Differential rebuild

[Spriteracer]

Happy new year all. Can anyone who has replaced the pinion bearings on the diff let me know if it is possible without a press please. i.e. by freezing the bearing outers and tapping the pinion through etc. I've done several sprite diffs like this but the Z4 diff is considerably bigger and uses a crushable spacer to set the preload which my sprite ones don't. (solid spacer and shims)

Most of the Youtube offerings seem to use a press but not all and I wondered how easy it actually is. I have the diff in bits and got all the old bearings off without a press and don't want to have to buy a press if not necessary. Many thanks for any advice.

 

[grannyknot]

It might be easier to take your diff to a local mechanic and have him press them out, you may be able to do it but it wouldn't be tapping, it would be hammering. The pinion bear are pretty big and deep.

 

[Spriteracer]

Thanks for the suggestion. The old ones came out / off without any difficulty - the outer pinion track drifted out of the casing ok although had to hit quite hard. There is no way I could see to use a press on this anyway. The inner track on the pinion itself, was partly cut with a small cutting disk and then it split by itself and came off by hand. One side bearing track came off with a bearing puller and the other one where there is no access for the puller by cutting as above. Scary but actually quite easy with a steady hand.....

The only mechanic I know and would trust near here is 40 mins away hence my question and I don't want to waste time going there or buying a press if it is not necessary. The only part I'm asking for advice on is putting the pinion back through the nose (outer) bearing.

 

[grannyknot]

I have a 20 ton press and when I rebuilt my diff it took maybe a 1,000 lbs pressure to install the pinion bearings.
If you are going to have a go at it just make sure the pinion shim or washer is on the pinion shaft before pressing the pinion shaft in, then you will need to support the pinion gear through the rear of the diff while you press in the front pinion bearing.
Also don't forget to drop the crush sleeve over the shaft before that front bearing goes in.
How are you going to measure the preload on the pinion nut?

 

[Spriteracer]

Many thanks for that, very useful info and tells me what I needed to know. A press will be on order very shortly... It will also be useful for other projects.

Re the preload, I bought the bearings kit from Bearingkits.co.uk, which comes with everything including a tool to use with a spring balance to measure it. I also had a long chat with Mark, the owner, who was extremely helpful and happy to assist further if needed.

As a bit of additional info I got from him, I also asked about a kit for the earlier auto diff as I have one of those in the car at the moment. His advice was that one of the pinion bearings for that diff was a special and is no longer available. He did also say there may be a way around it but as I don't need one at present we didn't go into detail. Maybe worth a call to him if anyone has in issue with one of those.

 

[grannyknot]

Re the preload, I bought the bearings kit from Bearingkits.co.uk, which comes with everything including a tool to use with a spring balance to measure it. I also had a long chat with Mark, the owner, who was extremely helpful and happy to assist further if needed.

Nice preload tool, would you mind posting the measurement between the centre hole and the spring pull hole?
I could make a mock up of that, good to have on hand.
Thanks

 

[Spriteracer]

Yes certainly will do when I get to the garage at the w/e. Press arrived yesterday - 20T Chinese £179.95 delivered. Not put it together yet but the parts seem ok. It uses a bottle jack rather than a hydraulic ram but should be fine for most things. Annoyingly I also have a 10T body jack (don't ask) which is very similar to the ones used on the more expensive presses. I looked at building a frame to use that, but the cost of the steel was more than the whole Chinese press and I couldn't easily lay my hands on enough scrap.

 

[GuidoK]

Nice preload tool, would you mind posting the measurement between the centre hole and the spring pull hole?
I could make a mock up of that, good to have on hand.
Thanks

It doesn't really matter how much the distance between center hole and spring hole is. It's just a parameter in the calculation.
It's best to set the distance in such a matter that you get a reading on your spring balance/scale that is somewhere in the middle of the range/scale as that is the spot where a spring balance usually is most accurate.

So for instance if you have an 188k with Koyo bearings, you need to get a pinion bearing preload rotational drag of 1,27Nm - 2,65Nm (these numbers are manufacturer specific (and of course diff size/type specific) and can be found in TIS, but only under older model bmw's!), and if you install a new seal, add 20Ncm to that.
So say you want to set the preload at 2Nm, and your distance to the centerhole is 20cm, then at 20cm you need to apply a force of (100cm/20cm)x2Nm=10N (=1kg). That would be great if you have a spring balance/scale that goes to 2kg.
If you have a spring balance that only goes to 1kg, and you want to have it about in the middle (so 500g) when applying 2Nm, then your distance to center needs to be 2Nm/5N=0,4m or 40cm.
So if you're making your own tool (which is indeed really easy, a strip of iron or wood will do and you need a drill), the length is best set according to what spring balance you have and what torque you want to apply

 

[grannyknot]

Nice preload tool, would you mind posting the measurement between the centre hole and the spring pull hole?
I could make a mock up of that, good to have on hand.
Thanks

It doesn't really matter how much the distance between center hole and spring hole is. It's just a parameter in the calculation.
It's best to set the distance in such a matter that you get a reading on your spring balance/scale that is somewhere in the middle of the range/scale as that is the spot where a spring balance usually is most accurate.

So for instance if you have an 188k with Koyo bearings, you need to get a pinion bearing preload rotational drag of 1,27Nm - 2,65Nm (these numbers are manufacturer specific (and of course diff size/type specific) and can be found in TIS, but only under older model bmw's!), and if you install a new seal, add 20Ncm to that.
So say you want to set the preload at 2Nm, and your distance to the centerhole is 20cm, then at 20cm you need to apply a force of (100cm/20cm)x2Nm=10N (=1kg). That would be great if you have a spring balance/scale that goes to 2kg.
If you have a spring balance that only goes to 1kg, and you want to have it about in the middle (so 500g) when applying 2Nm, then your distance to center needs to be 2Nm/5N=0,4m or 40cm.
So if you're making your own tool (which is indeed really easy, a strip of iron or wood will do and you need a drill), the length is best set according to what spring balance you have and what torque you want to apply

Thank you, makes perfect sense.
Whenever I rebuild a diff I set the preload by feel, then take it up to my local diff specialist and have him check my work, so maybe I can save myself a few bucks next time.

 

[GuidoK]

The other 2 ways to measure the friction is with a precision beam type torque wrench:

or with a special preload gauge:



That last tool is the style of tool bmw recommends, but these are hard to come by and very expensive.
They are however the easiest to use. You have to measure the preload when the axle is in motion. So you have to read the tool while you're slowly dragging it, turning the input shaft at ~50rpm, so little under 1turn per second (with a spring gauge this is not the easiest tool, but the range for the preload is quite big (about 100%)).
So it's not the starting torque you're measuring. This is why you also can't use any click type torque wrench for this setup.