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MG Midget and Sprite Technical - Knock knock.. at the back

Not a knock, knock joke unfortunately!

I have a knock coming from somewhere around the back axle which coincides with 1 rev per wheel. It can only be heard clearly at very low speed as it soon becomes lost in exhaust noise.
I've jacked the car up and tunred the wheels.. nothing
I've run the car on axle stands both wheels off the ground and one wheel off the ground. Nothing.
On the road it does it under power and on overrun and doesn't change if I apply the brakes.
I have wire wheels if that is relevent and the bearings and diff have been rebuilt/replaced about a year ago.
I don't think it's the diff as I've been told that nothing in the diff turns at the same speed as the road wheels at leat not in the way that a damaged tooth would knock at that rate.

So either I pull half shafts, bearing and drop the diff or I could leave it until it a) goes of it's own accord b) gets much worse so it is easy to find, or c) falls off.
Graeme W

The suspension is loose on the leaf springs... give it a good tighten and your good to go

Prop
Prop and the Blackhole Midget

I meant to say,

leaf springs hardware is loose on the diff mounting location

But sometimes can be loose on the leaf springs attachment locations

Esp where new rubber ((not poly)) is used as it takes a bit of time for it to shrink and begin the parishing process

Prop
Prop and the Blackhole Midget

Is it possible that an out of balance tire is causing a loose damper bolt to make a pop sound?
Trevor Jessie

Prop: would that result in "once per wheel rev" knock?
Trevor: my wheels aren't balanced so I can't have lost a weight. However, I'll tighten up all around and see whether that stops it.

The knock is very regular and quite sharp and "hard" sounding.
Graeme W

What rear dampers are you using ?

I a nice clear clean knock because my LA dampers were not tightened up.
Malc Gilliver

Graeme,

Some things in the diff have to turn at the same speed as the wheels since the half shafts are stuffed in the diff and the wheels are connected to the half shafts.

To clarify, are you saying you get one knock per one wheel revolution?

Charley
C R Huff

Charley: my original thought was damage to the pinion that the half shaft plugs into. One rev per wheel rev! I have had it explained to me that it doesn't work like that and that those pinions only rotate relatively when the rotational speed between the two wheels differs. THe explanation came from someone well respected in the field of diffs and gearboxes.
But yes, one knock per wheel rev.
Graeme W

One knock per rev - you state no wheel weights, so the only thing left outside of the axle is the rear drum - maybe rear bearings allowing the drum to touch the backplate - wouldn't show on stands, but would with weight on ?
Malc Gilliver

This comes up from time to time and I was shocked how loose mine was when it happened to me,

But id bet good money that its loose hardware... like you said it only does it when the car is moving, never stationery or with the wheels up,

The good thing is, it only takes a few minutes to fix.. try it 1st, then start taking stuff apart if its not the case.

Prop
Prop and the Blackhole Midget

wire wheels and their hubs are always more hassle and potential for problems than steel wheels and hubs but I wonder about the rear UJ being a bit loose or worn or needing greasing but I can't remember what they sound like
Nigel Atkins

I would suspect something just catching inside the brake drum. I am not familiar with the wire wheel hubs but l have had a similar occurrence caused by the domed head of a wheel stud at the inner face of the hub flange. It just clipped the brake cylinder as it went round. But for some reason it didnt catch when there was no weight on the wheel so it took some finding.
Guy Weller

Nigel: thanks for your (and everyone elses suggestions). I have ruled out anything upstream of the diff because that all turns around 4 times faster and I'm sure the knock is too slow for that.
Today I shall be mainly laying on my back with a spanner or two!
Graeme W

Surely the crown wheel rotates at wheel speed (provided you are steering in a straight line) Thats how the diff ratio is calculated (no. of crown wheel teeth / no. of pinion teeth)

Back in the late '70's I had an escort that knocked just as you described, and it gradually got worse where it was 'grabbing' once every wheel rev when rolling slowly. Turned out it was one of the crown wheel bolts coming loose.

