Manual - Chapter 1 Engine

24. Examination and renovation: output/balancer shaft

  1. To dismantle the output shaft assembly, remove the retaining circlip from its front end and withdraw the needle roller bearing inner sleeve. On 75 models, mark the drive gear front face at the point where it mates with the shaft itself. so that the shaft and drive gear can be refitted in exactly their original positions.
  2. Hold the drive gear/ball bearing assembly in one hand and use a soft-faced mallet to tap the shaft out of the drive gear; do not allow the shaft to drop clear, but allow it to rest on a wadded rag or similarly soft surface.
  3. On 100 models withdraw the damper rubbers from inside the shock absorber body and tap the shock absorber inner and end cover off the shaft. Note the thrust washer.
  4. To dismantle the drive gear assembly, clamp the gear in a vice with soft jaw covers to prevent damage to its teeth, then remove the large retaining circlip. Obtain a bearing puller of sufficient size and an adaptor that will bear against the drive gear boss so that the puller can exert pressure (see accompanying photograph 24.4b).
  5. Draw off the bearing noting the diaphragm spring (early models) or the shim (late models) behind it. Discard the diaphragm spring (where fitted); this should not be re-used.
  6. The anti-backlash gear should be removed using BMW tool 12 4 600 whose protruding lugs engage in the gear holes and permit it to be rotated slightly clockwise and lifted up to disengage the spring at the same time. If this tool is not available, check that the anti-backlash gear teeth are clear of the vice jaws and cover the gear with a thick layer of rag. Slip a thin-bladed knife between the two gears and gradually work the two apart until a screwdriver blade or similar can be inserted to lift the gear up and disengage it from the spring; use the thick rag to prevent any risk of personal injury caused by the gear jumping off. With the spring disengaged, withdraw the anti-backlash gear and use a pair of circlip pliers to remove the spring.
  7. Check the output/balancer shaft components for any sign of wear or damage and renew any components necessary. Check particularly the gear teeth and all bearing surfaces. On 75 models, note that the drive gear and anti-backlash gear were modified from late 1987 to remedy loud gear noise in this area. For a limited period the new components were marked with a spot of blue paint on their end faces for identification purposes. The later components can be fitted to earlier 75 models. On 100 models renew the shock absorber rubbers if they are at all compacted or if they show any signs of wear or deterioration; always renew all five together, even if only one is faulty.
  8. On reassembly, owners should note the following two modifications. The first was introduced on machines produced late in the 1985 model year (ie 100 models only) and consisted of a rear bearing with a thick shoulder (3.52 mm/0.14 in) integral with the bearing outer race at its rear end instead of the previous bearing which had a thin (1.75 mm/0.07 in) locating circlip around its outer edge. This was to improve the axial location of the output shaft. Earlier 100 models cannot be converted to the later type; their bearings must now be glued in place to ensure correct bearing location.
  9. While the modified bearing was fitted to 100 models from late 1985 (model year) on and to all 75 models, a follow-up modification consisting of a retaining circlip with a conical spring cross-section was not fitted until early in the 1986 model year. The new-type circlip can be identified easily by the large square tangs protruding from it, which press firmly against the bearing inner race to ensure improved axial location of the drive gear assembly.
  10. At about the same time as this subsidiary modification, the anti-backlash gear was changed and, on 100 models only the tension spring. Also the previously-fitted diaphragm spring was replaced on all models by shims to provide better anti-backlash gear retention. All these modified components can be installed in earlier machines.
  11. It will be clear from the above that machines built in the late 1985 and early 1986 model years will have anyone of a number of modified components. Owners of such machines should refer closely to the following text and to the accompanying illustrations; the photographs are of a late-1986 K100 RS and therefore show the fully modified assembly.
  12. Clamp the drive gear in a vice with padded jaws to avoid damaging its gear teeth and fit the spring, using a pair of circlip pliers; ensure the spring fits around the drive gear boss and that its locating pin engages in the hole provided. Refit the anti-backlash gear.
