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MG MGB Technical - advice on prop shaft

Deall All,
I have been noticing a noise from the rear end, putting it down to a wheel bearing, but would like some advice in case its something else. Its a 73BGT, daily drive, kept in good order. The noise is as follows - below 30mph, not there, above 30mph, a sort of shushushushushu that increased and decreases with road speed. What concerned me about the prop was that with the car stationary, switched off, in 1st gear, I noted that when i put my hand on the prop and try to turn it, I am getting about say 20degrees rotation. Yet upon take up when driving away, there is no clunk, no suddent jerks, and likewise it drives fine without any untoward jerks or clunks. Is this normal for a prop shaft to be able to rotate as much as this - I should add that the diff is full of clean oil (correct EP90), and when drained, there was no 'bits' in the old oil. Maybe its just wheel bearing and I am getting paranoid, but like to be sure andd safe.
Thanks in advance for any assistance you can give me, its much appreciated.

Your noise sounds more like dragging brake shoes. 20 degrees sounds very bad. When you turn the prop shaft watch the mating flanges to see if they are moving with the shaft, if they don't the U joints are bad. If the U joints are good, this much slop in the trans and rear end is a lot. It would be strange for the trans and diff to both get real sloppy at the same time. Pull the prop shaft and check the U joints by rotating them fully to their limits. It can be very hard to actually tell if there bad when still installed in the car.
John H

These types of noises are hard to diagnose - even if you are driving the car.

Some other things to check:

Does the noise increase under acceleration or deceleration? During a left or right turn? Is it there while coasting in neutral? In all gears? Cold or warm? Does the noise change if you lightly touch the brake? While braking? Any change if you exchange front and rear tires?

With the rear wheels off the ground (car safely on jack stands), can you hear the noise with the engine running and rear wheels turning at various speeds?

Wayne Pearson


the U-joints on both of my roadsters (1973 and 1975) lasted for about 60 Mls until i had to renew them.
If you can hold the flange at the gearbox with one hand and try to spin the shaft with the other hand, there shold not be any free play. The same test must be made at the rear flange too.

If you can move the center part of the prop shaft with the flanges held in location, new U-joints are a must.

If there will be other noises after this repair ('noises' on MG-B's are most common to all of us), you will find out the reasons soon.

Hope this helps



Thanks for all of the replies - here is some more info in case it helps:

noise is unchanged by accelaration or decellaration.
unchanged by braking, or cornering. Brakes dragging - thought of this, serviced the rear brakes, no change. experimentally de-adjusted rear brakes so that they were definitely not dragging, no change. Coasting - no change. 4 new tyres, no change (needed them, had hoped it might make a difference, no luck).

I will get it up in the air at the weekend and have a proper look at the propshaft - its an unknown quantity to me as regards previous care, so will be good to get a proper look at it in a well lit place.

thanks again


Here are a couple of thoughts if you end up renewing the u-joints (sorry if they are obvious to you!). The yokes on the prop shaft and sliding end have one set of shallow relief radii so the grease zerks (nipples I think to you) in the spiders clear the yokes. Find them before you set the new u-joints in place and make sure your spiders are in the correct position and facing the correct directions. The MGB propshaft runs pretty straight without a lot of deflection, so don't be concerened if after assembly the grease zerks (nipples) don't allow much flexing. They will be fine on the car if you have prositioned the spiders correctly.

Secondly, the yokes and flanges should be in the same planes end to end, which means the front end with the sliding splines should be reassembled so everything is squared up with the rear set.

Finally, a lot of folks don't lubricate the u-joints or the sliding splines fully after assembly. I thought I did, but decided to take my assembly in for balancing after I finished it. The balancing folks were kind enough to do a great job AND properly educate me on the needed amount of grease in the splines!

take care,
Dave Braun

nice one Dave, thanks for the tips - this probably wouldn't have been obvious to me, so its much appreciated.
(am now caked in muck, old grease, underseal, oil etc, the sun is shining, and all is well with the world, if not my hardy-spicers!)

The kind of noise you describe sounds very much likes brakes dragging- but it doesn't have to be the rear brakes. Have you checked to see whether the callipers are releasing fully?
Miles Banister

You say unaffected by braking - is this with handbrake as well as footbrake?

The drums could be rubbing on the back-plates. Should show on the drums and back-plates with the drums removed. This quite likely won't be affected by braking - light braking anyway - and if turning the wheel by hand when jacked up it may no longer rub, same as with the fronts.

Another test is to support the axle on stands, chock the fronts or make sure you have plenty of run-off(!) and run the engine in gear and see if you can hear the noise by each rear wheel. Unless it is very securely supported, i.e. more securely than even the axle on axle stands, I wouldn't get underneath the car with the engine running and wheels turning to listen to the diff unless you have a long 'listening stick' like a broom-handle, and even then make sure it is in a forward gear.
Paul Hunt 2

BTW, there are two types of EP90 - one labelled as gear oil (e.g. for V8 gearbox) to GL4, Hypoy, and the other labelled as diff oil to GL5, Hypoid (Hypoy B)which is for the diff and steering rack. Castrol for example EP90 is gear oil, its diff oil is EPX90.
Paul Hunt 2

Mick - Just an added thought, I had what I thought was a rear bearing noise in our MGB after changing over to disk wheels from wires, complete with a used rear axle. After much searching and not finding anything wrong with the rear axle bearings, I finally figured it was just some rear axle noise that wasn't hurting anything and put up with it. On my last oil change, when replacing the right front wheel (removed to gain access to the sump), I noticed that the outer bearing in the hub was worn out and replaced it. You guessed it - the noise that I though was coming from the rear of the car, was in fact that front bearing. Noises have a funny way of telegraphing through a car to lead the owner astray. Cheers - Dave
David DuBois

I was thinking as I was reading ths thread that this noise could be telegraphing from somewhere else and I see Dave from Brenerton thought the same way. While you are checking "up front" look also for loose mud covers and such that may be forcing wind into an area in normally doesn't go
Peter Murray

Dear All,
many thanks for all of your suggestions. Got car up on ramp, found that the 20 degree rotation on the prop was the whole prop, icluding the universals, so would indicate a combination of a bit of slack in the diff presumably - in the light of day its not as much as 20 degree anyway, and as no clunks on take up, I am going to leave alone for time being - would you agree this is the thing to do?

As regards the noise i heard, upon a careful check out, it is indeed wheel bearings. What was throwing me is that both sides appear to have worn at the same rate, hence my inablility to say which side the noise was coming from, hence my concern that it couldnt be a wheel bearing. Have changed both sides, all is now quiet again. relatively speaking.
Anyway, thanks all once again, this is place is like having an older, more experienced brother hanging over your shoulder, and i appreciate it.
Mick Rae

Wheel bearings *should* wear at the same rate :o) However experience is usually that one will tap you on the shoulder before the other. Had the same thing on my Celica recently - all the evidence pointed to the off-side, so we changed that, and whilst it was different it didn't completely cure it. Then further testing revealed the near-side had 'feelable' roughness although the noise still seemed to be coming from the off-side. Changed that and all was quiet again. Then a ZS180 came my way and I sold the Celica.

I agree that prop-shaft play, as it is not in the UJs, can be left for the time being. It's one of those thinks that can make a noise until 'Jesus comes again' (as John Steinbeck wrote).
Paul Hunt 2

This thread was discussed between 07/02/2008 and 15/02/2008

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