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MG Midget and Sprite Technical - Gearbox wear

Is sideways wobble of a smoothcase input shaft indicative of wear? Or is this permissable since it will be supported at the front end in the crankshaft spigot, when installed.
GuyW

I presume there is some sort of oil seal that the input shaft runs through and am concerned that this may have become worn with excessive radial wobble of the shaft
GuyW

no oil seal Guy, just another scroll in the 'box front cover, a bit like the rear main hence they are prone to leak a bit. Too much wear in the input shaft bearing equals play in all directions and leads to excessive scroll seal wear. A bit of play is present even on a rebuilt box, the tricky bit is defining and measuring how much is too much! I'd guess that 2mm is OK but more than that will lead to leakage eventually, while remaining drivable.
David Smith

Guy,

A small amount of sideways movement is OK, but do make sure that the bush is actually present in the end of the crankshaft! And in good condition too.

As an aside, a couple of years ago I had to remove the special spigot bush (T50 gearbox) from the end of our 1380 crankshaft to replace it, and for the first time tried the 'fill it with grease, and drive a well fitting shaft into it' method to remove it.

Worked a treat!

Richard
Richard Wale

Sideways movement is ok but how much is the question? A rebuilt smooth case and for that matter a ribcase does have a small amount say 1/16" to 1/82 at the input (clutch end)Any more and I would investigate the spigot bearing in the first motion shaft. This is a needle roller that sits inside the ist motion shaft on the end of a spigot of the 3rd Motion or output shaft. Its quite small and does take a bit of punishment if the crank spigot bearing is worn. A worn one means you gets a noise rather like a zizz from the gearbox on the overrun in top.
Personally unless the history of the gearbox is known I would strip it and replace all the bearings anyway. The layshaft needle rollers and the layshaft itself can wear quite alarmingly. I generally end up replacing them.
Bob Beaumont

Bob, doesn't the smoothcase use a PB bush rather than a needle roller between the 1st and 3rd motion shafts?

As a related aside, when I rebuilt a ribcase for use in my Frogeye (not installed yet) the 3rd motion shaft spigot was a bit pitted, caused by lack of oil probably. I think this is quite common and 3rd motion shafts are hard to find / expensive so I decided, with a friend who has a lathe, to machine a PB bush to substitute for the needle roller as it should be "kinder" to the spigot I think. It may not last as long (which is why they changed to a needle roller presumably) but we'll see.

Guy, with the above arangement and a new front input shaft bearing lateral play at clutch end of input shaft is about 1/32".
Bill Bretherton

Hi Bill

The smoothcaase I rebuilt had a needle roller as do all the ribcases as you say.
Wear on the spigot on output shaft is quite common. I had 2 or 3 output shafts knocking around which I recently dumped with nasty pitting. I think the pitting is also due to the stress on the needle roller bearing by a worn crank spigot bearing. They do work hard for such a small bearing especially if the intermediate gears are used quite a bit.I see denis welch remanufactures ribcase output shafts at an eyewatering £395 plus vat.
Bob Beaumont

Ah, yes, a worn (or missing...) crank spigot bearing is bound to put stress on the needle roller in the g/box. I'm surprised there isn't some company refacing the third motion spigot, the price they are!
Bill Bretherton

I think Heathrow Transmissions do reconned output shafts....
David Smith

Interesting to see that you can buy a reconditioned main shaft for £395, when I was an apprentice at a Morris/MG main dealers in the late 60s early 70s,we only repaired these boxes if the car was under warranty, as an factory exchange box was only £32-00
Andy Tilney

What David said.

I had an immaculate 1098 Minor third motion shaft which I listed on eBay a number of times. I can’t remember if I ever sold it.
Dave O'Neill 2

I think the Minor output shaft is different as the Spridget ones have needle rollers for the intermediate gears whereas the Minor one has brass bushes
Bob Beaumont

I remember Heathrow transmissions listing "re-tipped" mainshafts and I wondered who might have machined these (certainly not Heathrow themselves - they are gearbox and diff rebuilders) as you'd need a fully equipped machine shop with case hardening facilities. But how is it done? Removing material from the tip (spigot) would reduce its diameter. It may be possible to use metal "spraying" techniques or build up with weld and turn down followed by case hardening but this could be quite expensive. It really is an issue with these old mainshafts.
Bill Bretherton

I need to check through the condition of what l have and what is best to use. I have a smooth case which is original to the car. It's clean inside and all gears select reasonably well though a little notchy which l put down to no oil. (On the bench) there is some radial play in the input shaft which is why the original question. For clarification, when quoting acceptable limits of wobble are you (plural) giving full side to side movement, or the deflection from a centre axis?

I also have a ribcase, but it's a Morris Minor one. Looking inside l can see that the 1st/ reverse cog is damaged and it appears to be stuck in, I think 2nd gear though the detent balls don't appear to have popped out. It may be locked in 2 gears, if that is possible.
In theory I understand this should be the stronger box but given tis first assessment it doesn't look good. I may strip it to check a bit more and also as a training exercise.

I also have some genuine NOS internal smooth case parts; laygear, synchro hub/ gear assemblies and selector shafts.
GuyW

Hi guy, I would say full deflection of up to 1/8 would be ok.

The rib case MM box is fine for a 948cc engine but you will need a 1098 minor flywheel, clutch, release bearing and thicker Minor backplate.

The main bits (apart from the bottom gear and laygear) that go on the smoothcase are the 2nd/3rd gears as they have shrunk on synchromesh which wears and can be 'beaten' rather than the ribcase baulk ring set up.
The main shaft bearings and laygear bearings are the same for both boxes.
Bob Beaumont

Bob, it's going on a 1098 engine.
GuyW

Where best to buy bearings?