Jim
J Smith

Jim: although the pinion gear can't be suspect (knock would be 4x faster), as you say, the crown wheel would appear to rotate at wheel speed. A damaged tooth on that would have that effect as would, I guess, a loose bolt.
Could be diff out again! I wouldn't want to risk a bolt dropping out.
Graeme W

Unfortunately, since the noise only occurs when the wheels are under load, that it is a knock noise that occurs with each revolution of the wheels, tends to eliminate drive shaft (U joints), suspension or brake drums, or worn wire wheel splines, because they should knock under different conditions (accel/decel, in the case of loose spring hardware or WW splines, or would still knock even in the air, in the case of a drum dragging on something).

It does begin to seem like the ring gear (crown wheel) has a bad tooth, sadly.

Norm
Norm Kerr

"Sadly" is an apt comment!
I was rather hoping that it wasn't the diff but I rather supected it may be.
I wonder if I pull the half shafts, disconnect the uj I might feel it as I turn the input shaft, but then again if I do, it's diff out and if I don't it's still diff out, just to check.
Such a b**g*r to do laying on the floor under the axle casing.
Graeme W

just in case it becomes relevant to someone that reads (or originated) this thread here's a post I put in another thread earlier today

and comments there –

Nick Nakorn, Devon, United Kingdom
>>I think the recon units are a bargain (I got mine from Moss) because the setup procedure requires a lot of work and one or too workshop items I don't have.<<

Rob aka MG Moneypit, Cheshire, United Kingdom
>>Only £60 surcharge on the 3.7 diff. It would be worth putting on eBay rather than re-claim the surcharge with a returned diff.

Rob<<

my earlier post -

I recently saw that MGOC Spares is doing 3.7:1 crown wheel and pinion or in recon diff

"3.7:1 midget CROWN WHEEL AND PINION
Previously unavailable MGOC Spares has commissioned new 3.7:1 crown wheel & pinions (BTA535) manufactured to the original design and specified for use on Midget 1500 models. Owners of earlier MG Midgets & AH Sprites can benefit from upgrading to a 3.7:1 rear axle ratio, which will lower their car’s cruising RPM by up to 10%, improve fuel economy and reduce cabin noise particularly when keeping up with traffic on the motorway. New 3.7:1 Crown Wheel & Pinions are priced at £249.95, alternatively MGOC Spares can supply reconditioned Midget differentials built with the 3.7:1 ratio priced
at £398.25 + P&P (exchange surcharge applies)."

http://www.mgocspares.co.uk/pdfs/hotline.pdf
Nigel Atkins

THis particualr diff I bought second hand and had it checked over and the bearings replaced about 18 months ago. It's a decision whether to get the c/w and pin swapped or go for a complete refurb unit. This is a 3.9, which is what I would stick with for this car.
Graeme W

Graeme,

Wire wheels can make some odd sounds when they have spoke problems, and producing the noise would require that the weight of the car be on them. Though the wheels may not be a likely cause, I would swap the them front to rear to eliminate this as the cause before pulling the diff apart.

Charley
C R Huff

Well, the diff is out!
I filtered the oil and it seemed to have very small metallic particles in it but no lumps like the odd tooth or too.
I have attached a photo of one area of the crown wheel which looks dubious. About 5 teeth are showing a different surface finish compared with the rest of the wheel. No lumps missing but it looks as though the surface has been wearing compared to the remainder. There is also an interesting burr-like edge to the inner end of the root of the teeth.
No clonks or anything suspicious when I turn it.


Graeme W

and another

(The images are in the reverse order because I tried to upload two together.)

Could this be indicative of the cause of the knock? It's almost as though the crown wheel has a distortion and is running too close to the pinion at that point.


