Gearbox Ribs

[Nourish]

Why did BMW add ribs to the gearbox sides on 1979 onwards bikes - were they cracking there - or just a styling job?

[barryh]

The ribs were introduced to stiffen the case. A usually authoritative source claims the longitudinal ribs reduced the tendency for the case to stretch in use which would impact on the input shaft end float.

[Duane Ausherman]

I was out of it by that time, so don't know if it worked or not. However, it is a very good idea. The input shaft end play would vary quite a bit with temp and that was critical. Too many were too loose and a grabby clutch resulted.

[Ray from Indiana]

It would seem that the ribs reinforce the case laterally as well as longitudinally.

If shaft end play was noticed as a problem I wonder why they didn't reinforce the rear cover? Adding some ribs there would have increased the section modulus of the cover and make it more resistant to bending. Easy for that cover to flex.

[ME 109]

Quite so Ray. My experiments show the cover to flex several thou with just hand pressure.

[Duane Ausherman]

It is so tempting to do the spacing while the case and cover are hot from the shaft insertion heating. Not going to work out OK. One should put the box at the lowest temperature that is expected to result in actual use. Otherwise the shafts are in a bind when the assembly cools off. That can only shorten bearing life.

As usual, ME is right, a stiffer cover would be a very good thing and probably eliminate more expansion than the ribs added to the case. They tend to keep the case from bulging more than expanding lengthwise.

[ME 109]

As usual, ME is right

'Usual' must extend down to 51% then?

I was only agreeing with Ray.

[George Ryals]

ME did you check the cover flex with it bolted to the case?

[ME 109]

ME did you check the cover flex with it bolted to the case?

Good point Ray, no I didn't. Would be interesting to see if the flex was still possible with the cover bolted.
I suspect it would be, perhaps not as much.
I have found that input shaft 'free play' can be lost by torquing just one last cover bolt. That was even after a basic lapping of the rear cover.

[Duane Ausherman]

In case it hasn't been mentioned, the rear cover can be less than flat. One should at least file of the bumps at the holes. They vary a lot too.

Once it is together, we always would use our plastic hammer on the shafts to further seat things. We once found the shafts to still be tight. We heated it up, did some banging and cooled it off again. It was just fine then. Our math was OK, but something didn't seat the first time.

Always, always bench test a transmission before installing it. Don't ask.