The MGA With An Attitude
THE BAD U-JOINT, What It Looks Like Inside -- PS-102A

On 5/12/2015, Gertjan Yntema wrote:
"I was driving a week ago when suddenly metallic noises could be heard every now and then from the rear differential, but could not pinpoint the exact source. On the internet people immediately conclude it MUST be the gearbox or the differential. I also noticed once that a vibration could be felt in the steering wheel. I took the propshaft out of the car (easy) to try to turn the differential by hand and found that the front coupling was only able to move a tiny bit, no play in turning direction by the way what everyone suggests testing first.
Failed universal joint Failed universal joint Failed universal joint
Failed universal joint Failed universal joint Failed universal joint
"The front U-joint of the propshaft (one axis) could barely move (but still a bit). Two bearings were dry as what (probably as old as the car?) and the needles came out broken, so was the collar of the bearings. This must explain the metallic sounds. You can also see the needles having embedded in the bearing cages. I could have sworn that the noise came out of the differential! Probably the hollow propshaft tube acts as a soundboard, in combination with the battery cover being less insulated compared to the tunnel, makes it sound like it comes from behind".

Universal joints are a fairly common failure in older cars. You may notice this one has no grease fitting. This is called "lubricated for life". What that means is, when the lubrication fails it is end of life for the U-joint, because you cannot grease it. The yoke or frame of the U-joint has a relief notch at one side where a grease fitting should be located. When replacing the u-joint you need to get the orientation right to position the grease fitting in line with the relief notch in the yoke (so you can get the grease gun fitting onto it later).

Symptom of failure of the propshaft U-joint is vibration at road speed. For a rear U-joint failure you may feel vibration in the seat as it tends to shake the rear axle assembly. For front U-joint failure you would feel the vibration on the gearshift lever as it tends to shake the gearbox. Vibration of the gearshift lever my also result from failure of the tail bearing in the gearbox.

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