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MG Midget and Sprite Technical - Diff Decisions!!

After breaking and replacing a half shaft on my frogeye I am thinking about changing the diff. The current one whines very badly, it is a 4.22 and the car has a 1330 A+, 276 cam, HIF 44 and stage 2 head with ford 5 speed box (I can't be more specific the P.O. did the engine and box).
With the car came 2 spare diffs, another 4.22 and a 3.7.
1st question; which should I use? At the moment I am up to 6000 rpm in first in just a couple of seconds and it all seems quite frantic even in 5th, but is the 3.7 too big a jump?

2nd question; how do I tell the condition of the diffs? Neither show signs of wear that I can see. The 4.22 seems to have marginally more free play, backlash? at the propshaft connection, by eye something like 1/16". Other than that they look comparable. So is there a relatively simple way to judge the condition?
Thanks for any input, Graham.
Graham P

"6000 in first" First gear wasnt made for such cruel behaviour! First gear is ment to get the car from standstill to a couple of MpH.

3.9 would be great but with that spec engine a 3.7 would do the job with ease but you wont be able to compete with a Golf GTI at the lights anymore.
Bas Timmermans

Graham,

I have a 1275 produces just over 80bhp last time I had it rolling road tested @ 1200miles after engine rebuild. its freed up a bit since then so would reckon a few more BHP. I have a Toyota 5 speed box and have just changed to a 3.7 diff. As the lower ratios 1,2,3 are lower in the Toyota box than a MG box I have lost nothing in acceleration but the cruising is much more relaxed. With 155 R 13 tyres 5th gear gives me 20mph per 1000 rpm which makes a reasonably relaxed 3500 @70mph. Two up with camping gear to Le Mans I never found myself having to change gear too often on long motorway up hill drags and got 40mpg. Give the 3.7 a go, what you loose on acceleration you will gain in fuel consumption and more relaxed cruising.

Mike.
M J Pearson

There are a nummber of useful checks you can do.

Backlash occurs in two main places: Between the crown wheel & pinion, and between teh gears in the differential cage.

For the CWP, the original backlash is amrked on the back of the crownwheel. If you can borrow a dial indicator you can see if it has changed significantly.

For the differential gears (the most common cause of backlash, particlarly in cars with no LSD and observing the time-honoured tradition of 6000 in first) the usual source is wear in the thrusts behind the gears. Usually as a result of wheel spin :-)

If there is any lash there it would be worth replacing the thrusts before fitting it, as once the are worn through the gears start to grind into the cage.

If you can get some bearing blue, smear some on some of the crownwheel teeth and rotate the drive flange by hand to turn the crownwheel a few times, in both directions. The blue will show up a print of where the pinion is contacting with the crownwheel. It should be a patch along the centre of the tooth, slightly nearer the inside edge of the crownwheel than the outside. Bad mesh can occur in two ways - either too high or too deep on the teeth, or too near either the outside or inside edge of the crownwheel. Misalignment of the teeth from poor meshing is what causes the whine, so have a good look at this.

The misalignment can also be brought about by poor bearings, sometimes in conjuction with a lack of pinion pre-load, which allows deflection of either crwonwheel or pinion under load with the result that they drop out of perfect mesh. There are some simple checks of the bearings you can do yourself:
Feel for any sign of roughness as the diff is turned over by hand.
If you remove the crownwheel & cage assembly, you can better check the feel of the pinion bearings. They should be slightly stiff to turn, definitely not loose or rough. (The original drag on the pinion required a torque of 11-13 lb-in to overcome it)

While the cage is out, you can better feel the diff cage bearings for roughness.
Paul Walbran

Sorry, forgot to add a thought about the ratio:
The 3.7 is worth thinking about seriously.

One of our midgets (a Mk3) has a similar spec engine, and has the 3.9 with the original gearbox. And goes very well through the gears. A number of 5 speeds (but not all) have a first gear lower than the Midget's 3.2:1, so it may be that in going to the 3.7 you would be replicating the overall gearing of the standard mk3. (Example: Our other Midgets with 5 speeds are both around 3.5:1. A 3.5 first gear on a 3.7 diff [more fully 3.727:1] gives 13.05:1 overall, whereas the standard 3.2 on a 3.9 diff is 12.48:1 overall - a slightly higher ratio)

In any event, the mk3 is heavier than the frog-eye, which helps handle the higher gearing.