Graeme W

Graeme,

Im sorry my suggestion wasnt the cure, I really thought it was...I imagine you have given all those nuts and bolts a good kick by now and all were stiff

Some days its just going to be pain and no essy gold stars are going to be awarded for an easy victory

Hang in there

Prop
Prop and the Blackhole Midget

Those marks are odd, I wonder if, after the bearings were done, the CW&P were correctly assembled. But even if there's odd wear due to misalignment I can't see it would produce the knock you've experienced - it's all very mysterious Graeme.
Nick Nakorn

I wonder if it just a case of the (I cant think of the term...back spacing???) Wasnt set correctly with the DTI... maybe off by a couple of degrees

You did say its only 18 months scence this was rebuilt

Tough break... but, from my perspective, that parts all look in great shape, maybe some emery cloth and a recalibration and your good to go

Prop
Prop and the Blackhole Midget

I do have a spare 4.2 diff (this one is a 3.9) which I could put in to see if the knock has stopped. If it hasn't then it would prove it isnt the diff. It is has I'm not sure that it proves the diff guilty because disturbing everything may have indavertently cured the problem.
Graeme W

I would still suspect something in the hub. Hand brake operating lever? Or maybe the bolts that secure the backplate are loose, allowing the backplate to rotate against the stops and clonk as the shoes catch slightly on each revolution?
Guy Weller

I had just this a few years ago, and it was a cracked diff cage. But that's ruled out now.

It's a nice day. Go for the 4.2.

Or of course you could get in the car and ask a beautiful assistant to push you up and down the road while you listen to the drums.
Nick and Cherry Scoop

Im always confused by this

Does the 4.2 make you faster off the line,but higher engine revs and less speex at the top end say around 70mph comparred to a 3.7

Or is it the other way around... slower speed off the line but lower engine revs at the top end at 70mph.


Prop
Prop and the Blackhole Midget

4.2 is lower geared so better acceleration but lower top speed and protentially not so good economy.
Swapping between 4.2 and 3.9 (or any other ratio swaps) causes speedometer issues.
The early cars had 4.2 and the 1500 came with 3.7 AFAIK
Graeme W

Thanks graeme,

I wonder how that would work with a 5 speed datsun...

Im guessing 5th would be like orginal 4th in the ribby

Now theres a thread we havent seen before... hahaha

Prop
Prop and the Blackhole Midget

not read all the contributions to this thread, but have you checked the Prop shaft and UJ ?
P Bentley

I came to the conclusion that as a propshaft-related knock would be 4x faster than a wheel-related knock, that was too fast for the circumstances.
Graeme W

THe saga continues!

I have put the old 4.2 back in! Of course, the knock has gone. Unfortunately the only certain result would have been the knock continuing. Now I can't be sure whether the 3.9 diff had developed a knock, or simply taking everything apart and putting it back cured the problem.

Due to get an expert look at the diff on Friday.
Graeme W

Useful to have an expert to look at it.
But if your premise that "simply taking everything apart and putting it back cured the problem." then perhaps if you now put the original diff back in it will remain "fixed" ! ?
Guy Weller

Guy: I'd wondered about that too. But to be honest, taking the diff out and in, out and then in again is tedious and quite hard going, particularly when it involves laying flat on your back under the rear axle getting imprints of spanners on your backside because you are laying on the spanners you can't find in the first place.
Nothing was evidently loose or "out of place" when I stripped it all down, so I can't see how it could have gone back differently, which in my mind still points to diff issues. I have only taken the drums off and pulled the halfshafts. Didn't need to touch the bearings or pull the hubs of course.
Probably the appropriate time to get another set of eyes on the diff and check internal settings. THEN I can put it back.... and the knock will reappear!
Graeme W

Graeme,

I'm sure that Guy's tip would work for the diagnosis, but I bet the problem was in the part that you replaced. Now that the noise has gone, I think that different look on part of crown wheel was relevant. It doesn't take much to muck up the works.

Years ago I took apart a 5-speed Fiat transmission because it was growling. I could find nothing wrong with it. All the bearings spun nicely, and all the gears looked nice. I couldn't think of anything else to do, so I replaced every bearing in it. When I stuck it back in the car, everything was fine.

