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MG Midget and Sprite Technical - Checking a gearbox

Wet day. First proper rain for 6 weeks. Started 24 hours after the water companies said we should be careful about water use or there would have to be hosepipe bans. Seems to have worked.

Anyway, what to check on a gearbox? I have 2 smooth case boxes in the shed which I have never used. One is an original Frogeye one and the other looks the same but is an A35 box. I have not had either in a car so they are untested. What should I inspect as at least a first check to see if either is worth considering for my Frog. Both seem to select gears OK. I will remove the side covers and turn the gears to look for chipped or damaged teeth. And I can look for tell-tale debris in the oil. But beyond that, what specifically should look out for?

Its difficult to properly check a box without taking it to bits. A view through the sidecover will of cousrs let you see obvious damaged teeth. The first/reverse/laygear gets the worst damage as there is no synchro but its difficult to see them. The amount of lateral movement in the 3rd and first motion shafts may also give a clue to the condition of the spigot bearing and main bearings. The big problem is you cannot inspect the layshaft/laygear bearings (the Achilles heel of A series boxes)or the synchromesh. The early smooth case boxes do not use baulk rings for the synchromesh as the later ribcase does, instead relying on a brass cone shrunk and machined onto each gear. The brass wears especially on 2nd and its a major and expensive bit of machining to replace them. I think some firms (Heathrow Transmissions)can supply the smooth cases with later baulk ring synchro. The oil will probably have a nice brass sheen given the way the synchro works so not easy to spot any other issues. The A35 and Frogeye smooth cases are identical internally apart from clutch mechanisms.
Bob Beaumont

Pretty much was Bob says.

You can (just about) see the first gear teeth on the laygear. They will often be chipped or broken. The first speed gear suffers similar damage, but it is hidden by the selector fork.

If you're lucky, you may be able to make one good 'box out of two, but as the same bits tend to wear, that may not be possible.
Dave O'Neill 2

Guy,as Bob says. I have a smooth case which I haven't stripped as I'm using a ribcase, which I've rebuilt. Of course if you're putting a 948 engine in then you'd ideally use a smooth case. The problem is availability of parts for smooth case boxes plus the synchronisers are virtually impossible to replace. The "dog" teeth are important, these are the smaller cogs attached to the gears which engage with teeth in the synchro sleeve for 2nd, 3rd and 4th. Wear in these teeth or the sleeve can cause jumping out of gear AIUI.

Ribcase gears can be fitted to a smooth case box but machining is needed. You could, of course, try a box to see if it works but I realise the effort needed to remove/ install g/boxes. Or maybe strip the two boxes, making one out of two and replacing what you can get, especially 1st/ reverse gear, bearings, laygear/ layshaft and bush between 1st and 3rd motion shafts.
W Bretherton

So, would it be better to strip them down to assess parts? Or maybe just leave them both for now and test them by fitting in the car when the time comes? Its a quick enough job to fit and remove an engine/gearbox in a Frogeye to test them.

(Sorry Bill, I was typing that whist you posted your advice - saying pretty much the same thing, though I seem to think to R&R a Sprite engine is easier than you do!)

If you don't mind removing/replacing engine/box two or three times then maybe try them out. I don't think it hard, just a bit tedious to repeat!
W Bretherton

I know I can remove the engine and gearbox from my '71 car in just over an hour and replace it in 1hr 20, and believe the Frog to be easier, especially if I don't refit the bonnet between times.

I have some time to consider it anyway! I was just tempted to do something mechanical rather than bodywork on a wet afternoon. Probably best not to start until I have cleared some workshop space anyway!

As suggested I be tempted to strip both and choose the best bits although you may end up with big pile of worn out bits... I recently rebuilt a rib case using 3 donor boxes and still ended up replacing the the laygear/first gear and reverse gear and layshaft. All the bearings were renewed as a matter of course. Because the synchro was not machined on the other gears they were ok and was possible to just renew the baulk rings providing as Bill says the selector teeth were not worn . With the smooth case you have to replace the whole gear as the synchro is part of it.

