Originally Posted by deanwoolsey
It's 1.4 mm, about the diameter of a pencil lead. I do not think that a failure would cause this, but I've never pulled one apart until yesterday. My bearing is still pretty smooth. I was quite surprised at how far it is off when John told me .05 to .10. Amazing it lasted as long as it did.
I have cut open many bearings that felt so smooth I doubted that they needed to be replaced. You really do have to cut them open to visualize the races.
I'm not sure what you measured to come up with 1.4mm but that makes no sense to me for either the preload amount, or the total shim thickness. In over 50 drives, I have removed only one 1.0mm shim; I have never seen a combined shim thickness of 1.4mm. It doesn't make sense to me.
(Addendum: Hummm..... maybe you are measuring the shim behind the tapered roller bearing? In that case 1.4mm makes sense. However, that shim is used to adjust crowngear/pinion gear backlash, it is not the preload shim. The preload shims are thinner, much larger in diameter, and usually found stuck in the crownwheel bearing recess in the FD cover.)
What was the existing shim thickness? There would have been one or two shims in the crownwheel bearing seat of the FD cover. If there are two, one of them is very likely a 0.15mm shim. The other is likely to measure between 0.20 to 1.0mm.
Now that you have the FD disassembled, I suggest you pull the bearing off the hub if you have not, and cut it open for inspection. Since your chip detector warning is the reason you are into the FD, we are very curious as to what the extent of bearing degradation is. I hope you'll be able to post pics of the bearing.
Inspect the FD housing for the "creeping input pinion needle bearing race" (see earlier posts for pics of this). Also check for the "loose tapered roller bearing".
Having ruled out the presence of either of these two occasionally coincident problems, press on a new crownwheel bearing. Press the new bearing on using only the inner race or you will damage the bearing. Alternatively, heat the new bearing to 250F and put the crowngear hub in the freezer for 30 minutes and the bearing should drop on without resistance. Careful you don't get the bearing canted or cockeyed on the hub.
Once you have the new bearing on the hub, you are readly to start measurements for shim thickness. My video, which has a few errors and poorly edited still shows a valid and reliable method of measurement as originally shown to members of this board by DMAN.
Alternatively, you can use the method described in the BMW Service manual which requires a special tool to stabilize the bearing for accurate measurement. The bearing, being a "sloppy" Class C bearing, will tilt significantly when one side is pushed down. This tilting of the bearing will cause a huge measurement error. (I suspect that this factor may have been a factor is BMW have set up so many FDs with excess shim thickness.) Because of the tendency of the bearing to tilt when making measurements for the BMW Service Manual method, the bearing needs to be stabilized and multiple measurements taken on opposite sides of the bearing (180degrees from each other) to ensure that identical measurements are obtained around the bearing.
Addendum: I just reviewed the Tapered Roller Bearing Preload Check section in the Clymer's Manual. (I have always used the BMW Service Manual and had been unfamiliar with the write up in Clymer's.) Clymer's is basically a repetition of the BMW manual and simply states: "A measuring ring (BMW part No. 33 4 601) and a vernier caliper or depth gauge are required for this procedure." Does anyone have this BMW tool? I've never seen one, and only a diagram rather than a photo of it is shown in the manuals. I have wondered to what extent this tool prevents bearing tilt during measurments.