Man, I know what you mean about the wrenches under your back. It is hard to find something more irritating than crawling out from under a car to get a wrench only to find that under you all the time.

Charley
C R Huff

Even worse, Charley, is using a "crawler"! The spanners get under the wheels and you can't go forward or backwards.

I am going to put the "new" diff back to reduce the uncertainty. If it knocks again I can be "reasonably" certain the diff is to blame. It came down to balancing fitting the diff again or a return trip around the M25, I've gone for refitting.

Most likely the trip round the M25 will be next week with Charley saying "Told you!" ringing in my ears. I shall have to remind myself of Guy's philosophy!
Graeme W

I used to use a crawler, until one day I accidentally used a sheet of corrugated cardboard instead and found it was so much better: more head room, easier to get on/off of, slippery enough that movement was about the same.

Putting the old diff back in to see if it makes the noise again reminds me of 2001, when HAL suggested they put the "faulty" antenna unit back in. Chaos ensued (murder, intrigue, drama)…



Norm

Norm Kerr

If it's not a bearing failure, rear gears can make noise for a long time and still keep pulling.

So Graeme, now I can say "told you so" whether you make it or not.

Charley
C R Huff

Man, I know what you mean about the wrenches under your back. It is hard to find something more irritating than crawling out from under a car to get a wrench only to find that under you all the time.


Now thats funny, ive been working on a cool new idea, (in my head so far), ....

An UPSIDE DOWN magnetic tool tray that a fixes to the bottom of the car by way of... magnets. I was thinking having 2 trays built into one,,, with one for tools, one for screws/bolts and parts then it would have hanging cords to hold spray can fluids fluids, a flexiable shafted flash light, and a place to hold your smartphone....it would also have a special easy get to slot for a magnet on a stick

only the USA version would have built in radios and cup holders :-)

Prop



Instead of a creeper crawler I use old carpet... much warmer and comy
Prop and the Blackhole Midget

Oh and the tray would also spin so you have even better access.to parts and tools above your head

I came up.with this the last time I had to put the sump pan back into place and one of the bolts got kicked way to the back side of the car and then noticing all the disorganised crap and tools I needed under the car when I extracated myself

Prop
Prop and the Blackhole Midget

Prop: why not magnetise the whole chassis! You could park spanners, nuts, bolts and so on ANYWHERE. A coil wound round the vehicle and a flash of current should do the job. An additional benefit would be direction finding! Take your hands of the steering wheel and the car would tend to head north. Southern hemisphere owners should wind the coil in the other direction.
Graeme W

Graeme,

By using electricity and magnatizing the car, it would have an adverse effect on the production of rust, if you dont have rust to repair, life gets boring

Prop
Prop and the Blackhole Midget

Graeme, also check that the brake shoes are on correctly, that the springs are not at odd angles and that the brakes are correctly adjusted. The knock could have been an out-of-line brake shoe snagging on the drum.
Nick Nakorn

Put the rebuilt diff back, having discovered there was no knock with the old diff and....... there is still no knock!

All good news except I don't know what was knocking. But at least its stopped. Goodness knows why turning everything by hand didn't show up a problem somewhere. On balance, I would rather find it stopped and not know why, than discover it was still there and have a large bill for new bits.

So is that a group "we told you so" then?

But, wow, when it comes to "how fast can you get a diff out?" I must come close to being the record holder, even with my eyes shut.
Graeme W

Well, that is good news, Graeme. I second your attitude. Why look a gift horse in the mouth, and all that. You probably just accidently fixed it.

Charley
C R Huff

Graeme,

Glad to hear your woes are behind you (or no longer behind you, if you see what I mean).

I need to swap the diff before too long because of a drip from the nose seal. Now that you are a record holder, do you have any handy tips for doing this job please, apart from where not to put the spanners while lying underneath the car?

