The BMW K75 & 100 from 1983

Manual - Chapter 1 Engine

25. Examination and renovation: auxiliary drive shaft and starter clutch

Remove the a-ring and thrust washer from the auxiliary drive shaft rear end, then hold the main assembly and twist the starter clutch gear pinion clockwise to release it from the locking elements so that it can be withdrawn.
On early 100 models remove the three rollers and the spring and plunger from behind each. Unscrew the three retaining screws and separate the clutch body from the auxiliary drive shaft.
On all later models remove the six bolts and withdraw the end cover, the sprag clutch assembly (noting which way round it is fitted) the clutch body and the conical spring washer. Note which way round the spring washer is fitted.
Thoroughly clean and degrease all components, then check each item for signs of wear or damage, looking for chipped or damaged gear teeth, worn bearing surfaces or locking elements (rollers or sprags). If any sign of wear or damage is found, the components concerned must be renewed. On the early roller-type clutch check the rollers themselves are free from pits, flats or other signs of wear and that the plungers move easily against spring pressure in the clutch body. If in doubt about the condition of the springs, they should be renewed; they can be checked only by comparison with new components. The sprag-type clutch fitted to all later models should be checked similarly and renewed as a complete assembly if worn or damaged. On both types of clutch check carefully for wear of the starter clutch gear pinion boss.
Above all be careful to wash thoroughly all components in a high-flash point solvent. Most problems with these components are caused by the locking elements slipping on deposits of oily sludge which build up inside the clutch assembly. Even new components must be thoroughly degreased before reassembly to remove the preservative applied at the factory.
Modifications to the clutch assembly are as follows: Early 100 models were fitted with a roller-type clutch. This was replaced in early 1984 by a fourteen-element sprag-type clutch. At the same time a conical spring washer 1.15 mm (0.05 in) high was fitted inside the clutch assembly to control the gear pinion endfloat and the idler shaft assembly was narrowed so that a spring could be installed, also to control endfloat. Note that the idler shaft is now 21.3 - 21.5 mm (0.84 - 0.85 in) wide, measured across the outer faces of its pinions; if a new component is any wider than this the spring must be omitted.
In early 1985, components of the sprag-type clutch were modified to improve the flow of oil through the unit and reduce the build-up of sludge which could otherwise render the clutch inoperative. The most important of these was the starter clutch body which had three 4 mm (0.16 in) holes drilled in it radially, at 120° intervals, to permit sludge deposits to be thrown out under centrifugal force; this should have been fitted to any machine whose owner encountered the starter clutch problem. Also modified were the clutch assembly (two flats machined in its outer diameter to improve oil flow and its components nitrided to harden them) and the conical spring washer ,now 1.40 mm/0.06 in high); these latter two components need only be fitted when the originals require renewal.
On reassembly, thoroughly clean and degrease all components using only clean engine oil as a lubricant during rebuilding. Check that the modified components (where applicable) are available if any are to be renewed.
On the roller-type clutch fit the body to the auxiliary drive shaft apply a few drops of Loctite 273 FL or similar thread-locking compound to their threads and refit the three retaining screws tightening them evenly to the specified torque setting.
Fit the springs and plungers together, insert each assembly into its respective bore in the clutch body and use a shim screwdriver or similar to compress the plunger into its bore while the roller is refitted.
On the sprag-type clutch place the clutch body on the auxiliary drive shaft and insert the conical spring washer with its convex face to the rear, ie so that its outer edge rests against the drive shaft and its raised inner edge projects rearwards, towards the starter clutch gear pinion. Fit the sprag clutch assembly with the coil springs towards the rear, followed by the end cover. Apply a few drops of Loctite 273 FL thread-locking compound or similar to their threads and refit the six retaining bolts, tightening them to their specified torque setting.
On both types of clutch hold still the main assembly and insert the starter clutch gear pinion with a clockwise twist to help it enter the locking elements. Refit the thrust washer and O-ring.
To check the operation of the clutch, hold the body and drive shaft while rotating the starter clutch gear pinion; seen from the rear, the pinion should be locked when rotated anticlockwise and should be free to turn when rotated clockwise.

25.5 Starter clutch components must be thoroughly cleaned whenever possible

25.11a Reassembling sprag-type starter clutch - refit clutch body to drive shaft ...

25.11b ... and refit spring washer - convex face rearwards

25.11c Sprag clutch assembly is fitted with coil springs to the rear - note notches in outer cage showing modified type of clutch

25.11d Refit end cover and bolts - secure bolts as described in text

25.12 Twist clutch gear pinion clockwise to refit to clutch assembly