MM Gearboxes

[smithskids]

Does any one know if the speedo drive on the std MM gear box was different if you had one of the alternate axle ratios instead of the standard 4.55 :1. ie. 5.286 :1. or 5.375 :1.I know the speedometers are different. My MM came with the 4.55 axle and speedo x70620/2 1600 tpm. I have fitted a 5.375 axle and a x70620/7 speedo and it doesn't seem to work. The speedos are identical inside except the gear wheel that drives the oddmeter has a 20 tooth gear driven by the worm on the 1000 tpm one and 25 teeth on the 1600 tpm one. Any ideas lads and lassies. Cheers smithskids.

[mike.perry]

The alternative axle ratios are listed in the manual and the Series MM parts book lists 9/41 (4.55) and 8/43 (5.375) diffs and 5/12 and 6/17 pinion gears. Only two speedos are listed, MPH and KPH so presumably the difference is in the gears and not the speedo TPM .
The early SII lists 8/43 (5.375) axle only and no ratios for the speedo drive. Two Speedos are listed with different part no. to Series MM.
Can find no reference to 7/37 special in parts book

[smithskids]

Hello Mike, I have a MM gearbox that I bought with the 8/43 axle, I will check the speedo drive pinions on both gearboxes and see what is what!! Thanks Iain.

[mike.perry]

I would check mine if I could get a socket on the pinion nut

[sparesman]

Hello Mike, I have a MM gearbox that I bought with the 8/43 axle, I will check the speedo drive pinions on both gearboxes and see what is what!! Thanks Iain.

Could the MM axle with the 5.375 diff ratio actually be one from an early Series 11 which used the same axle casing? MM speedos are 1600 tpm, Series 11 are 1000 tpm.

[smithskids]

Bryan, the axle No.is 19596. 8/43 5.375:1 all unf threads including the wheel bolts. The spare gearbox I bought at the same time is in good condition and No. is 27756. They had been standing in a garage for 20 odd years. I stripped them both and wrote all the numbers down. I had to change 1 ball race in the box as it was rusted up but everything else was as new. I didn't count the speedo drive spindle teeth as I thought they would be all the same
Regards Iain.

[smithskids]

Checked both my MM gearboxes and they both have the 5/12 speedo drive pinions in, I was hoping one may have had the alternative 6/17 pinion in but out of luck. Cheers folks and thanks for all the info. Iain.