Drive train vibs

[Ford Blue blood]

Good friends 67 Fairlane Ranchero has a vibration starting about 68 - 70 MPH.  It has a 5.0 roller, C-4 and a stock 8".  Car has been fine for years with the drive train in it now.  The vibration started about a year ago.  No work had been performed on the drive train other then routine maintenance. 

He has gone through the rear with a fine toothed comb.  Pumpkin replaced with a known good piece, both axles checked for true, drums are true, wheels and tires off a 67 500 put on with the same results.  Drive shaft and u-joints have been checked and found true.  Steering wheel is rock solid, vibrations are felt in the seat. The only thing left is the transmission output shaft bushing and the flex plate.  Might be an internal transmission failure?

Looking for suggestions/ideas of what you guys might think.

[alvin stadel]

I doubt  that any of you remember this but about 3 years ago I had a bad vibration in my 65 Falcon, 289, 4 speed. Like you I checked everything. It almost caused me to go crazy.  In desperation I changed the rubber transmission mount on the cross member and the problem went away.  Vibrations can drive you  nuts.

[rmk57]

I've heard of harmonic balancer's slipping and creating vibrations. Never seen one that has happened to but it should be easy enough to check out.

[hiball3985]

I had a similar problem and it was the tail shaft bushing on my T-85. Could be the mount also as was suggested, I would change that first..

[Fairlane62]

Does it have the two piece driveshaft with the rubber insulator sleeve between an inner and outer steel tube?  I have seen these slip in the rubber, especially with added horsepower, and cause a vibration.

James

[lowrider]

Transmission tailshaft bushing is a good thing to ck. Also you might want to ck the driveline angle. I had a 57 that developed a vibration after I installed a 5sp trans in it. New trans thru the driveline angle off. Small adjustment to the rear axle housing fixed it.

[djfordmanjack]

I was troubleshooting crazy steering vibrations @ 65mph on a Mustang II frontend last year. after replacing all rubber bushings it's all smooth again. None of the bushings were really worn or torn. just hardened from age. who would have thought this could cause such strange vibrations. your friend may as well replace the engine mounts along with the trans mount.

It is interesting to note, that according to my calculation this was a speed where certain engine rpms and rear axle ratio resonated exactly with the circumference/rpms of the front tires. they had gotten out of round from being parked for an extended period. I also shaped the tires and had them pro balanced. I also found that one of the front discs ( the side that had more wobble) was extremely out of balance. the hub cast was offset a lot to the machined surfaces. replaced that. I still have to put wedge shims under the rear axle mounts, to properly realign u joint angle front and rear.

[Ford Blue blood]

Thanks for the tips guys.  I have passed to him your thoughts.  He did admit he failed to check for any "wiggle" in the output shaft of the transmission.  He also stated that the transmission mount was at an angle, like the engine was tilted?  Have not looked at that myself yet but it is a good place to start.  Motor mount failure?  I'll let you know if/when things get checked out and the results.

[glr7533]

He might want to check the fan. There could be a crack starting. I was at a car show a few years ago and there was a car there with a gash in the hood from a fan blade that broke. He had developed a vibration he couldn't find until it let loose.

[Ford Blue blood]

Update....drive shaft is one piece, no "wiggle" on the output shaft.  Transmission mount is new.  Got a "small" bit of info from him yesterday, it would have been nice to have had this a week ago.  The transmission mount flange on the C-4 is at an angle.  Told him to check the cross member, it is level.  So check your motor mounts, that flange on the transmission should be level if all else is square!  Waiting on an update from him.......

[Ford Blue blood]

Final.....bad axle bearing.