  13. On 75 models it is necessary to use the special tool 12 4 600 to refit the gear unless a suitable substitute can be fabricated. Place the gear over the drive gear assembly and engage its protruding pin with the hole in the free end of the spring. Lock the tool's lugs into the holes in the gear and rotate the tool clockwise while pressing down until the anti-backlash gear centre fits over the drive gear boss and its protruding pin fits through the hole in the spring free end into the hole in the drive gear; check that the anti-backlash gear is free to move against spring pressure.
  14. On 100 models the tool is not necessary but care must be taken. The spring free end has a bent-over lug which can be pressed into the large hole in the drive gear (see accompanying photograph) so that the spring is effectively anchored at both ends. Take care that the spring does not fly off; keep a thick wad of rag over it at all times, during reassembly. Fit the anti-backlash gear, aligning its centre with the drive gear boss and engaging its protruding pin with the inner hole in the spring free end. After assembly, check the amount of gear overlap, particularly if excessive engine noise has been noted. Turn the backlash gear clockwise to take up any free play and measure the overlap between the teeth of the drive gear and output gear. This should be at least 1 mm; if less, note that adjustment of the spring lug will be necessary. Adjustment is made by grinding a calculated amount off the spring lug, details of which should be sought from an authorised BMW dealer.
  15. On early or unmodified models, fit a new diaphragm spring with its convex surface outwards, ie its outer edge in contact with the anti-backlash gear. On later models measure the height of the drive gear bearing locating boss above the anti-backlash gear rear face and fit shims as required to eliminate the clearance that will exist between the anti-backlash gear and the bearing (maximum remaining clearance 0.01 - 0.15 mm/0.0004 - 0.0059 in on 100 models). Shims are available in thicknesses of 1.00, 1.15, 1.30 and 1.45 mm (0.0394, 0.0453,0.0512 and 0.0571 in) on 75 models and 1.60, 1.75, 1.90 and 2.05 mm (0.0630, 0.0689, 0.0748 and 0.0807 in) on 100 models.
  16. Thoroughly wash all oil from the bearing and heat it to approximately 80 - 100 °C (176 - 212 °F). Using suitable protective clothing drop the bearing over the drive gear boss with its thin locating circlip (early models only) towards the front (ie next to the gear) or with its thick shoulder (later models only) away from the gear (ie to the rear). Check that the diaphragm spring or shims (as applicable) are seated correctly around the drive gear boss and use a hammer and a tubular drift such as a socket spanner which bears only on the bearing inner race to drive the bearing fully on to the drive gear boss. Refit the circlip.
  17. On all models it is essential that the circlip is seated absolutely correctly in its groove to take the high axial loads; on early models clearance between the groove and the bearing is minimal, therefore great care is required to ensure that the circlip is seated correctly. On later models with the square tang type of circlip, fit the circlip with its convex surface outwards, ie so that the square tangs around its outer edge are firmly in contact with the bearing inner race. Allow the bearing to cool, then lubricate it thoroughly.
  18. On 75 models align the marks made on dismantling when refitting the shaft to the drive gear, so that the balancer weights are correctly timed. Tap the gear assembly firmly on to the shaft locating tangs.
  19. On 100 models fit the end cover and shock absorber inner to the shaft, lubricate the shock absorber rubbers and refit them to the body. Do not forget the thrust washer when refitting the shaft assembly to the shock absorber body/drive gear.
  20. On all models fit the bearing inner sleeve and its locating circlip.

24.4a Use suitable pliers to remove retaining circlip ...

24.4b ... before using puller as shown to draw off output shaft rear bearing

24.12 When refitting anti-backlash spring, anchor fixed end on locating pin (arrowed)

24.14a Refitting anti-backlash gear, 100 models - press spring free end into drive gear hole ...

24.14b ... then fit gear so that its locating pin engages with hole in spring free end (arrows)

24.15 Fit diaphragm spring (early models) or shim, as shown (later models) around drive gear boss

24.17 Rear bearing retaining circlip must be seated correctly - check carefully

24.19a 100 models only - lubricate shock absorber rubbers to aid reassembly ...

24.19b ... insert output shaft and shock absorber inner into shock absorber body, as shown

24.20 Do not omit front end bearing inner sleeve and circlip from output shaft end