Diff rebuild pointers
For this, you need a strong bench vice and a clean workbench, lots of patience and a ‘feel’ for things mechanical, because this is not like changing discs and pads. Take extreme care at the cleaning, examining and reassembly stages because there is no room for error. If it doesn’t ‘feel’ right, that’s because it isn’t. There are no official torque figures and the mechanics at Gear Change rely on decades of experience. With the diff in pieces, you may decide it’s too far gone and either buy a reconditioned unit or send it to Gear Change to be rebuilt. If the planet gears can be used again, you’re looking at £576 for a rebuild; if they’re not, it’ll cost £720. Bear in mind that BMW will charge £2000 for a new one and, with used units, you’ll still be buying a diff with a finite lifespan before it starts to whine.
Removing the bearing inner races can be a problem. One approach is to grind them flat on one side and split them with a cold chisel. Refitting is greatly eased by leaving the diff and pinion in a freezer for a few hours and heating the bearing races in very hot water for a few minutes before fitting – this expands them enough to make them easier to refit. Conversely, when fitting the pinion outer races, get the diff casing as hot as you can and freeze the bearing races. Spend half-an-hour using emery to clean the bores in the diff casing where the races sit – just one burr or speck of dirt and they won’t sit perfectly. Our fully rebuilt diff will hopefully last another 100,000 miles. Even filled with oil, these ball bearing-type differentials are much easier to turn over than the older taper roller bearing ones. Many people say the ball bearings were never really up to the job, but we think they should last a lot longer given new oil every 25,000 miles.