Diff removal and refit - leave the cage or not?

[356DNA]

2017 420r - BMW diff.

I need to remove my diff for an LSD rebuild (carbon plates failing) - is there any reason not to remove the diff from its carrier cage / bracket and therefore leave that aligned in the car or will this cause me an unforseen headache?

 

Many thanks

David.

[Jonathan Kay]

Private Message sent.

[ScottR400D]

I have had a lot of experience with this recently, having removed and refitted my diff 3 times.

It doesn't make a lot of difference in all honesty. The first two times I refitted the diff having left the carrier in place. The third time the diff was already fitted to the carrier.

All three times the diff ended up central but leaning down at the front at an angle of approx 1.7 degrees more than the engine. That's how it is, there's nothing that can be done. Yours may be at a different angle, there's no reason why they should lean down, it's presumably just sloppy tolerances in the chassis.

In terms of difficulty there's little in it. Taking out and refitting, carrier attached, means the handbrake cable needs to be released from the brackets up in the tunnel and fed through the bracket on the carrier to release it. Refitting is the reverse.

I made a simple carrier from a piece of shelving, about 250mm x 200mm with 3 little blocks on that locate on the flats at the rear sides and nose of the diff. When the diff is sat on them it's just about horizontal in both planes. I cable tie that to my jack and can then simply jack up near enough into the correct position to get at least the front bolts in, then jacking or lowering to get the remaining one in. Simple but means it's an easy one man job.

 

[ScottR400D]

By the way, there a story behind why my diff has been out so much, which I'll be posting about soon. You might not want to rush into having you diff rebuilt just yet.

[ChrisC]

Was waiting for your reply because I thought you're the best person to comment on the process.  Has your original thread been deleted?  I tried to find it this afternoon.

[ScottR400D]

Hi Chris. No but it was a long way down the list. I’ve updated one of them anyway now!  

[Shortshift]

ScottR400D said:

"All three times the diff ended up central but leaning down at the front at an angle of approx 1.7 degrees more than the engine. That's how it is, there's nothing that can be done. Yours may be at a different angle, there's no reason why they should lean down, it's presumably just sloppy tolerances in the chassis"

Well, that's interesting.  I got a digital level on mine last night and that's pretty much exactly what I found.  The diff sits in the car with around 1.8 degrees more nose-down attitude than the engine and gearbox.

I, too, had a look (and feel) around and could see no reason why this should be the case - there appears to be plenty of space (relatively speaking) for the diff to have been installed with correct orientation, on an axis parallel to the engine.  But short of having the diff carrier remade to position the diff at the correct angle it seems there's nothing that can be done - there's no apparent adjustment possible either between the diff and the carrier or the carrier and the chassis. 

I didn't look at the engine end of things (raising the gearbox tailshaft, rotating the engine around the front mounts) but I suspect there's no room for that sort of thing.

I suppose the question is:  is 1.7 or 1.8 degrees of misalignment excessive for a Hookes-jointed installation?

James

[ScottR400D]

The thing is James, that 1.7 degrees is relative to the Duratec, which itself slopes down at about 1.5 degrees to the chassis to enable it to clear the hood. 

If a car has a Sigma, which I understand is installed horizontally, and the diff is installed at the same angle that your car and mine appear to be, then the difference in angle is more like 3 degrees. 

Is that enough to cause a problem? I don’t know, they seem to work OK but who knows what effect that has on joints and diff?

i took this up with CC but didn’t get a clear answer, they don’t seem to know. 

I’ve had a good look, believe me, there seems no way to get the adjustment needed. 

[Shortshift]

Yup - as you say, that's 1.7 or 1.8 degrees difference between diff and Duratec; I hadn't appreciated that a Sigma could be nearly double that.  It's clearly not best practice and the objective should be zero misalignment to avoid vibration, noise and wear issues.  Maybe someone will come along and tell us what is considered to be an acceptable amount of misalignment.

Interestingly, I came across a reference that said the diff should be 1.7 degrees nose down relative to engine/gearbox...  But, this was to allow for rotation of a diff on a live axle car where the axle and diff rotate (wind-up) as driveline torque is applied.  Clearly not the case where the diff is mounted immovably to the chassis frame!

James

 

[Doctor]

James, maybe there is a rotation with the fix mount as well - by flexing the whole chassis. 

[Shortshift]

"...by flexing the whole chassis"

An extreme point of view!

James

[356DNA]

Thanks all - Scott - can you PM the details re the diff before i remove mine? Many thanks

[ScottR400D]

Done!

I would seriously recommend you consider the SPC diff for a 420R, if you’re faced with a full rebuild in any case.