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MG Midget and Sprite Technical - Crown wheel differences

I have 2 diffs both are 4.55 to 1 ratio 9/41 The crown wheels are stamped part no. BTA1347 and the other ATA7091. The BTA1347 is a much thicker unit than the ATA7091. Does anyone know which vehicles these were originally intended for. What crown wheel and pinion assembly numbers did these sell as?

P Batty

The same is true of 4.22 diffs, having either ATA or BTA part numbers.

I can't say which vehicles they were specifically fitted to. The BTA version may have just been a later, stronger development.
Dave O'Neill 2

Hi Peter,

The 4.55 ratio (9:41) came in the Morris Minor 1000. I use this diff (modified to fit) in my road going TC. (I use 4.2 or 3.7 in my race TC depending on the track.)

I have converted more than 40 Morris / Spridget diffs to fit TCs and TC racers but have never noticed different thicknesses in the same ratio. Just speculation, but perhaps the thicker crown-wheel is designed to fit that ratio in the housing made for the taller ratios. They have a bigger offset of the pinion towards the RHS of the car.

From what I have seen, the 5.3 , 5.1, 4.8, 4.5 ratios all come in a housing with the smaller pinion offset and strengthening webs only reaching the inside of the gasket flange. The 4.2, 3.9, 3.7 ratios all come in a housing with the bigger pinion offset and the strengthening webs reaching the outside of the gasket flange. The small offset/small web diffs have an oil filler/level hole in the housing but the big offset/web diffs don't have a hole. (They rely on a filler/level hole in the banjo.)

Has your diff with the thicker crown-wheel got the bigger offset (and also the bigger webs and no oil hole?)

Happy problem solving,

Bob Schapel
R L Schapel

Thanks Dave and Bob for your comments :-
Further info now that I have looked more closely at my collection of diffs.
A) 4.55 to 1 has thick crown wheel BTA1347 - Diff casting number is BTA548 - has strengthening webs that go to the flange - but has a hole blanked with Allen headed plug on the centre line of the axle (higher than where the normal filler plug would be) My understanding is this diff may have been bought brand new from the Morris Minor Centre but back in the early 1980's

B) 4.55 to 1 narrow crown wheel ATA7091 - Diff casing casting No ATA7033 has boss cast with filler plug and web strengthening does not go to the flange (as Bob mentioned)

c) 4.22 to 1 Crown wheel is thicker stamped BTA547 9/38 has wide strengthening webs - diff casting BTA548 but has no filler plug.

Question was diff A a very late 4.55 diff made for the Morris 1000 - this is the one I need to get rebuilt

All the manuals quote assembly not individual part numbers.

P Batty

Although the Minor saloon diff changed from 4.55 to 4.22 in 1962, I have a vague recollection that vans were still fitted with 4.55 after that.

Also, the A35 van was in production until 1968/9 and, although they used the same 1098cc engine as the Minor, there was also an 848cc engined version, which used a higher ratio diff.

I used to know all the details, but it's been a long time!
Dave O'Neill 2


It seems my speculation might have been right. I guess the more heavily webbed diff is stronger and your midget has a filler in the banjo, so it will suit your car well.

I have heard of people putting a "pad behind the crownwheel" to improve impact strength. I have never seen one of those either but is that what the Allen headed plug is? If so, the crown-wheel would run only a few thou from its inner end.


R L Schapel

This thread was discussed between 23/07/2015 and 25/07/2015

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