5. Front forks: reassembly

  1. On K75 S models refit the spacers to the fork lower legs, ensuring that they are the correct way up.
  2. On all models refit the fork seals. Check that each housing is free from burrs or raised edges, then smear grease over the seal's outside edge and tap it squarely into its housing until it is just flush with the top of the fork lower leg. Do not attempt to drive it in any further as this will merely distort the seal and promote leaks. Use a hammer and a tubular drift such as a socket spanner which bears only on the seal's hard outer edge to tap the seal into place.
  3. If the damper rod assembly was disturbed, it must be rebuilt following the accompanying photographs and illustrations to ensure that all components are correctly refitted. Note particularly that the valve fitted at the bottom of the rod on later models has an 0-ring around it which should be on the lower side of the valve.
  4. When the damper assembly is complete. the piston must be refitted to retain the components; make a final check that all are refitted and degrease the piston and rod threads. Apply a single drop of Loctite 638 or 273 thread-locking compound and screw the piston on to the rod until the rod's overall length (from the top of the piston to the bottom of the rod's lower end) is 258 ± 0.5 mm) (10.16 ± 0.02 in). Note that a specified figure is not available tor K75 S models; these must be rebuilt to the length noted on dismantling. When the piston is correctly set, either dry the Loctite with a hot-air blower or leave it to cure for 24 hours at room temperature.
  5. Fit the damper piston ring to the piston groove so that its notched end is downwards then wrap a sheet of thin (the metal in the photograph is 0.35 mm/0.014 in thick) metal or stiff plastic around the piston and ring to hold the ring securely in its groove and to act as a guide to lead it into the stanchion bore. Withdraw the guide and push the damper rod into the stanchion.
  6. Fit the valve housing over the rod lower end and insert it into the bottom of the stanchion. The housing is retained by a circlip but the clearance between them must be eliminated by careful shimming to prevent an annoying rattle. Shims are available in thicknesses of 0.1 and 0.3 mm (0.004 and 0.012 in) for early 100 models but can only be used in conjunction with a modified circlip which was subsequently fitted to all other models. For later models shims are available in thicknesses of 1.6, 1.7, 1.8, 1.9 and 2.0 mm (0.063, 0,067, 0.071, 0.074 and 0.079 in).
  7. Smear the stanchion assembly with oil and insert it into the fork lower leg, using the spring(s) to stop the damper rod from disappearing inside the stanchion- Refit the damper rod Allen screw arid its sealing washer. Either pass, a slim wooden dowel, with a coarse taper ground on one end, down inside the stanchion to bear against the damper piston or refit the spring(s) and spacer(s) and use these to apply sufficient-pressure to stop the damper rod from rotating. Tighten the damper rod Allen screw to the specified torque wrench setting, where given. Refit the drain plug and tighten it to its specified torque setting. Smear the sealing lips with grease (BMW specify Gleitmo 805) and refit the dust excluder.
  8. If required, the fork oil can be added at this stage rather than risk wasting any in trying to pour it through the rather small filler hole. Refit the fork spring(s) ensuring that the spring seats (where fitted) are correctly installed and that the springs are refitted the correct way up. Refit the spacer(s) and install the fork top plug with a new 0-ring.
  9. Push the plug into the stanchion, fit the retaining circlip to its groove and allow the spring pressure to push the plug back up against the circlip.
  10. Fill the fork leg with exactly the specified amount of the correct type of oil, as described in Specifications of this Chapter, then check that the fork leg is fully extended before refitting the filler plug; the forks are designed with the cushioning effect of the trapped air in mind. Hold the top plug with an open-ended spanner and tighten the filler plug to the specified torque setting.

5.5a Wrap piston ring around piston groove ...

5.5b ... and use fabricated guide to ensure assembly is inserted into stanchion without damage.

5.6a Fit valve housing over damper rod lower end ...

5.6b ... valve housing may have to be tapped gently into stanchion.

5.6c Clearance between valve housing and circlip must be eliminated using shims.

5.7a Lubricate all components before refitting stanchion assembly to lower leg.

5.7b Prevent damper rod from rotating while Allen screw is fastened.

5.7c Refit dust seal to top of lower leg - do not forget gaiter (where fitted)

5.8a Ensure springs are refitted original way up - do not omit spring seats, if fitted.

5.8b Refit fork spring spacer ...

5.8c followed by top plug - note new sealing O-ring

5.9 Push top plug into stanchion until circlip can be refitted.

5.10a Add exactly the specified amount and type of oil to each fork leg.

5.10b Hold top plug as shown while tightening filler plug - fork leg should be fully extended.