R180 runout question

[djwarner]

My R180 has 180K miles on it and has the developed the usual whine between 45-55 mph that changes pitch between driving and coasting.

The manual says this is an indication of excessive runout between the pinion gear and drive gear. I have previously replaced the wheel bearings all round and have been impressed with the change in ride.

When I first got the vehicle 18 months ago, the side seals showed evidence of leakage and one half shaft was installed backwards. After the mechanics replaced the forward differential mount, side seals and re-installed the half shafts, I noticed a clunk when shifting from driving to coasting and vice versa. The u-joints were okay but there was a noticeable amount of free play (read runout) in the differential.

After my success with the wheel bearings and I have read through the FSM several times in anticipation of replacing the bearing in the differential. I have learned several things:

A. It appears that the shims on the pinion shaft serve to establish preload on the pinion bearings and to establish the correct height of the pinion gear to the side shafts. The preload is required to minimize the axial movement of the pinion shaft. The correction to the height is needed due to variation in the various machined dimensions of both the pinion shaft and housing.

B. The left/right position of the differential case assembly is established by shims installed under the side bearing retainers. These shims establish the preload of the side bearings and the clearance between the drive gear and pinion gear. The variation in clearance of the drive gear and pinion gear determines its runout. These shims come in various thicknesses from 0.2mm to 0.5mm. Again these shims account for variations in machined dimensions including bearing heights. The various machine dimensions are stamped on the parts to determined which shims are to be used.

C. The shims in A. correct heel or toe contact errors in between the pinion gear and drive gear.

D. The shims in B. establish the clearance between the pinion gear and drive gear. This is a major contributor to runout.

E. Page PD-8 of the FSM manual says to inspect the pinion and drive gears for surface defects and runout in excess of 0.08mm and to replace the pinion and drive gears as a set if needed.

F. Pages PD-12-13 describe checkout after re-assembly and specify drive gear/ pinion gear runout of 0.1mm to 0.2mm - This is greater than the allowed runout in E. ?!?

G. Correction for excessive runout or improper preload under F. is to adjust the shim thicknesses under B. by trial and error. The side bearing preload value is less on re-used bearings to account for wear and may require adjusting shim thickenesses.

The various shims are sold in sets of 10 - if you can find them. The pinion/drive gear matched sets are rare and can cost more the a used differential. The side bearings are relatively cheap. Pinion bearings are expensive.

Considering the above, I have the following questions:

1. If the pinion gear preload is acceptable and no axial runout found, would you consider changing the side bearings only?

2. Since one half shaft was installed backwards, what are the odds that the left and right side bearing retainers and or shims were inadvertently swapped resulting in a shift in the differential carrier assembly and an excessive runout?

3. Assuming bearing heights should be pretty consistent, would you consider changing bearings and re-using the same shims?

4. Has anyone replaced bearings without swapping the pinion/drive gear set and what was the result?

I am also open to the opinions of our more experienced members.

[Zedyone_kenobi]

You are done a wonderful job of explaining things. The diffs are something I have never had the guts to tackle. The amount of spare parts and shims are just to hard to come by. I have been amassing spare diffs over the past 6 years, I am up to three. My OEM diff. Another 71 diff I bougth but never used, and the 2004 STI diff I am currently using in my car. I would love to find a 3:54 diff from an automatic Z. It would be darn near perfect. However, I am also amassing transmissions. I am about to pay a buddy for his 5 speed.

Back to your question. I have always consdered the rear diff to be a LRU (line replaceable unit). I do not pretend to have the knowledge base or time to tackle a proper rebuild. My advice would be to pick up a used diff first and then try. so you have a back up.