Thanks.
Jonathan Severn

Jonathan
I'm sure lots of contributors will have something to add but these are my random thoughts:

I assume you are working with the car on the floor, which is fine but a little bit of a struggle. Make sure you jack it safely - I put axle stands under the spring mounts forward of the axle, and stands under the axle, and wheel chocks.

Drain the oil and remove wheels, brake drums and pull the halfshafts. One silly point - if you are in a garage, make sure you have room to completely remove the halfshafts both sides without moving the car (which can't be done!). You need the halfshafts out to replace the gasket and rubber seal. Don't mix them up and reverse sides!!!

Before you pull the shafts it's prudent to remove brake shoes because oil from the diff runs along the axle tube and will drip in the vicinity of the shoes and will ruin them if they get "oily" (been there, done that).

You will need to unbolt the prop shaft. Beware the prop shaft can drop out of the gearbox end. I have a flexible cable which I feed through a convenient hole in the inner wheel arch, through the uj and out the same hole on the opposit side. I pull it tight so the prop shaft can't drop forward and then tighter again to lift the free end of the shaft when the bolts are out. Mark the uj flange and the diff flange as you may not realise the bolts are not equally spaced!

When you unbolt the diff and pull it forward it is just light enough to handle it from the side by reaching around the road spring. But only "just"! Weight lifting is not my strong point.

The minimum you will need is a diff gasket, two hub gaskets and o ring seals. If you have wire wheels you may want to replace the locking tabs on the brake drums.

This is one of those jobs which opens a whole can of worms! Hub bearings, lip seals, nuts on axle ends? If you are going to tackle the leak the job you are contemplating has issues. THe pinion nut is VERY tight (140 ft lbs) and you will disturb the preloading in the diff bearings. Read Norm's piece on this work here: http://www.mgexp.com/article/mg-midget-differential-rebuild.html

I think that unless it is leaking puddles, I would leave it ("we can't believe he said that!")
Graeme W

Jonathan,

Is all that is wrong a leaking drive flange seal? I can't recall doing it on a Spridget, but on my B I just dropped the driveshaft from the rear flange, removed the flange, and replaced the seal. I can't imagine swapping the rear because of a seal leak.

Charley
C R Huff

Charley: I read that as J's problem being a leak at the seal where the uj bolts on. There is a lip seal around the input shaft. The problem is that the bearings on that shaft have a preload which is why, if it is set up correctly, the shaft doesn't spin freely.
Between the bearings there is a spacer which crushes as the pinion nut is tightened and the recommended method is to measure the torque to turn the shaft as you progressively tighten the nut. In theory the correct torque to TURN the shaft coincides with a tightening torque on the nut of (IIRC) 140 lbs ft but the quality of the spacers nowadays is iffy and you can't just tighten the nut and rely on that.
Norm has it clearly described in his item I refer to. At that pinion nut takes some shifting. Lawrence gave me a hand and it needed a scaffold pole, together with a weight on a bit of string wound around the shaft to measure the turning force on the shaft itself.
Graeme W

Graeme,

The cheater's way out is to feel, mark, measure what you have, and then try to duplicate in on reassembly. My theory is that is risky to put a new crush washer in with old bearings, and then preload them like they were new. So, if just replacing the seal, I try to get it back to what it was before I pulled the flange nut off. Besides marking, you can get some sense of it by removing the nut with a torque wrench.

Charley
C R Huff

I know what you mean Charley. What does the least damage - running with min/no preload or placing new loads on old bearings? Some will argue just replace the bearings as well.
Graeme W

Hi guys,

if your bearings are good you should be just fine with a new crush spacer (good bearings won't know the difference if they were new or used, if they've got no wear on their races or the needles). Inspect them carefully to be sure (after cleaning, in a bright light go over all of the surfaces and feel them rotate perfectly smoothly in your hands). If there is any change in glossiness in the race, or any roughness to the needles, they need replacing, even if they haven't begun making much noise yet, now is the convenient time to do so.

