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MG Midget and Sprite Technical - Ribcase gearbox differences

Hi all

What the the key differences between a 1098cc and a 1275 rib-case gearbox for MG midgets and Austin Healey Sprites please?

I've got Richard Wale's very useful table on gearbox ID from an earlier thread - thanks Richard (did Nigel later suggest this has some errors? If so what are they please): http://www.mg-cars.info/mg-midget-sprite-technical-bbs/smoothcase-gearbox-numbers-2017080712090219150.htm

My reason for asking is can you fit a 1275 gearbox (22G229 on input shaft) onto a 948 engine? How does this differ from how you fit a 1098 gearbox (22G172) to a 948 engine? (follow the recipe in: https://www.scottysretroworkshop.co.uk/wp-content/uploads/2020/08/MIDGET-SPRITE-SPECIAL-TUNING-MANUAL-948CC-ENGINES-Ref-C-AKD1021C.pdf & https://www.scottysretroworkshop.co.uk/wp-content/uploads/2020/08/TUNING-BMC-SPORTS-CARS.pdf - of thick backplate from a 1098, 1098 flywheel, 1098 clutch cover, plate, release bearing & arm. Thanks to Alan A for pointing me in the direction of this info).

My reason for asking is that 1275 Midget gearboxes seem more easily available compared to 1098 Midget ones, noting the 1098 ribcase (and the 1275) has the same ratios at the MkII 948 box.

Thanks
Mike
M Wood

PS I forgot to mention getting the correct gearbox front cover as well as clutch arm etc http://gerardsgarage.com/Garage/Tech/TO_Bearings/tobearings.htm
M Wood

AFAIK none except the reverse light switch on the remote housing
David Smith

ISTR the gears have different angles between 1098 and 1275. The 1275 is superior as you'd expect. Someone else will comment more on back plates and clutch pushrods hopefully.
Bill Bretherton

Mike
If fitting ribcage gearbox onto a 948 you do not change the gearbox front plate to a smooth case one as they are not interchangeable.

Use the ribcage clutch release yoke: be it 1098 or 1275 although you need these from a Sprite / Midget as these are for a hydraulic clutch whereas a Morris Minor is mechanical, although a Morris Minor one can be adapted.

The same procedure is followed for fitting any ribcage to the 948 as it is the ribcage bell housing that differs, in dimensions, on the two gearbox types. This is why I made my own bell housing from a smooth case gearbox when fitting a type 9 gearbox to my 948 (ish) Sprite.

Last year I did as you are want to do in fitting a ribcase bell housing to a 948 as type 9 bell housings are ribcase. In this case the conversion was for someone living in France.

An article appeared in the March Edition of the Midget & Sprite Club Magazine. Members can access the archive on the club website however I cannot post pictures for you as I get rid of them after publication.

(the attached picture appears to be a later ribcase bell housing that I made for another type 9 conversion)



Alan Anstead

Bill is half-right int hat there were Type A and Type B gears, using different helical cut angles. However this change was fairly early on in 1098 production to remedy reports of noisy gearboxes from new. I'm not sure the actual change point is known. Anyway it won't make any difference to the fitting to a 948 engine as the changes were purely internal. Just be aware that Type A boxes are virtually impossible to rebuild due to the lack of new spares.
David Smith

Thanks all, really helpful.

My follow up daft question follows on from Alan’s very helpful explanation about bellhousing and backplate differences. So have I got this correct, that if I already have a thick (1098) engine backplate on my 948 engine and a 1098 clutch (and likely 1098 flywheel) which is currently fitted to a smooth case gearbox, then I should be able to put a 1275 ribcase gearbox on using the same 1098 clutch, flywheel and backplate setup (making sure I have the correct clutch arm and 1098 release bearing?).

Thanks
Mike
M Wood

The short answer is yes!!
Bob Beaumont

Thanks Bob

I am a slow learner!

Cheers
Mike
M Wood

This thread was discussed between 23/01/2021 and 27/01/2021

MG Midget and Sprite Technical index

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