diff rebuilding

does anyone have any experience in overhauling the rear diff? mine is sweating a bit, but its not broken. just thinking forward u know.

Old thread, but this job is on my to-do-list. A distinctive slack in the drivetrain has prompted me to do the rebuild. Both the side bearings are quite wobbly and I'll have to check the pinion bearings aswell. I heard the helical gears which are connected to the half-shafts could be inverted (with a little machining) to make the whole diff "like-new".

Reading through the service manual makes the whole thing a little intimidating with all the preload measuring and pinion shimming :thinking-011:

If someone has pictures and advice on this one it'd be nice :wave:

I have opened the Q4 rear diff. As the diff were leaking, the plan was to replace the 3 seals in there and not change anything in the bearing set-up. Once open the bearings showed signs of heating and the plans changed.

I have done this years ago with success on an Alfetta, but it is not easy as you are always uncertain whether it is correct. This time I simply replaced all the bearings and seals with the spacers that were in position before. There were more slack on the pinion than I like, but it is as it was before and the same as on my spare diff.

If you have a problem with drive train backlash, I would go about it differently. The theory is rather simple once you decide how to go about it logically but it is a long process if I have to be certain of the adjustments.

I would strip the diff completely, change the bearings on a press with no seals fitted. Then with the pinion not fitted I would fit the Crown wheel/Torsen unit in its bearings and tighten by fitting the side plate. As it is taper bearings, it is possible to tighten the through 0.5mm, which will make them too tight. The theory is there must be a certain amount of pre-load on the bearings. It means if the bearings is too loose there will be a knock effect which will wear rapidly and unevenly, and may even break the bearing. If the bearing is too tight, it will even get tighter when it heats up and seaze.

What makes this tiresome is you can not adjust it like the front wheel bearing on a rear wheel drive car. Where you would have tighten the taper bearing until it would not turn and then slack it off till it is free with slight resistance. And you have the large wheel to turn and "feel" the resistance. The tightening on the diff bearings is done by stripping the diff and adding spacer rings to the outside of the bearings and you probably would split the spaced amount between the two bearings. You can push the side shaft stub in and even bolt a lever to it with 300mm per side, to feel how free the bearings are. When you are happy with the "tightness" of the bearings remove the unit again.

Now fit the pinion with new bearing but without the seal in the casing and tighten. This is not easy as you must hold the pinion(gear) still while tightening the nut outside(an air tool help). Now fit the crown wheel unit back and tighten. The crown wheel/ pinion adjustment is made by moving the pinion deeper into the casing, so that it mesh deeper on the crown wheel. If you smear some mechanics blue on the pinion gear, it will leave a trace on the crown wheel teeth. This line should be from the inside of the tooth, through the center to the outside of the tooth on the other end. The pinion flange(propshaft) should be not tight but have a small amount of slack when you move it side to side. You may have too strip the diff half a dozen times before it is perfect.

When you are happy that the clearances are good, you will have too fit the seals which are on the side shaft stubs and the pinion. The reason for not fitting the seals when you first assemble the unit, is the seals are so tight on the shafts, you will never be able to feel how tight the bearings are with them fitted.

The wear on the teeth of the crown wheel and pinion is also a factor in the adjustment, and will probably increase the amount of slack on the pinion flange. Unless you are very lucky, this can be a day's work if you have the equipment and a knowledgeable helper.

http://s1224.photobucket.com/user/co...rary/Q4%20Diff

I seem to remember from years ago, that the front diff was the first place to look when it came to driveline backlash. I'll dig some more info out later.

I read this in another Forum...
..."Both front and rear diffs have a pre-loaded pinion gear.The pre-load is achieved using a crushable spacer that deforms when you tighten it. The trouble is that if you apply lots of torque to the transmission the spacer can continue to deform leaving the pinion loose with the pre-load gone leading to movement of the pinion in use causing noise.
You can replace the crushable spacer with a solid steel one but it takes a long time to whittle it down to the right size to achieve the correct rolling torque."...

This sounds very very interesting, especially for high powererd cars.
If diff is open anyway, this is a good add-on Job in my eyes.
Juergen

Very interesting and useful stuff! Got started today as I dropped the rear axle and propshaft off. I have the seals and bearings for the side casing flanges and the front bearing to the "torque tube". Also the center support was totally ripped so I'll change that aswell. Also the u-joint showed some slack, but I'm not quite sure if that's ok. I will also change the rear axle bushes for PU ones. And there's also some rust in the boot area...

Flickr fotos

Nice Fotos,

Keep us updated, please

Juergen