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MG Midget and Sprite Technical - smooth case gearbox

What are the differences / implications - if any - of fitting a smooth case gearbox to a 1275cc motor - just undoing a PO's bodge.....
S G Macfarlane

The smmooth case unless it has been modified using 1275 Spridget internals is not as strong as the later rib case. The 1275 spridget box has needle roller bearings for the intermediate gears and closer ratios. The smooth case was originally designed for the 803cc engine pushing out 30 bhp. The 1275 puts out almost double that and more torque
Bob Beaumont

Thanks Bob
Are there also differences to the spigot and release bearing etc that would affect the clutch operation
Regards
Stuart
S G Macfarlane

The spigot is the same size but the input shaft is slightly shorter. The 1275 has a thicker rear engine plate than the 803/948 engine hence the slightly longer input shaft.
The release bearing fork is slightly different. The 1275 uses a diaphram clutch and the release bearing is different. The bearing is held in place in the fork with clips rather than springs.
Bob Beaumont

Stuart,

The pivot for the clutch release arm on the front cover will be at the wrong height (the ribcase and smooth case are different). I also think the pivot point is in the wrong place with respect to the distance from the centreline of the 1st motion shaft. The front cover plates are not interchangeable, the fixing screw pattern is different.

I may be wrong, but I seem to remember seeing a picture of a selection of release arms, and the distance between the pivot point and the centre of the carbon thrust pad varies between them.

Finally the carbon thrust varies between the 948, 1098 and 1275, but these are interchangeable, if you know what height you need.

As Bob has said, the smooth case was designed for an engine with less than 50% of the 1275 output, so is totally unsuitable for it.

The chart below allows you to identify which gearbox you are looking at, as fortunately the 1st motion or primary shaft part number is unique to each of the A-Series in-line gearbox variants. The part number is stamped on the primary shaft.

If yours has the part number AEA3032 stamped on it, it is from the later 948 Sprite/midget. This has closer ratios and needle roller mainshaft gear bearings, and is now quite rare. Still not suitable for the 1275, but if in good condition would fetch a good price on Ebay.

Richard



Richard Wale

Picture of the 3 carbon thrust variants below:

Richard


Richard Wale

Update of the part number table, now includes the release arms and carbon thrust bearings.

Richard


Richard Wale

Great, thanks for the table.
Very helpful when trying to piece together what i have and what i need. Although its enough to drive you bonkers.
The smooth case box has no part number on the primary shaft so cant confirm the type - anyway the box is on ebay starting at 99p if anyone wants it.
I have a couple of what i think are 1275 clutch forks - they are both marked 22G169 - not 22G168 as per the table
Regards
Stuart

S G Macfarlane

In BMC history, casting number and part number are often found to be 1 digit apart.
David Smith

The primary shaft should be stamped - worth rubbing it with a fine piece of wet 'n' dry.

The picture below shows the part number stamped on the primary shaft of a 1275 gearbox that I sold on Ebay a few years ago.

Richard




Richard Wale

Hi Richard
Thanks for that. There is definitely nothing on the primary shaft other than a MOWOG stamp, ive got other gearboxes so know how hard it can be to see sometimes.
Regards
Stuart
S G Macfarlane

I think the smoothcases did not always have a part number on the input shaft. I think it was introduced when the ribcase came in?

I guess one way would be to observe the number of turns the mainshaft achieves through say 10 turns of the input shaft when each gear is engaged.

Bob Beaumont

'observe the number of turns the mainshaft achieves through say 10 turns of the input shaft when each gear is engaged'

Yes, it would. The two sets of ratios are at the bottom of the part number table attached in an earlier post.

Richard

Richard Wale

This thread was discussed between 08/04/2018 and 10/04/2018

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.