3. Final drive: examination and renovation

  1. Dismantling the final drive is beyond the scope of the majority of amateur mechanics. Wear or damage will be indicated by a high pitched whine. Backlash between the crownwheel and pinion may be assessed by holding the wheel flange firmly and rotating the pinion shaft in both directions. Any lateral play in the crownwheel can only be checked with the assembly installed in the machine. by pulling and pushing the rear wheel.
  2. If any wear or damage is found or suspected, take the unit to an authorised BMW dealer for reconditioning.
  3. The only task which can be undertaken by the private owner is the removal of the cover to renew a faulty crownwheel oil seal or to attend to the speedometer impulse transmitter rotor.
  4. If this is to be undertaken, drain the oil from the unit and mark the cover and housing across the joint face so that it can be correctly refitted on disc rear brake models; on drum rear brake models use a stepped drift of suitable size to tap the brake camshaft sleeve out of the cover to the housing right-hand side. Working progressively and evenly in a diagonal sequence remove the cover retaining bolts or Allen screws and withdraw the cover, applying a few firm taps from a soft-faced mallet to break the seal.
  5. The crownwheel should be removed with the cover; since the shims are fitted between the taper roller bearing inner race and the crownwheel, or between the ball bearing and cover, they will not be disturbed at this stage.
  6. First check that the speedometer rotor is firmly fixed on the crownwheel end and that it is undamaged; this applies mainly to early 100 models since the fit of these two components was improved on all 75 models and later 100 models. If it is to be renewed, the taper roller bearing inner race must be extracted first, using a knife-edged bearing puller; take careful note of any shims found and ensure that they are fitted in exactly their original positions on reassembly. If the rotor is loose but the mating surfaces are undamaged, all components should be thoroughly degreased and all traces of oil deposits or corrosion polished away. Apply a few drops of Loctite 638 or RC/620 (62040) adhesive to the contact faces and tap the rotor onto the crownwheel with a soft-faced mallet. When the rotor is seated fully against the crownwheel shoulder, leave the assembly for four hours so that the adhesive can cure. This can be speeded up to 30 - 40 minutes by heating the crownwheel and rotor to 120°C (248°F).
  7. If the oil seal is to be renewed, tap out the crownwheel and bearing; BMW recommend that the cover be heated to approximately 80°C (176°F) to permit this. The ball bearing should stay on the crownwheel, in which case the shim behind it should be refitted exactly the same way round. The seal itself can then be driven out and the new seal fitted with a smear of grease around its outer edge. Fit the new seal by hand as far as possible, ensuring it remains square to its housing, then tap it into place using a hammer and a tubular drift which bears only on the seal hard outer edge. If a sufficiently large drift cannot be found, use either a soft-faced mallet with great care, tapping the seal evenly and squarely into its housing until it is flush with the cover, or take the assembly to an authorised BMW dealer to be rebuilt with the correct service tools.
  8. On very early (1984 models only) UK K100 models check that the breather is clear by removing the breather cap and inserting a length of wire into the breather passage. If the wire enters by more than 22 mm (0.9 in), down to 30 mm (1.2 in) approximately, the passage is clear and the wire can be removed and the cap refitted. If the wire cannot be inserted as far as this, the passage is blocked and must be cleared by the careful use of a 7.5 mm (0.3 in) diameter drill bit. With the breather cap and final drive housing cover removed, drill downwards to the outer annular cavity, taking great care not to damage the thin walls of the breather passage. Be very careful to clean away all traces of swarf and other debris.

    Note: This only proved necessary on very early models after which the breather was checked at the factory. It is also very unlikely that any machine has survived unmodified.

  9. Later 100 models were fitted with a modified type of breather incorporating an O-ring around the breather base and a cap with a 3 mm (0.12 in) hole drilled in it: note that the hole must face to the rear. This modification was to further seal the breather against the entry of water and is fitted as standerd to all 75 models. If water is found in the final drive oil on early 100 models, take the machine to an authorised BMW dealer for checking.
  10. On reassembly, grease the seal lips and do not forget the shim as the crownwheel is fitted to the cover. Fit a new O-ring to the cover groove and grease it. then place the cover assembly on the final drive housing and align the marks made on removal (disc brake models) or the brake camshaft passages (drum brake models) before tapping the cover into place; ensure that the O-ring is not damaged or disturbed. Working progressively and evenly in a diagonal sequence tighten the cover retaining bolts or Allen screws to the specified torque wrench setting. Use the stepped drift to tap the brake camshaft sleeve back into place in the cover (drum brake models only).


3.4a Disc rear brake models only - make reference marks to ensure correct reassembly ...


3.4b ... before removing final drive case cover


3.5a Crownwheel, bearing and speedometer rotor will be removed with cover ...


3.5b ... and must be tapped out if seal is to be renewed


3.7 Do not attempt to disturb final drive pinion assembly


3.10a Thoroughly clean housing mating surfaces on reassembly ...


3.10b ... and always renew sealing O-ring