Personally I would consider using rib case internals inside a smooth case. The synchro is far superior and the ratios even using morris minor gear sets better. The ratios of the standard frog/a35 box have a big gap between 2nd and 3rd. When the Mk2 sprite came along it had far better ratios which remained virtually unchanged until the 1500 midget
Bob Beaumont

I'm going to have the same problem. I've got 4 1275 boxes in total, none of which I know anything about. I've got two of them up for sale but I've only really chosen the ones to keep by the way they look!

So do I just fit one and hope for the best, or do I spend 400 and have it reconditioned beforehand? Both options have major pitfalls, engine out/in etc or 400 (and even then might not be right!).

Anyway, it will be months/years before I have to make that decision, in the meantime I suppose I could read up on gearbox rebuilding and give it a go.
John Payne

John, if you can strip and rebuild brakes, suspension etc. then you can strip and rebuild a gearbox imv. There are fiddly bits, of course, but you just have to be systematic. If you've 4 boxes and mess one up then so what? You'd get it right the next time.
W Bretherton

I have previously rebuilt a T9 box fitting new syncros, and main and layshaft bearings. All pretty straightforward but its a lot easier seeing what's what in a T9 with the top removed.

The stage I am starting with the smooth case boxes is to find out if they are useable with what I have. If I need to start replacing bits I would want to take a step back and reconsider other options like a rib case, or even buying an upgraded offering from Heathrow Transmissions.

First things first though - to identify if either of these boxes is worth using as is, or with minor replacement of parts.

I am not, incidentally, going for much beyond standard as regards the engine in this car. As I already have a heavily modified 1275 (1380) car the idea with this is to keep reasonably close to the 1950's character of it.

Delighted to hear it, Guy. That's the way to do it.
Nick and Cherry Scoop

Maybe I'll have a go then, another string to ones bow and all that!

I'm actually ditching the 948 (not literally, it will live under the bench!) in favour of a standard 1275. For starters the 948 is very tired, it would need a new crank and I already have a spare 1275. I also don't have a smooth case box so it's not going to be an original car.

John Payne

One thing if you have oil in the gearbox then you can put a drill on the input shaft and test the thing through the gears to check the synchro operation and get an idea on noise. IIRC you need a 1/2" drill to grip the spigot section.
David Billington

That sounds like fun! I have a 1.2Kw 18mm chucked wrist breaker of a drill that should power up a gearbox quite well. ;-)

You do need access to a press to take these apart and put them back together properly. A properly rebuilt box is going to cost at least will almost certainly need a new lay gear cluster (they are always worn on 1st/reverse) 1st gear hub and rebuid kit. That's around 360 in bits alone.
R Cohen

At this stage l was looking for advice on what to check, what to look out for and how to assess so that l can choose between what l have available and whether either is useable 'as is'.

The details on what is involved in reconditioning is a different question and may not arise anyway. Either because what l have is ok, or because what l have is so far gone that l just need to write out a cheque to Heathrow Transmissions! Their price for a recon smooth case is around 550 depending on the options chosen which is a lot but given the cost of parts and expertise isn't bad at all


There are 2 versions of the 948 smoothcase gearbox:

The one fitted to the Minor/A35/A40/Frogeye 948 engine cars.

The one fitted to the Midget MkI and Sprite MkII 948 engine cars.

The differences are the ratios for 1st, 2nd and 3rd and the mainshaft gears run on needle roller bearings in the Midget MkI / Sprite MkII box.

The way to tell exactly what you have is by the input/primary shaft part number in the table in the attached image:

22A141 is for the Minor/A35/A40/Frogeye

AEA3032 is for the Midget MkI/Sprite MkII

Richard Wale


Slight update to the table to show the original part number for the 948 primary shaft of 2A8359, which was superseded by 22A141.


Richard Wale

This thread was discussed between 15/05/2017 and 22/05/2017

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