Gearbox Input Shaft Shimming and Measurements

[DaveWL]

I'm about to replace my input shaft but can find very little info that details the shimming process or critical measurements. The Haynes manual "speaks" to these concerns but doesn't give much in the way of specifics. Is anyone aware the key points that have to be covered off? Thanks.

 

Dave

[smiller]

You will need the BMW factory service manual, but even that is intended to be more of a reference for someone already familiar with the procedure than a tutorial on how to perform it.

 

If you are only replacing the bearings then I would mic the width if the races of the old and new bearings, you will probably find that they are within a few thousandths of each other in which case you can probably OK in using the existing shim pack. If you are replacing the entire shaft though then I would advise that you or someone perform the shimming procedure.

 

If you don't have the BMW test jig then you will have to be inventive and come up with a way to perform the required measurements to within .001 or .002 inches and it's not all that easy. A couple of very accurate spacing bars and a depth micrometer is all you need, but there is a lot of technique involved in getting consistent readings. If you are good with such tasks (and good at arithmetic, you'll know what I mean soon ) than it is doable, but if you are not completely comfortable I'd suggest letting someone with the appropriate experience take care of it. The acceptable range is only .004" and that's not much considering that without the BMW jig your cumulative measurement error will probably be half of that. But a good repeatable job of making the measurements can be done in the home garage, it just takes a lot of care and patience.

 

If you want to give it a go I and a few others here can answer any questions as they come up.

[duckbubbles]

This is where a good relationship with a tech with all the tools pays. The guy told me, "don't worry about the shimming, just put it together and go with it."

Being the anal type, I used his transmission shimming jig and shimmed it to the Nth degree. It shifts just as crappy as it ever did. Maybe it's not super critical after all.

 

Frank

[smiller]

Maybe it's not super critical after all.

After doing a couple I really don't think it is all that critical (on the M97 models with caged ball bearings at least) and not likely to affect shift quality much unless it is really out of whack. The operating end-float must be more than zero (because caged roller bearings should operate without external preload) and it shouldn't be too ridiculously excessive, but I don't think it's super-critical either. That said, if I were replacing an entire shaft I would check it for peace of mind. You definitely don't want a negative value for end float on the shafts.

[DaveWL]

Thanks a lot for the comments. I would really like to try it myself, but having no information on the actual procedure, its looking more like I will have to have a shop do the work.