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MG Midget and Sprite Technical - Gear box noise

I have recently bought a Mk4 Sprite which is is fabulous condition with receipts for over £3000 for engine reconditioning
But the gearbox is noisy. When I lift off the pedal there is a vibration. This could be normal, or from the gearbox or the back axle. Also the car accelerates really well but is revving at 4000 at about 50mph. I wonder if the diff has been changed. Is there someone in the Glasgow area who has a similar car who would be prepared to drive my Sprite and comment on noise and the rear end ratio.

Jan T 07799667289
J Targosz

Jan,
certainly sounds like your diff might have been changed. Was the car going to be used for sport rather than road use perhaps.

Are you sure your speedo and tachometer are registering correctly?

4,000 revs in 4th should be 65mph with standard wheels and tyres, could you check the speed at 4,000 or any other revs against a SatNav or speed camera detector, don't bother with free apps for phones as they're not always accurate.

The vibration on lift off could be a few things.

You can put up two photos per post, if you can take good quality high resolution photos (in landscape please) of the engine bay, gear lever area and wheels on one side of the car it might save a lot of guessing and typing.

If I was up your way I'd gladly come over and compare Spridgets.

You will help yourself a lot if you have a copy of the Driver's Handbook (this isn't a workshop manual, hide any Haynes away for now).

Below, landscape, portrait - landscape please.

P.S. Glasgow, Central UK?



Nigel Atkins

Jan,

Welcome to Mk IV ownership.

Have you checked the oil level in your gearbox?

My MkIV's gearbox (it's actually a Morris Minor one!) has a slight leak and if the oil level drops the box starts to make a bit of noise to tell me it's time for a top up.

A friend re-built a lower ratio differential for me a couple of years ago. Unfortunately he never managed to get the settings correct so it made an awful noise. I'm currently running the original 4.22 differential which is a bit tired but otherwise fine and that just sings its own sweet song which I've accepted as being all part of driving the Sprite.

Colin
C Mee

You can usually find the diff ratio marked on the perimeter of the wide flange where it bolts into the diff housing/axle case.

Its written in the form of an equation i.e 10/39 = 3.9 etc.
9/38 being the lower 4.22

Worth checking for any play in the propshaft universal joints as that can cause vibration on accelerating and lift off also.

R.
richard b

Check the tacho calibration. The early electronic tachos suffer badly from calibration drift and often over read by heaps.
Also, the relationship beyween tacho and speedo is set ny the speedo drive and isnt affected by diff ratio changes as the speedo is effectively measuring RPM at the gearbox output but showing equivalent MPH on its dial. When the diff is changed the speedo reads fast or slow accordingly.
Paul Walbran

I was wondering by gearbox noisy and accelerates really well might be diff noisy and accelerates really well and one or both dials inaccurate. My thinking is that possibly there's more than one issue at play. I'm curious about accelerates really well.

Not that it really necessarily matters but it'd be interesting to know what year Mk4 it is.

I might be wrongly taking it that Jan is new to classics or Spridgets, but then the surname looks half familiar to me but I'm probably mixing it up with someone else.

Nigel Atkins

Hi Nigel,

You may have noted my name from TD/TF BBS posts. I had 1954 TF for a number of years but sold it a couple of years ago. I couldn't live without a classic car so recently purchased my 1966 Sprite. It is a 20 year old heritage shell rebuild with a slightly modified engine. Sports cam, Omega pistons, ported head, Magnaflow exhaust. It also has a Frontline telescopic suspension upgrade so someone may have lowered the axle ratio to make it more 'sporty'. Despite the possible problems with the drive train I am really pleased with the car. I am at an age where spanner work is fine but don't want to get involved with a welding torch!

Cheers

Jan T
J Targosz

Jan,
I feel good about actually remembering your surname as familiar but should have recognised your first name, sorry.

As yours is an earlier Mk4 it would have from factory a 4.22:1 diff so 4,000rpm would show about 60mph.

Don't read too much into the FL front suspension change as recon Spridget LA dampers from the usual suppliers have been crap for decades.

As a former TF owner you'll appreciate the value of doing a staged rolling full 36k-mile service, check, lube and change of fluids whilst also using the car and getting used to it.

Personally I'd clean/flush and refill the gearbox and back axle with fresh good quality oils after a hot drain, another excuse to run the car.

Gearboxes can be noisy, even when new, and diffs get a whine, oil top-up (or better still change as above) can help a little or a lot.

For the vibration spanners out, as put earlier prop UJs but also all over nuts and bolts, but it does depend on what and where the vibration is.

Have you got wire wheels (vibrations)?

Photos (videos) can help but are also of interest and often soliciting more and varied responses than just text.

My previous Spridget was a Heritage shell and my current midget is a Heritage shell.

I've got some basic notes I made up for maintenance and servicing info, non-technical, plus loads more Spridget info, if you want any just email me.



Nigel Atkins

I am reminded of when I bought my Sprite in 1978. It seemed to accelerate quite well, and was a demon on tarmac autotests. But I did wonder why everyone else was exceeding the speed limits. Only after a trip to Venice and back did I find out it had a 4.55 diff.