A further point to ponder - when opening the taps in first, does it wheelspin easily? If so, you are wasting the 4.22 gearing (not to mention risking busting your diff with the wheelspin) as tyre grip is the limiting factor. In this situation, there wouldn't be much downside until 2nd is reached.
Paul Walbran

My 1275 (75 bhp) has a(LSD) 4:2 with a Toyota Box I tried it with a 3:7 and a 3:9 and it wouldn't pull in high revs, once I changed back to the 4:2 I had full use of the revs, I put it down some engines are revers and others powers for some unknown reason..
K Harris

Thanks for your responses guys.
If I gave the impression that I'm in the habit of pulling 6k in first then that isn't the case, it's just that I have to change out of first idecently quickly if I give it a little wellie!
There is a chance that I may be able to swap my spare 4.22 with someone I know who has a 3.9, the impression I am getting is that 3.9 would be the one to go for if possible.

Paul, thanks for those checks, I should be able to do them, I have a dial indicator somewhere & bearing blue should be gettable, it'll probably take me a week or so to get the time to do it, if the thread is still on the board I'll let you know how it goes.

I believe the ford box has basically the same first four ratios as the earlier spridgets so should pull pretty well in first with either a 3.9 or 3.7, so it's a toss up between better acceleration or more relaxed cruising :?

As for wheel spin, yes, I can get it very easily in first but try not too, trying to protect the half shafts! I think this shaft broke because the wheel bearing momentarily seized, it's been rumbling for a couple of weeks. Bought the bearings but hadn't got round to doing it, that'll learn me!!!
Graham P

I think it all depends on what you are wanting to do, and where you want to drive....

if your in the mountians, and do alot of parking lot races, smoking the tires at the lights and drag racing Id say the 4.22


if your in laid back cruise mode....fuel economy, and pulling a camping trailer,,,then Id say the 3.7


prop
Prop

3.9
Daniel Thirteen-Twelve

Just to deviate a little, but still on the subject of back axles - I have been looking through various handbooks and workshop manuals to try and find out the service interval for changing the oil in the rear diff.
Nowhere is it mentioned, except for stating that the oil should not be changed during the first after sales service. Surely there must be some point at which the oil should be changed?
Dave
Penwithian

I have a service sheet for Midget, 1961 onwards which states for the rear axle "top up if necessary" every 6,000 miles or 6 months.
Mike
Mike Quilter

Is the rear axle and differential on a 1275 the same as a 1500 ???

I never thought about that...just wondering


prop
Prop

Mike,
Thats exactly what i'm getting at - information exists on topping up but no mention of changing the oil.

Prop,
The 1500 rear diff as far as I know is identical to the 1275 with a 3.9 ratio up to GAN6 200000 and 3.7 thereafter.
Dave
Penwithian

Graham,

Just saw you have a 5 speed box. That makes torture to the first less damaging.

The first gears of original boxes with their nonsynchro are just for getting it roling. I also use my first (Datsun 5sp) up to 4500 RPM with no pain.

Bas
Bas Timmermans

Leaving the EP oil inside without ever changing it isn't that rare in the industry, relying on simple top-ups to replenish leaks.

The type 9 has no drain holes, except those provided by the front and rear seals and they are fairly robust.

(note that I only said fairly!)

Bill

Yer average T9 might not have a drain hole but there are a few enterprising individuals who may have drilled and tapped their own ;-)
Toby Anscombe

Bill,
Does that go for the standard 1500 gearbox as well, as it's filled with EP oil, unlike the engine oil in the A series gearbox? Does the EP oil get changed?
Dave
Penwithian

I just looked up an Autocar report (reprinted in Lindsey Porter's super profile on Spridgets) which states in "maintenance"

-Gearbox 1.5 pints SAE 90EP. Check every 6000 miles-
-final drive 1.75 pints SAE 90EP. Check every 6000 miles-

So it seems to be unlikely*

As it happens I have a drain hole type9 which is up for chucking away because it ran its front gearset and had the mice in nibbling away at the lovely teeth and let its bearings get rubbishy.

drain hole was a fix too late


*AUTOCAR W./E 14 June 1975

quotes time allowed for serice 3.5 hours
cost at £4.30 per hour £1.05

doesn't that take you back then?
Bill

"*AUTOCAR W./E 14 June 1975

quotes time allowed for serice 3.5 hours
cost at £4.30 per hour £15.05"

yeta nuvver oops moment!
Bill

Talking about T9 gearboxes, I have to inspect and possibly change my UJ's, which of course means removing the prop and all/some of the oil draining out!!!
Where on earth is the filler on a T9, how do I get to it and what oil do I use?
Graham P

left hand side

half way down

large Allen Key to release it

and the oil has been discussed recently in here

The proper Ford oil is no longer available but ISTR that the same oil that Ford use in the Mondeo gearbox is acceptable as is the Comma Oil that Guy is using now

semisynthetic SAE 80EP ESD-M2C 175-A spec
Bill

Thanks for that Bill.
Graham P

Sorry for the hijack, another type 9er here!!!

