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MG Midget and Sprite Technical - Noise from differential?

Evening. I am experiencing a knocking type noise at low speeds (1st/2nd gear speeds) which seems to be coming from the area of the rear axle/differential. It goes away at higher speeds. Any suggestions please as to what the source of the problem might be? I topped up the differential with oil the other week so lack of oil shouldn't be the issue. Gearbox oil is fine. It is annoying more than anything else and I'm keen not to be damaging anything by driving it when it is like this. Thanks in anticipation.

Tim AH Sprite Mark 2 1961
TJ Prime

Thighten up the all hardware on the leaf springs

Let us know if that did the trick

btw... steel wheels or spoke ?

prop
prop

.
prop

Prop

Steel wheels.

Tim
TJ Prime

Knocks going from power-on to power of and vice versa, continues under constant acceleration, or when driving at steady speed?
Paul Walbran

Steel wheels just eliminated 50% of your searching ... congratz
prop

Prop shaft UJs?
Guyw

What Paul said and then I'd check the universal joints and the prop shaft attachment to the diff also.
Martin Washington

Knocks when idling or low speed but seems to disappear when at steady speed.

Tim
TJ Prime

"Low speed" could still cover a range of load scenarios, and different causes make a noise under different circumstances so without further info (have a look at my earlier post) it's very hard to be definite. However, two common causes of rear end noise are:

Propshaft UJs as noted above. When very flogged the yokes touch each other and clatter under acceleration. With somehwat less wear there is an audible click when changing load direction. Either way, this would be accompanied by a marked vibration at around 50-60mph.

Differential thrust washers - we do a heap of these in our workshop. Produced a lower pitched clonk when changing direction of load. Best to renew before they dissappear and the cage wears out too. Assess by jacking up one side at the back with the handbrake off (having taken measures to ensure the car doesn't roll away of course) putting it in top gear and rotating the wheel back and forth and measuring how much it can do so at the perimeter of the wheel. 15mm is good, 35mm means the thrusts have mostly gone and 50mm indicates likely cage wear.
Paul Walbran

yeah, I too have a loud clunk when you get on or off the power too quickly, especially in a low gear (I presume due to the power multiplication in lower gears). ujs are new so pretty sure it is diff thrust bearings. I have the bits and probably going to tackle this next week and will post pics when I do if it helps.

Malcolm
Malcolm Le Chevalier

If it knocks when idling and the car is not moving, it is unlikely to be the diff .... Exhaust pipe?

Bob Schapel
R L Schapel

Good point Bob!
Paul Walbran

Could be exhauste ... BUT

wrote this ....

Evening. I am experiencing a knocking type noise at low speeds (1st/2nd gear speeds) which seems to be coming from the area of the rear axle/differential. It goes away at higher speeds.

i still say tighten up your hardware on the rear suspenion leaf springs and lever shocks... it dosnt cost any thing and only takes about 10 minutes and cures this knocking sound 6 out of 10 times

like guy says... do the small easy stuff 1st before ripping into a rear axle assembly

prop
prop

Thanks all. Googling does seem to suggest that exhaust problems are louder/noisier when hot and this does tie in with the problem I have as the noise does seem louder when I've been driving for a while. I'll check out the exhaust fully and also the bolts to the leaf springs and take things from there. Tim
TJ Prime

A wee update. Bolts to leaf spring were needing to be tightened up on one side of the car but I've narrowed the problem down to the exhaust. Seems to be a very small on the long straight exhaust pipe near the engine bay and I can only assume that this is causing the excessive vibration, especially at low speeds. Next step is to see whether full exhaust replacement is needed. Thanks everyone. Tim
TJ Prime

A very small 'what' Tim?
Do you have the bracing strap fited which goes between the exhaust pipe and the bracket at the bottom of the engine backplate?
David Smith

Next step is to see whether full exhaust replacement is needed. Thanks everyone. Tim


WOW... That was fast ! It just smacked me upside the head and was gone before i could ever ask... what happened ?

Soooo how did we get to needing a completely new exhaust system ?


You tightened the hardware on the suspention which im sure cured the popping as this is a fairly common problem then your blaming the exhaust for something on a long pipe in the engine bay and then WHAMO you need a complete new exhaust system

what did i miss ... im lost

prop
prop

David. Sorry - a small hole is what I meant to say as I can see the exhaust gases leaking out. There is a centre mounting clip/strap but none at the front of the pipe. As far as I know there never has been one during my 13 year ownership. There is quite a bit of vibration at that point so inserting a strap there might prove helpful. Don't worry Prop, I'm not going to replace the exhaust unless it is absolutely necessary! Whilst the tightening of the leaf spring holders was needed it has not removed the knocking/clattering noise - the exhaust is definitely the source of this racket. Tim
TJ Prime

Paul.
My diff has just started to exhibit the lower pitched clonk you describe, and the play at the wheel rim is about 40mm on both sides.
I'm about to embark on a trip to Le Man's Classic on 4th July, plus touring the Loir valley afterwards. A possible 1500 mile trip.
I have a spare 3.9 diff and shafts, (wire wheels, mine is a 4.2), so, do I fit the spare or risk it? I think it's a no brainer, but I would just like confirmation from someone with more experience of how quickly it's likely to deteriorate.

Apologies to Tim for hijacking your thread.
Bernie Higginson

This close to the 4th ... id leave it

If its been a long while scence the fluid was changed id leave it also sometimes clean fluid can remove grim in the bearing and gears thats actting as gap filler wnd with it removed will leave the tolerances wider open

if the diff is really bad say more then 70% then id be tempted to remove bt id do it fast and then drive it every day for 100 at a time just to make sure you get allthe grimlins killed off

ive been driving my car almost everyday for over a month and im still getting attacked about once a week from grimlins ...luckly there getting smaller and spread out longer between mishaps

but id just be leary of such a big project like that so close to your departure date without alot of proving time and miles

prop
prop

Well. I fitted the spare diff, which turned out to be a 4.2 not a 3.9. and I'm glad to say it seems to be OK. Clunk has gone. Drives nice and smooth. I only did a short test run today, and I didn't get it hot, so I don't know if it whines or not. Now I'll have to rebuild my original diff because the spare that I've fitted belongs to my project midget.
Bernie Higginson

Sounds like a plan

are you going to be in the mountianous part of france... that 4.2 will work out well
prop

Bernie, if the diff doesn't whine when cold any whine when hot should be minor.
Paul Walbran

This thread was discussed between 18/06/2016 and 25/06/2016

MG Midget and Sprite Technical index

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