MGB Register Forum

My turn to ask for some of your experiences. My '72 BGT has it's original axle and has been making a drumming sound at 70mph (or so) on the motorways for some time now. Nothing much can be heard below 70ish, but always there above that. Sounds like a wheel bearing I think (from past experience) so I have removed and checked both outer bearings (at the hub end of the driveshafts), they seemed fine with no play/no roughness etc. so they were replaced.
I've also replaced the thrust washers in the sun/planet wheels (once recently, and once many years ago). I've also replaced the sun/planet wheels with a good s/h set.
None of this has made any difference to the drumming/throbbing at 70mph!

I am of course making a bit of an assumption that it is the rear axle. It could be coming from elsewhere but everything else has been checked, and it does seem to be coming from the back of the car.My engine is mildly tuned and the engine mounts fitted after the engine rebuild seem very firm/hard, so it may possibly be an engine resonance - sound plausible?

So my questions for you are:

# how quiet are your Bs/BGTs at 70mph, do you have any rhythmic drumming sounds (not counting anything from your loudspeakers)?
# does it sound like an urgent job to fix, or do these axles really go on forever as commonly believed?

Informed replies welcome,

Ian F.

Very difficult thing to find Ian. It could be anything that at that speed sets up a frequency and vibrates. Body panels, propshaft, engine and gearbox mounts exhaust, and many other items and the sound can travel from its original source. My BGT thrums at higher speeds which is the exhaust note.
The diffs can go on for years and give plenty of warning usually a high pitched whine. If it's not too annoying can you live with it?
. Vic Butler

Thanks Vic. A thrum would be a good description of the noise I get from my car, so possibly a similar thing. Certainly not a whine of any kind.

The B has been doing this for quite a while so I am used to it, my only real concern is having a failure at high speed. Not sure what the consequences might be if the diff failed or how likely that would be!

Ian F

Ian, if it's safe to do try switching the engine off when the noise occurs and coast in neutral and see if the noise persists.
. Vic Butler

Hi Ian,
I hadn't responded earlier as I have not experienced the sort of noise you have. I've had two of the later axles fail, on an MGC GT many years ago, and more recently on my MGB. Initially in both cases I was aware of a severe vibration through the car before any noise occurred. In the recent occasion it quickly became so bad that the mirrors were vibration so much I couldn't see in them. I swapped the axle for good second hand one, on taking the back plate off the diff shims are very badly worn so I've out that to one side to do sometime in the future.
It is difficult to describe noises but it was more of a brrrrrr sound, and I'm more inclined towards Vic's opinion that a 'thrum' is more likely a bearing. Since you have replaced the diff shims recently perhaps the pinion bearing have been damaged and are now a bit noisy, of the diff carrier bearing are noisy.

Perhaps first you need to check if the 'Mini overhaul' you did recently is still ok. You can check the backlash on the diff/propshaft flange by jacking the car up until the wheels are off the ground and putting a mark on the pinion casing, and then turn the propshaft back in a clockwise direction until the wheels start to move. Make a mark on the propshaft flange. Turn the propshaft anti-clockwise until you feel the load increasing or the wheels just start to move. Make another mark on the propshaft flange and measure the distance between them. If it is 4.5mm the axle is unworn, upto 8mm the axle is in good usable condition, and over 10mm the axle needs an overhaul. Doing this will tell you if your mini overhaul to change the shims is still ok, in which case don't worry about it too much. If it isn't then you have to question why in not many miles, it is again out of tolerance in which case it might be wise to consider an overhaul or an exchange unit. You need special tools to stretch the casing to dismantle the final drive completely. When my axle was vibrating it had 25mm of movement! The car was very unpleasant to drive.

Thanks Peter. That sounds like a useful check which I will do when back home. If I understand correctly the check is to measure the "free" rotation of the prop shaft before it takes up the drive? I'm not sure of the function of the mark on the pinion casing? Two marks on the flange would seem to be all that is required?

Thanks again,

IanF

Thinking about this for a nanosecond - I realise the mark on the pinion casing is simply the datum against which the shaft is marked. this allows a more accurate measure of the free play.
Apologies for not seeing this earlier!

Ian F

Make sure you don't make the same mistake as me...........Horrible noisy diff on my old BGT rally car. Changed the whole axle for a good one and....... noise still there. Turned out to be a worn front (!) wheel bearing with the associated noise travelling down the tunnel to clearly 'appear' behind the seats.

Thanks Steve. Useful contribution! I have recently changed both front bearings (and checked both outer rear beatings), so am tempted just to drive it a bit longer and monitor any changes.

Ian F

Checked my diff as per Peter's instructions, and was pleased to find around 5mm backlash, so no problems there. Thanks for the advice Peter.

So, if it is the diff I suspect a bearing, but will continue to drive it and see how it goes!

Ian F