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MG MGA - U-Joint Question

I just finished my frame-up restoration and got the car back on the road. I now have a concern, however. The car has a (minor) vibration that I take to possibly be a drive shaft vibration. Fairly high frequency, no shimmy, just vibration, continues when the clutch pressed and the RPMs go to idle.

Inspecting the drive shaft, I have a concern about one of the u-joints. Both are new (which makes them suspect). The front u-joint is very firm with no slop in any direction at all. The rear has no “twist” slop like you would get from bad or missing needle bearings. The rear u-joint will, however, slide back and forth within the bearing caps. I was able to tap the bearing cap a little over 90 thousandths of an inch into the yoke and away from the circlip to tighten. This occurs on both axis of the u-joint. Is this normal/acceptable? How much tolerance should there be?

One more note. I damaged one of the u-joints during assembly (lost a needle) and had to replace it. So there is a chance that they are not identical.
F. J. Bruns

Why dont you jack up the rear wheels (Safely) and run the car in gear to see if there is any obvious out of centre movement on the driveshaft?
If you rig up a pointer next to the driveshaft, you should soon see if there is a problem there.

Colyn
c firth

Colyn - It may come to that, but I'm a little timid about crawling under the car while it is on jack stands and the car is running, but it may come to that. There isn't visibility of the drive shaft inside the tunnel without getting right under it and up close.
F. J. Bruns

Just yank the driveshaft sideways (engine off) There should be no play at all.
Art Pearse

A driveline shop can check this for you, it's what they do. Their balance machine will get you set up properly. Until then, the joint should have zero play anywhere. Of course, any side movement will cause the shaft to spin off-axis and vibrate due to imbalance.

Short of taking it out and having it properly checked, you're guessing.
MAndrus

Without thinking too much, I had assumed that the drive shaft was balanced without the U-joints, still leaving me with the concern about them. Realizing that they are balanced with the shaft, balancing the drive line is a no-brainer. Removed last night and heading to the driveline shop this morning.
F. J. Bruns

FJ,
I'm a bit confused as you say that vibration "continues when the clutch pressed and the RPMs go to idle".

In this state, are you overunning, i.e. the rear wheels are turning the prop shaft, or are you parked and thus the prop shaft is not turning?

regards
Colin
Colin Manley

I had a similar problem with my B. I fitted new UJs only to find that one had some endfloat.

The supplier eventually agreed to supply some replacements, however I was a little reluctant to fit them and bought some from a propshaft specialist.

They were a lot more expensive, but they seemed to be well made.

I also discovered that one of the eyes in the rear flange was oversize, so had to replace the flange.
Dave O'Neill 2

FJB, check that the U-joint axes are parallel front to back.
Art Pearse

Just an update - Colin, Driving the car at about 65 MPH and depressing the clutch. The prop is still rotating at speed (just driven from the other end)axes are. Art, yes the axes are parallel.

The driveline shop called and said that both u-joints are WAY too loose. He doesn't know if they are the incorrect parts or just cheap and poorly sized. According to Barney's site, this part has the correct dimensions. (NOW I know ;-(

I will measure the removed parts when we swap them out.
F. J. Bruns

Sorry, I meant that the parts the shop is ordering are the correct dimension.
F. J. Bruns

Just to close out this tread, I have an answer to the original question. According to the driveline specialists, there should be NO lateral or end-to-end play with the U-Joints. The tolerances are very tight. Barney has an extensive cross reference list for compatible U-joints for our (and many more) cars. http://mgaguru.com/mgtech/propshaft/ps103.htm Some of the numbers are old, but the most important information are the correct dimensions. The diameter of the cap should be 0.938” and the width across the caps should be 2.411” to 2.412”. The parts I received from Scarborough Faire as appropriate for the MGA had the correct cap diameter. The width across the caps, however, was 2.350”, about 62 thousandths too short. I got these about 2 ˝ years ago, so I don’t know what is in the pipeline right now. The U-Joints I have now are Spicer Series 1100 5101-X.
F. J. Bruns

This thread was discussed between 10/11/2015 and 19/11/2015

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