It’s no secret that the small case 168 differential on the 4-cylinder petrol 1 Series and E90 3 Series (plus the Coupé, Convertible) as well as the 2004-2007 M47N engined 118d, is a bit troublesome. On the face of it there is no reason for this to be so. It’s a very well made unit machined to incredibly fine tolerances and bearings from well respected manufacturers are used. Yet many are whining merrily by 70,000-miles and they can get really grumbly. It’s rare for a diff to actually fail but I recently replaced a diff in a 2008 320i E92 Coupé that was very close to a big bang and a loss of drive.
ABOUT THE TYPE 168
The Type 168 has been BMW’s small diff almost forever – the smaller engined E30, E36 and E46 cars used this diff size and the 168 apparently refers to an internal measurement. The medium size diff is 188 and the really big one, 210.
The 168 as fitted first to the 2004 1 Series – 116i, 118i, 120i and 118d – differs from the previous E46 type unit in one major respect as it replaces the previous taper roller bearings for the pinion and crown wheel side bearings with ball bearings. This was done to drastically reduce friction and to make the diff easier to turn – you really notice it when you have the older and newer type diffs side by side. Also, the oil used was a much thinner synthetic 75/90 as opposed to EP90.
Lastly, the sun and planet gears used in the actual differential unit are sintered steel, doubtless cheaper to make than the older type gears used in E30 and E36 units.
WHAT GOES WRONG?
A lot of the blame could be levelled at the lack of a drain plug for the diff. The factory fill oil is there forever unless you have a tool to suck all the old oil out and nobody ever bothers. Having said that, I’ve seen diffs with 150,000-miles (one 118d with over 200k) and they’re still perfect.
Whilst the differential side bearings very rarely wear, the pinion ones certainly do with the smaller end bearing nearest the propshaft flange being the worst. BMW use three bearing manufacturers and these are INA, FAG (German) and Koyo from Japan.
I’ve rebuilt several diffs and found that Koyo bearings to be the least durable – I’ve also found that the newer the car, the more likely it is to have these bearings. They are undoubtedly very high quality but they seem less able to cope with longlife diff oil. The sintered sun/planet gears also start to pit and eventually break up – the ones in this 2008 320i are an extreme example due to insufficient oil.
CAN I REBUILD MY DIFF?
You can, but they’re pretty difficult, requiring both a very good tool kit, a big bench vice and a ‘feel’ for things. The differential assembly is secured into the iron diff housing with a big circlip each side and these double up as shims to set the crownwheel and pinion lash correctly. These must be marked with paint so they go back the right side. The pinion bearings are very hard to remove and replace. Once the pinion is out, the big and small bearing outer cages need to be driven out using a mixture of brute force and careful skill.
Refitting the new big bearing outer cage is really hard work and I’ve found the best way is to heat the casing in very hot water and have the bearing cage in the freezer. There is a spacer shim under the bearing cage as well and during bearing removal it’s very easy to mark the soft cast iron diff casing – this must be rectified or the new bearing will not sit correctly.
Think that’s bad? Now you have to get the big bearing’s inner cage off the pinion and it’s pressed on with several tons of force. A puller will not grip it so you have to carefully grind away enough of the bearing cage until it will split of its own accord but without marking the pinion.
Then you have to fit the pinion with the new crush tube and set it up. I do these without the pinion oil seal fitted and you have to grip the prop flange in a big bench vice and tighten the nut with a big breaker bar. Periodically, remove the diff from the vice and check for play.
When it gets close, nip it up a tad more until there is none – it’s hard work. Then mark the nut against the pinion with paint, remove it and the flange, fit the oil seal all the way in and refit the nut, lining up the paint marks.
IS IT WORTH IT?
No, it’s a waste of time – 4-5-years ago when good used diffs were £500 plus there was a case for it. E90 and 1 Series cars were rare in breakers and good diffs were hard to find. But now, scrapyards are brimming with stretcher case 1 and 3 Series cars, ten to fifteen years old and good used diffs are down to £300 – don’t pay more. As the values of the cars has dropped, so have parts prices and nobody is paying £400 to put a used diff in a 2006 318i. Early cars are now in scrap territory but can be the source of a good used diff to live on in a newer car.
HOW DO I TELL A GOOD ONE FROM A BAD ONE?
Easy. Turn the pinion flange and the turning motion should feel pretty smooth. If it feels rough, noisy or graunchy, it’s scrap.
Get the right diff as well. The 168 unit has seven bolts securing the rear alloy cover and the prop flange is circular with four bolts. Look on the white top sticker and you can see the ratio and part number. The various ratios are 3.91, 3.73, 3.64, 3.45, 3.38, 3.23 and for the 118d, 2.47. Put the VIN from your car into Realoem (realoem.com) and you can find out what diff your car has.
Cars with auto stop start can be very fussy on ratios. Fit a car which had a 3.64 from new with a 3.91 or a 3.23 and the car’s ECU will pick up the difference in road/ engine speed and object, rendering the cruise control inoperable and often put the car into limp home mode.
BMW diff part numbers are often one digit out on original fit and replacement (super session) numbers. BMW swapped diffs around from model to model and it would take a month of Sundays to list what each model had fitted. Again, Realoem is your friend so enter the last seven digits of your VIN to see what diff it should have. This is a guide to diff part numbers and super session numbers plus some of the cars they were fitted to.
2.47: 118d only. Original part number 7556794, new number 7556795.
3.23: Part numbers 7529108 and 7529318. Early 318i and 118i.
3.38: Part numbers 7524319 and 7524320. Later N46 and N43 318i. N46 118i.
3.45: Part numbers 7524321 and 7524322. 120i N46 and N43 320i saloon/Touring.
3.64: Part numbers 7519925 and 7519926. 1126i manual N43, 320i manual N46 and N43 320i Coupe manual.
3.73: Part numbers 7524323 and 7524324. 116i N45, 120i N43 and N46, manual including Convertible models.
3.91: Part numbers 7524325 and 7524326. 116i, 118i and 120i N43 2.0 auto, 318i and 320i auto.
4.10: 7555316 and 7555316. 116i Auto N45 and N43