Les
L B Rose

I have only had the sprite for a couple of weeks and only driven it with the hood up. I am comparing the noise to my old TF but I had fitted a Ford 5 Speed to that. The Sprite does appear to run out of steam and become noisy at about 55 mph and I suspect I have a 4.5 diff fitted. I am currently away from home visiting family but am due home this week and am looking forward to turning the prop shaft whilst my wife checks one of the rear wheels turning. Perhaps it would be better for her to lie under the car and I will watch the wheel!

Jan T
J Targosz

good luck with that proposal!
Bob Beaumont

Jan,
hood up you should be able to hear tappets, exhaust, (induction depending on filters) gearbox, back axle whine, anything rolling around the boot and squeaks and rattles not attend to (or already attended to).

Roof down you forget to listen.

Unless your speedo is wrong even with a 4.55 diff it should be showing national speed limit and above before the car runs out of puff. Standard cars top speed when new was in the 90mph range.

As yours is an earlier 1275 it could have the original tacho or if the car has been converted to negative earth perhaps a later tacho, or existing one changed from positive earth as in a recent MGA thread, the numbers on the speedo and tacho dial faces might give you a lead.

There were thousands of changes to Spridgets over the production life of the model and even lots of changes on just the 1275 models, that's without changes over the years by previous owners of course.

I think it's very unfair of you expecting your wife to be counting the wheel turning as it's a very boring job and easy to lose count and then you'd moan, let her have a lie down and turn the prop.
Nigel Atkins

Jan

For diff ratio checking, I prefer to turn the wheel, rather than the prop.

If you mark the diff flange, then lock one rear wheel and turn the other wheel two complete turns, while counting the turns of the diff flange.
Dave O'Neill 2

Now checked the ratio. Lifted one rear wheel with a bottle jack and could see both the rear universal joint and the wheel at the same time. I put a dash of white paint on one of the UJs and lined it up with one of the diff bolts. I then chalked a line on the bottom of the tyre and a matching one on the ground. After turning the wheel one revolution the UJ had turned for times and almost a quarter. I must have a 4.22 crown wheel and pinion. I did note that I had to turn the wheel about 1" at the circumfrence to take up play before there was movement at the propshaft. This must be from the bevel gears or the crown wheel and pinion or perhaps both. A friend has given me a 3.9 diff which I will rebuild. Backlash appears to be fine but the bevel gears do have some play. Are standard and oversize thrust washers available? Should I set them with engineers' blue so all four touch in the centre of the teeth.

Jan T
J Targosz

Clutch thrust/release bearing would be my bet....
Oggers

Actually, that brings up a good point, why not drive the car regularly on reasonable length journeys and see what quietens down with use (and servicing and maintenance).

Or go to a local meet where people actually drive their cars and jump in and out of each others cars to see how they compare on the road rather than cosmetically in the car parks.

Last time I ever when to a Spridget meet a young lads just happened to say he was surprised how heavy the clutch pedal felt on his car and I thought he was joking but he wasn't so I suggested he try the clutch pedal on mine. We surmised there must be something wrong with his. Had we not been hemmed in by so many shiny cars I'd had taken him out in mine to get an idea of what a reasonable mechanical model went like.

Nigel Atkins

If there is excessive movement at the wheel rim, as you describe, it isn't necessarily lash the diff crownwheel, The "olive" bushes on the planet gear cross shaft wear. Replacing them is an easy job - no measuring skills required, and can make a lot of difference to haw it feels, especially when driving at a steady speed.
GuyW

The thrust washers behind the sun gears can wear especially the later tufnol ones which I have seen worn to nothing. They have been produced in steel, bronze, and tufnol so if they need replacing getting a bronze set would be the best option.
David Billington

David
Is there any particular bronze ?

I'm thinking of doing this and have quite a lot of phosphor bronze (don't know exactly what grades unfortunately !)
I assume brass is out due to the hypoid EP oils.

R.
richard b

Richard,

I've got some 20swg (0.036") PB102 sheet which I got to make some as this seemed a suitable alloy for the job and was available. I no longer have access to a portable "brick on a stick" xray analyser or its larger variants so can't test the bronze washers I have to determine their alloy.

Regarding attack of yellow metals by EP additives I have read that this only occurs to any significant degree at elevated temperatures which aren't going to be encountered in our cars.
David Billington

I see Brown and Gammons have the thrust washers for the bevel gears for sale and they are inexpensive. I have watched a posting on You Tube when an American rebuilds a full diff. He recommends fitting the carrier bearing shims onto the outside i.e. large outer race rather than small inner race. This enables setting of the crown wheel meshing without pulling the bearings off. Does anyone know where these large dia shims can be purchased from? I have tried before to cut shims from shim stock but couldn't do so without creating burrs.

Jan T
J Targosz

Have a look at Accu https://www.accu.co.uk/en/704-shim-washers , there may be others that's just one I know of from the recent FWB thread.
David Billington

Jan I've had work done on my car and parts from B&G - do NOT think that they always necessarily supply the best parts, often they're from the usual crop.

And I wouldn't rate their general overall service above average for a Spridget at least, and possibly for RV8. but possibly they have a sweet spot for As but I wouldn't know.
Nigel Atkins

This thread was discussed between 27/05/2021 and 02/06/2021

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