When l rebuilt a T9 box some time ago l got bearings from a local bearing supplier just by quoting part numbers. They were a fraction of the cost that was being asked at a well known specialist ( begins with
BP) who quoted them by their application / position in the T9 box.

But these are a good deal older and maybe didn't use standard component sizes?
GuyW

I really think you have to strip the box down to properly assess condition. It isn't that difficult providing you are systematic and careful with the various ball bearings that come out. What puts me off the smooth case is the lack of spares available and the fact that the synchronisers are shrunk on. I did a lot of reading before deciding how much renovation I did (to a ribcase). The consensus seems to be to renew (at least) laygear, layshaft, 1st/ 2nd gear, reverse idler, front and rear bearings, rear seal and bearing between first (input) and third motion (main) shafts. You can fit 1275 internals in a smooth case AIUI - Heathrow transmssions do this.
Bill Bretherton

Does the smooth case box need any machining to take ribcase internals then ?
GuyW

Guy, I don't know for sure, I just know it is possible. Bob will hopefully confirm.
Bill Bretherton

"Does the smooth case box need any machining to take ribcase internals then ? "

That's my understanding, although I've never done it.

When I rebuilt my first rib-case gearbox, the local gearbox specialist supplied me with individual needle rollers to fit into the cages. That's what he used, as it was considerably cheaper than a complete needle roller assembly.
Dave O'Neill 2

This something I'll be looking at as well in the near future. I've got a couple of 1275 boxes that look ok but are unknown. To save time and hassle I could send one to Heathrow and have a full rebuild but with postage both ways and the oil seal front housing I want it will work out at about £850!!

So I have been looking at the rebuild kits that Heathrow sell on eBay. They contain all bearings, gaskets, seals and lay shaft for about £100 (they do one for the smooth case as well) and the oil seal conversion is £80. So I could potentially have a semi reconditioned box for a couple of hundred quid. Problem is I've not really done much gearbox work so I'm wondering if I should just bite the bullet and spend some cash - I'm in the same sort of position with the diff.
One thing is certain, I only want to fit the engine and gearbox once!!

So anyway Guy, if you want a kit for rebuilding yours then Heathrow can supply you one - not sure if it would be cheaper to buy things separately from somewhere else but you get decent help from them in my experience.

John Payne

John

As well as the parts you've listed above, you would really want to be changing the synchro rings - at least the one for second gear - as well as any other worn parts, such as gears with worn or chipped teeth, or worn synchro dogs.
The laygear itself often has a lot of wear/damage on first gear.

The rebuild kit is probably only good for keeping an otherwise good gearbox running for many years to come.
Dave O'Neill 2

Guy,

If you need a first motion shaft I am 99.99% certain I have a very good one (had it for over 40 years !) came with a cardboard box of mk1 gears as the chap had rebuilt the g/box I was buying with Mk2 internals.
It grieves me to remember chucking out the rest during a house move about 25 years ago - only kept the first motion shaft to line up clutch plates.

I think you have my e mail.

R.
richard b

The smooth case will accept the ribcase internals but it does need machining. From memory the ribcase drive train is longer than the smooth case so the smooth case casing needs to be relieved slightly.

As regards bearings I get mine from Peter May. he does two varieties, the standard and heavy duty ones. I have always gone with the HD variety as the 1275 engine puts more stress through the gearbox. Guy, you may be ok with the standard ones given yours is 1098.
I always use however the uprated layshaft and bearings and this part takes the most stick!

The other firm is Hardy Engineering in Leatherhead Surrey. They have just rebuilt a 3.7 diff for me and its quiet.

I don't always replace the synchro rings (ribcase only) The original BMC ones are superior to the remanufactured ones. Sadly NOS is quite rare as Minis use them.
Providing there is a 1/16-1/32" gap between the sychro ring and the gear then all should be fine. The synchro dogs on the gear should not be rounded or worn down.
Bob Beaumont

Thanks everyone, this has been a most useful discussion for me!
Given that both of my gearboxes would need some attention my first decision is which one to go for. There has always been criticism that the smoothcase box is 'weak', and after all it was fairly quickly upgraded for the original cars. But would it be ok with a 1098 in standard tune, driven moderately for low mileage as that is my reality these days. If this is a yes, probably, then together with my NOS parts that I have for this, plus maybe Richard's offer of needed, I will focus on a rebuild of this.

If a smoothcase isn't up to it then I will turn my attention to the ribcase that I have . Except as it's a MM one it maybe isn't much better plus so far it appears to be in worse condition.

Maybe I should ignore both and search for a decent Spridget ribcase but they are becoming rarer and pricey these days.
GuyW

The smooth case has a poorer synchromesh and the ratios compared to the Ribcase MM box are worse. parts for the ribcase even S/H are easier to source. Ideally a midget box is best as its the strongest and has the best ratios but a minor box is fine given yours is a 1098 and your not thinking of driving hard.
Bob Beaumont

Guy,
I also have a scrap ribcase MM box - it eat a gear, its dismantled now, so if you need any bits that are O.K you are of course welcome to them. As is the FMS I mentioned.

R.
richard b

If I manage to get to Spridget 60, there may well be some NOS synchro rings for sale.
Dave O'Neill 2

I have a copy of a table of part numbers for these A series gearboxes. But on close inspection there are errors. e.g. same part number for MK2 input shaft and MK3 3rd gear cannot be right! Then I remembered some discussion here about errors on this table and some corrections made.
Does anyone have a copy of an updated and correct version please?
GuyW

I believe Richard Wale did update it.
Dave O'Neill 2

It's actually Richard Wale's table as he posted in Oct 2017 that has the error l mentioned. Maybe he has corrected since then
GuyW

This thread was discussed between 09/08/2018 and 11/08/2018

MG Midget and Sprite Technical index

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