Edit; Off topic, I just mistakenly found out if you type : ) without the space between them you get


"The Master Cylinder"

Enjoying life at the beach in SOCAL

I think the phazing of the drive shaft joints is a different, although related, subject. I do know that I have had two experiences with vibration in the drive shaft that was reduced by rotating the drive shaft at the rear end by 180 degrees. The people that I have had build shafts, a very old company, are the ones that first told me to try this. It helped. Chuck

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Garry, I don't think rotating the driveshaft 180 Deg would have changed anything. This is a pretty complex subject and I think I may have miscommunicated by implying that the two U-Joints should be 180 Deg out of phase. In fact, they should be in-phase. In-phase would be established by rotating the transmission and rear end yokes so that they are parallel with each other (while the driveshaft is out of the car). For example, positioning the front and rear yokes so that they are both in the vertical plane and then installing the driveshaft (without rotating either yoke) would achieve the "In-Phase" condition. Also, it is very important that the front and rear operating angels are near equal. Here is a link I found on the subject: http://www.hotrodhotline.com/md/html/drive_shaft_harmonics.php

Rotated the driveshaft 180 degrees, no change, 65 70, put it in neutral, still vibrates. The drive shaft is relatively new, built for the tranny conversion. Before I got my suspension adjusted the driveshaft hit the floorboard a couple times, no dent in anything, just paint rubbed off? All fixed on that end. How much up and down or side to side play is allowable in the pinion yoke and tranny tail shaft? Tomorrow it gets put away for the winter, then I'll check tires for roundness, etc. Does the fact that its so smooth at 55 rule out anything? Thanks for all the suggestions, Gary

       

Gary, 55 Vicky, St. Germain, Wisconsin

 

Allow me to try again. When the two U-joints are clocked properly (phased 180 Deg), for a given 1/2 revolution, one is increasing in RPM and the other is decreasing in RPM. Thus the vibration is cancelled out within the drive shaft tube. This is happening every 1/2 revolution, which results in a 2 x drive shaft RPM vibration. If the two U-joints are in phase with each other, the cyclic RPM is additive and is transferred into the rear end and the back of the transmission.

Was the driveshaft modified for that three speed conversion? If so, then insure that the U-Joints were clocked or aligned with each other when any welding was performed. The balance of the shaft is yet another consideration.

Lorena, Texas (South of Waco)

I understood what you were saying. Its a different question. For example, I had a new drive shaft made for my Dodge van. When I first drove it it had a cyclic vibration almost all the time. I went back to the shop and they checked the shaft. The balance was dead on so the guy put it back in rotated 180 degrees from the previous installation. He had marked the carrier and shaft when we took them out. The vibration was gone! Thats what I was wondering about. Apparently this is common enough that the shop was familiar with it but couldn't explain why. Chuck

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Sorry, I should have worded it more clearly. The two U-joints being rotated 180 Deg from each other is what I was referring to when I said, "For a drive shaft with a U-joint on each end and out-of-phase with each other, this rotational unbalance will be absorbed by torsional deflection in the drive shaft tube."