If you drainfrom the left hand side drain plug, how it the box refilled?

Cheers

BC
B Cavagan

BC - Bill described the position of the filler plug. There is no drain plug on the type 9 casing unless an owner has fitted one or the Ford iron main casing has been exchanged for an aluminium alloy casing.

Some boxes don't in fact have a hex filler plug but a square filler plug.
Daniel Thirteen-Twelve

BC, there isn't a drain plug, Bill describes the filler plug (I know that and I don't even have a Type 9!)
David Smith

so does the oil have to be 'sucked' from the box before fillimng again?

I didnt fit the box myself (although I had it rebuilt etc...) so i have no idea how to service it!!

Thanks for your help.

BC
B Cavagan

thats crazy...dont tell david benttly on the other page...he will loose it forsure

was the cars that the T9 was installed in where never meant to outlast a gearbox oil change before going to the salvage yard???


what ever you do dont just tap a drain hole indiscrenitly...or else that cast aluminum will spiderweb big time

prop
Prop

I doubt if you are expeced to change the oil, only keep it topped up, same as the 1500 gearbox and the diffs since time immemorial

Additives in EP oils are supposed to last for the life of the machinery, the only real maintenace is to keep the levels right.

So my advice would be to "dont worry"

If they needed emptying they would have a draining plug

When I fitted the drain plug to my type9 I drilled and tapped on the opposite side to the flat area where the internal magnetic scavenger ring sits.

I wrapped my arm in polythene to protect my skin from any pollutants then drilled and tapped the hole whilst the residual oil drained away, taking the tap thread cuttings away with it into the debris!!
Bill

Cheers Bill, I wont bother making a drain hole then, I will just keep filling until it pours out!!!

Never have been to confident of gearboxes, happy to have them rebuilt and fitted If I have a whim of as change, but for me, best left to those in the know!!! Engine and body are more my ticket!!! (thats why I didnt fit the thing myself!!!)

Whats crazy Prop? Diod I miss something mad??
BC

Aha!

one thing has just re-occured to me

there are two internal oil levels listed in the Ford Haynes book for the Sierra, one is when flowing out of the filler hole but the other is either 15 or 25mm below the hole (inside obviously)

but I'm having an old gits moment here, I can't remember how to tell which level you use with what kind of gearbox

You will need to check it out in a Sierra Haynes book

which will only work if you can work out what year of Sierra the box came out of...

If I can get my hands on the Sierra book soon I may be able to remember how to work it all out... but for now? Sorry
Bill

>>>>>>>Whats crazy Prop? Did I miss something mad?? <<<<<<<<

crazy is a tranny with NO drain hole....thats something Id do if designing the tranny...lol


prop
Prop

Ok, I will gove you that one, it is a bit daft!!!

Must just be Ford, dim americans ;-)

BC
B Cavagan

Prop,

If you look at it from the point of view that the initial oil fill will last the design life of the gearbox, with possible occasional top-ups then it's not such an odd decision.
David Billington

haha, they never thought we would be using them 20 years later in MGs I bet!!! They sohould have added a drain hole though, just for the incase!!

BC
B Cavagan

>>>>>>>Must just be Ford, dim americans ;-)<<<<<

Dont blame us, they CAME from YOUR side of the pond...we only have a couple thousand of them here from an 87 ford EXP...lol

prop
Prop

ok, I will take one on the chin for this side...lol!!! Very true is the point of boxes outlasting the car in that case, have you seen th eweather here? Wet most of the time, and more salt than McDonalds chips spread on the road for even a risk of snow!!!

england = RUST!!!!

For all americans needing T9 boxes and struggling to find one, we probably have more than props estimate rotting in our local scrappy. I got 2 for £50 thinking one good box cold be made. I have two boxes now as they were both very good!! no harm in a spare!!

BC
B Cavagan

BC

If your smart, you will learn to rebuild those boxes and start exporting them over here....you could make some bucks.....I fear there are only a handful of datsuns 5 speeds left....I have only seen about 5 scence january on e bay.


Oh dont forget to pput a drain plug in ...lol

prop
Prop

This thread was discussed between 01/08/2008 and 06/08/2008

MG Midget and Sprite Technical index

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