XSeries-forum

The insides of my 3.91 rear diff have tried to escape through the case rendering it scrap.

There are none on the bay of e.

I've phoned 18 BMW breakers and none have a 3.91 but two have now told me you can fit the 3.73 so long as you do both front and rear together.

Aparently the front is a direct bolt on and you just have to swap over the 3.91 output flanges to the 3.73 on the rear diff. I've been offered a matching pair of 3.73's for £450 and there are loads out there should a diff fail again.

Anyone have any experience of this swap or know either way whether it can be done?

Yes, I saw this post but Quarry bmw breakers thought the swap dubvious at best and FAB bmw breakers specifically told me it can't be done unless you change the transfer box too.

Does anyone have a working, none leaking 3.91 rear diff sat in their garage/shed?

If both front and rear diffs are the same ratio then the transfer box will be happy


The problems will be if you have different ratios front to rear

Its my car in the other post and it definitely works, it's still driving perfectly and I've given it plenty of beans to try and make it fail.

Edit to say I didn't change the transfer box but had to swap the flanges over on the front and rear diffs.