New Diff 3.91 OPEN - Now the fun begins!

[e34.535i.sport]

So I got the new 3.91 open diff (Aka tyre shredder! ) from a 735i today.

Looks in pretty good condition - the oil came out pretty clean too which is a good sign. I'm hoping to get it in next weekend, so ordering new hardware/seals etc this week.

Question's:

1. What diff fluid should I use? After looking through the archives Redline seems to be the #1 choice, however it seems this is less readily available in the UK. I contemplated buying some Castrol (semi-synthetic) 75w90 today but thought I'd check first. If my best choice is go to the dealer I'll do that.

2. How much fluid will I need? (I searched extensively on this but absolutely no clue on any thread I found and the haynes is eluding me)

3. What should I renew... I'm thinking new bolts for the flanges... #6 in this diagram.
http://www.realoem.com/bmw/showparts...73&hg=33&fg=10

4. I plan to keep it for a little while to ensure the 3.91 works out, but is my 3.64LSD worth a shiny penny? If/when I sell it how much should I be looking for bearing in mind it's in absolutely perfect working order?!

Thanks... I'll keep updating this thread and get some pics of the swap when i do it!

[632 Regal]

75w-90 is fine for an open differential the LSD takes the thicker 140 weight dope. Try to use a full synthetic instead of the semi.

Are you going to replace the front input seal? If so good luck in getting the proper preload on the bearings.

I think mine took 2 1/2 litres to fill it

3:64 is a perfect gear for the 530 and I suspect a lightening upgrade for a 540. I paid around a hundred for mine a few years ago.

[Paul in NZ]

why not swap things over so you get a 3.91 lsd?With the extra go you will need it even more I suspect

[e34.535i.sport]

Thanks - I will make sure I get fully synthetic, I want to do this thing properly!

No I'm hoping I don't need to mess with the front end, if my flange is the same it can stay as it is. Fingers crossed!

[e34.535i.sport]

why not swap things over so you get a 3.91 lsd?With the extra go you will need it even more I suspect

Hmmm... The dude who I got the diff off suggested using the LS unit out of mine in this diff but suggested you need someone with experience to do it. I'm not sure if I'd trust any of our local 'specialists'! Also it would, I assume mean removing the front flange and having to preload the bearing. If my flange is different and I have to swap any way I might do some research and consider doing the lsd swap myself.

[632 Regal]

In theory it would be easier to drop lsd into the new case assuming the input flange is the same. As that would need to be done is setting the lash with proper side shims and preload. I wouldn't be afraid to do it as long as I didnt need the unit back in case the shims or shim kit needed to be ordered. Without any experience I wouldnt even fathom of such a project.

Thanks - I will make sure I get fully synthetic, I want to do this thing properly!

No I'm hoping I don't need to mess with the front end, if my flange is the same it can stay as it is. Fingers crossed!

[shogun]

75W-90 1,9 Liter

[e34.535i.sport]

Without any experience I wouldnt even fathom of such a project.

That's a roger

[e34.535i.sport]

75W-90 1,9 Liter

Brilliant thanks!

[e34.535i.sport]

Just been out to further clean the new diff up, and also got rid of the two pesky flange bolts that rounded... Angle grinder then molegrips made short work of them!

Also went underneatht the car and sprayed the bolts with WD40 ready for next weekend when I'll do the swap.

Seals will be ordered tomorrow, and I might ask the dealer if they do fully synthetic oil for it as I can't seem to get hold of it anywhere else... Bar ebay which will cost me FAR too much!

The countdown begins!