Manual - Chapter 3 Gearbox

7. Checking and resetting the gearbox shafts endfloat and preload

  1. If any of the gearbox shafts are renewed, or if any of their bearings are renewed, the fit of the shafts in the gearbox must be checked and adjusted, if necessary, using shims.

Layshaft and output shaft

  1. With the gearbox housing stripped and heated evenly, in an oven or similar, to approximately 100 °C (212 °F), check that each shaft is completely rebuilt and that its bearings are pressed as far on to the shaft as possible, then mesh the two together and install them in the housing. Tap each shaft firmly into place with a soft-faced mallet; the heated casing should ensure that the shaft rear bearings are fully and correctly seated.
  2. Lay a straightedge across the housing mating surface next to the shafts and using a vernier caliper depth gauge or similarly accurate measuring device, measure the distance from the front edge of each shaft's front bearing outer race to the straightedge, ie the total amount each shaft projects beyond the gearbox housing mating surface; let this measurement be called dimension A.
  3. Record this measurement for both shafts, then lay the straightedge across the front cover over the shaft front bearing housings. Measure the distance from the straightedge to the shoulder in each shaft's bearing housing against which the bearing outer race will locate; let this measurement be called dimension B.
  4. For each shaft subtract dimension A from dimension B to give the total amount of endfloat available for that shaft. Subtract the specified standard endfloat tolerance from this total to give the required thickness of shims.
  5. Shims are available from BMW dealers in thicknesses of 0.30, 0.40 and 0.50 mm (0.0118, 0.0158 and 0.0197 in); use whatever number and combination of these that will reduce the endfloat as much as possible within the specified limits. Store the shims carefully with their respective shafts until reassembly.

Input shaft

  1. Assemble the input shaft as described in Section 5 but do not yet fit the front bearing. Check that the raar bearing is correctly positioned against the circlip and that its outer race is pressed fully into the gearbox housing, then heat the front cover evenly, in an oven or similar, to approximately 100 °C (212 °F) and fit the front bearing outer race, ensuring that it is driven squarely into its housing to rest against the locating shoulder. Allow the cover to cool.
  2. Fit the input shaft into the stripped housing and lay a straightedge across the housing mating surface next to the shaft. Use a vernier caliper depth gauge or similarly accurate measuring device to measure the distance from the straightedge to the shaft collar (the locating flange against which the shims and bearing will be fitted); let this measurement be dimension A
  3. Place the straightedge across the front cover mating surface over the shaft front bearing outer race and measure the distance from the straightedge to the rear edge of the outer race; let this measurement be dimension B.
  4. Subtract dimension A from dimension B to give the total amount of endfloat available for the input shaft; let this be dimension C. However, since the input shaft must be preloaded (ie slightly compressed when the front cover is fitted) the required shim thickness is C plus the specified preload tolerance.
  5. Shims are available from BMW dealers in thicknesses of 0.30, 0.40, 0.50, 1.42, 1.44, 1.46, 1.48 and 1.50 mm (0.0118, 0.0158, 0.0197, 0.0559, 0.0567, 0.0575, 0.0583 and 0.0591 in); use whatever number and combination of these that will preload the shaft as closely as possible within the specified limits. Fit the selected shims to the shaft and drive the bearing into place against them.
  6. To check the preloading, assemble the shaft into the housing, lubricating its bearings with the specified oil, and refit the front cover, tightening its retaining screws to the specified torque wrench setting. The shaft oil seals must not be fitted yet or their drag may produce a false reading.
  7. The amount of effort required to rotate the shaft can be measured with a friction gauge or with an extremely accurate spring balance attached to the end of a lever of known length from the shaft centre, ie the setting is correct if the pull required is in the range 0.13 - 0.38 lb measured at the end of a lever 1 ft from the shaft centre. If the setting proves to be incorrect, the measurements or calculations must have been faulty at some point and the operation must be repeated until the setting is correct.

 

Next Section: Gearbox: reassembly


7.11 Input shaft preload shims are fitted between bearing and shaft collar