M56, M59 Gearbox differences

[claymore]

Right, can someone explain to me exactly what the difference is, why can the 850r viscous coupling not be fitted to a T5?

[Jamest5r]

The shafts are thicker, thats about it i think

[p fandango]

the external casing is different on the M59, think the viscous/driveshaft locks into the casing somehow

[claymore]

Could a hybrid driveshaft be made?, and why is the M59 deemed weaker? what brakes on them?

[Dangerous Dave]

The M59 has an extra splined output shaft like the M58 AWD one does, which comes straight off the ring gear/diff case. The offside driveshaft has a viscous coupling on it which locks up when there is a difference between the two outputs from the gearbox. So its an alternative to a LSD.

Driveshaft with viscous coupling


Splined output shaft (note very worn, from my AWD)


Ring gear and splined shaft (again worn splines)


Other than that the gearboxes are the same, so I can't see why people say they're weaker TBH. The only weak point is the splines on the output shaft can wear.

[Yosser]

I *think* the diff is the perceived weak point, possibly due to extra torque loading from the viscous coupling arrangement.

[claymore]

Thanks Bomb, just the sort of reply I was hopeing for

[Dangerous Dave]

Oh and the ratios might be different (been a while since I last saw them), think pedro posted a list somewhere.

[Yosser]

I'm pretty sure the ratios are the same as the M56H. My M59 was certainly rebuilt using a gear set from an M56 (and I assume it was an H).

[Dangerous Dave]

I'm pretty sure the ratios are the same as the M56H. My M59 was certainly rebuilt using a gear set from an M56 (and I assume it was an H).

Ah yes, you're right bud, found the table.