The crush spacer is just a time saving device, the early pumpkins used a solid spacer and shims. in either case, the point is to get a 12 in-lb drag on the pinion when the nut is torqued (the nut torque will be somewhere in the neighborhood of 110 ~ 130 ft lbs). Note that the torque applied to the nut is immaterial, what you want to confirm is the drag on the pinion. The tolerance is only +/- 1 in-lb, and just a degree of rotation of the pinion nut can change it more than that, so marking the nut position is very unlikely to get it close enough.

Note that you will need a very small torque wrench to measure that small of a number (your normal in-lb torque wrench will not be sensitive enough to use). These are available for rent from most good shops, or you could borrow mine, if you were near enough to my house.

The above is for how to do it right.
If you want to take a chance, then the best method for doing this job "quick and dirty" is to measure the pinion drag torque before starting, and then turning the nut until exactly that drag is measured again.

Note that to measure the drag you really ought to remove the crown wheel (just 4 nuts to take it off, none of the adjustment will be upset by doing it). However, I measured mine when it was stripped down, and then again as a complete diff and the difference was very small, in pinion drag, so just pulling the half shafts ought to be enough to make this method work reasonably well.

Note, though, that you still need the little in-lb torque wrench to use this quick and dirty method.


By the way, a differential will run for a very long time even if it was poorly put together. I put 15,000 miles on mine after I cranked the pinion nut down to 140inbs (no understanding at that time the correct way to replace the seal), and it got gradually noisier and noisier. I finally pulled it, saw the ruined pinion bearings (caused by my over tightening), but the rest of it was still as good as new. After rebuilding it the right way it is silent now, though it will really take a few more years to be sure.



Hope this helps,
Norm





Norm Kerr

Once I had a rear wheel brake cylinder that had moved sufficiently for the hub inside the drum to contact it. Nearly wore through the brake cylinder before I found it.

That was in '92ish. I still have the cylinder on the shelf to remind me.
Dean Smith ('73 RWA)

Graeme (and others), many thanks.

If anyone is interested, I have had a quote of £200 + VAT from Hardy Engineering to fit new bearings and seal (assuming the gears are OK). I also had an estimate of £375 from A1 Gearboxes in St Neots to rebuild the diff (not sure exactly what that includes and whether or not that price is + VAT).
Jonathan Severn

Hardy are good, I think.

I had a diff from them that gave out a little howl - just a little one - and they swapped it without having to listen to it.

(I lived in London then)

Never heard of A1.
Nick and Cherry Scoop

Hardy are supposed to be good although I recently met somebody who'd had a bad experience - possibly they're not what they once were. Still f expensive though!
I've used Charlie's marking the nut method with good results.
David Smith

For the Saab 99 that I used to have, there is a large idler gear in the train that transfers engine power to the transmission below the engine. The book gave two specs for setting the preload on the taper bearings for that gear. One spec was for new bearings, and there was a lesser load spec for used bearings.

Charley
C R Huff

THe problem I had is that there was no noticable preload on the bearings to start with so although I marked the nut, when I got round to that position there was still no resistance to the shaft turning (although the nut was extremely tight!) I concluded that the spacer was balancing the torque from the nut and needed to be overloaded a gnat's so it just crushed a little more to load up the bearings!

So, I measured the radius of the body of the pulley, wound a chord around it and then calculated the weight I needed to hang on the chord to just turn the shaft. I carefully tightened the nut until the weight started to show a reluctance to drop under gravity. At that point I called it a day! Beware a very small additional rotation of the nut will crush the spacer and build up preload very quickly. And you can't go back - if you over do it,loosening the nut again will take of all the load.

I have no complains about Hardy Engineering. They have a good reputation in the Kent group as far as I'm aware. Not cheap but good workmanship.
Graeme W

This thread was discussed between 27/06/2014 and 09/07/2014

MG Midget and Sprite Technical index

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