Different transmissions question

[katy]

Our ’61 MM has a ribbed case transmission and I have a spare that is a smooth case.
I understand that they are interchangeable, but I also understand that the ribbed case is a better choice. Can someone explain the differences?
Is it gear ratios? Or?
TIA, Ken

[MColes]

I'm sure there's a difference in the syncromesh. Not sure what else.

[PSL184]

The ribbed case box is from a 1098 and the smooth case from the earlier engined cars (803 or 948 etc). The ratio's for the 948 box are the same as the 1098 but different to the 803. The big advantage is that the 1098 box is much stronger than the previous boxes so if you are running a 1098 engine or above you really need to stay with the 1098 box. I have one on my 1275 engine and it is coping well with the extra BHP but I doubt a 948 box would do so well.....

[bmcecosse]

The smooth case box is from the earlier 948 cc engine. It's a much weaker box - and yes -weaker syncromesh. If your engine is a 1098 - then it's not a straight swap. You will do MUCH better to source a good rib box if you need a replacement.

[mike.perry]

The ribbed gearbox I believe has a larger bell housing to take the larger clutch of the 1098 engine, however I think that you can just fit a 948 box on a 1098 engine. 2nd Gear is higher in the 1098 box, 2.172 to 2.374

[bmcecosse]

Only if you fit a 948 flywheel/clutch assembly.

[IslipMinor]

Apart from the improved 2nd gear ratio (well worth having), the main benefit of the 1098 and later gearbox is the change to baulk ring synchromesh from the earlier simple cone type. The 948/803 smooth case boxes had weak synchromesh, which was very noticeable, especially if swapping from a 'modern' to the early box, but the 1098/1275 is very good, assuming that it is in good condition as well.

[David53]

Is the early 1000 gearbox same as the 803? I always thought they changed boxes when they upgraded the engine - does the 1000 box not have a remote selector section wheras the 803 has the stick straight into the box?

[mike.perry]

803, smooth box, direct gear lever, poor ratios, weak.
948, smooth box, remote gear lever, better ratios, stronger box
1098, ribbed box, remote gear lever, higher 2nd gear, improved synchro.
1275, ribbed box, remote gear lever, closer ratios, straighter cut gears, reversing light switch

[bmcecosse]

Don't think the gears in the 1275 box are in any way 'straighter cut'! In fact - I'm sure they are NOT!

[IslipMinor]

There are 2 versions of the 1275 gearbox with 'A' and 'B' gears, the difference being the helix angle of the gears, and I think the later ones had less of a helix angle, hence Mike's comment about straightER, not straight (other than reverse!).

Being even more picky, the Spridget versions of the boxes both 948 and 1098 had closer ratios than the Minor/A35/A40 and needle roller bearings instead of bronze bushes for the main shaft gears. The 1275 box was only ever used on Spridgets.

[katy]

I have no way of knowing exactly where the 2 different transmissions that I have are from, other than the fact that the car came with a 1275 engine and the ribbed case box.
Is there any casting numbers or other identifying marks on them that can be used to positively ID them?

[bmcecosse]

A gearbox from a Spridget has a built-in reversing light switch. There is also a unique input shaft - sorry I don't know the number on the shaft - but someone who does know will be along shortly!

[mike.perry]

It appears from the Haynes Midget Manual that all the Midget boxes except the Frog Eye Sprite have the same ratios. Can anyone check this in a BMC Midget Manual?
The 1275 box when installed in a Minor and with a Minor gear lever has a shorter throw into 1st gear. The gear changes feel to me to be lighter. This is because the box normally has a short lever and the gear change is more of a wrist action. I have thought about fitting a Midget lever in the Morris but I think it would be a bit of a stretch

[bmcecosse]

Yes - all ribbed case Spridget boxes will have the same ratios - but not the same ratios as the ribbed case Minor box. Someone published a very useful note about all this not too long ago.

[mike.perry]

The standard Minor ratios plus the 1275 box are on the Series MM website under car. ident.

[MColes]

Is this the info you were on about BM?

It came in earlier than the change to 1098, as apart from the Frogeyes, all Sprites and Midgets had the closer ratios and needle roller bearings - in both smooth and ribcase.

The Spridget ratios are:

1 3.200
2 1.916
3 1.357
4 1.000

In comparison to the 1098 Minor:

1 3.628
2 2.172
3 1.412
4 1.000

There were also two different gearsets 'A' & 'B' used in the ribcase, which makes swapping parts more limited.

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