Differential

The differential is located on the bulkhead side of the engine/transmission unit and is held in place by nuts and studs. The crown wheel or drive gear together with the differential gears are mounted in the differential unit. The drive pinion is mounted on the end of the mainshaft in the gearbox.

All repairs can be carried out to the component parts of the differential unit, only after the engine/ transmission unit has been removed from the car. If ‘it is wished to attend to the pinion it will be necessary to separate the transmission casing from the engine.

Specification

The differential unit on all transverse engined Mini variants is identical, with the exception of the ratio which varies from model to model.

Differential Removal

  1. Remove the engine/transmission assembly
  2. Undo the four bolts which hold the cover plate to the underside of the gear change extension and remove the plate.
  3. Undo and remove the bolt which holds the shaft lever to the remote control shaft and then with-draw the shaft.
  4. A split pin locks each of the drive shaft flange securing nuts. Remove the pins and holding the flange stationary with the special BMC tool (or by means of a screwdriver inserted through one of the holes in the drive flange) undo each nut in turn and pull the flanges from off the gear shafts.
  5. Undo and remove the five bolts and spring washers which hold the end covers in place. Take off the end covers together with their cover gaskets. Carefully note the number of shims fitted between the drive gear bearing and the end cover housing.
  6. Undo the 7 nuts which secure the differential casing to the main transmission casing.
  7. Pull the differential casing up-off the transmission casing studs and then lift out the differential assembly.

Differential – Dismantling, Examination and Reassembly

  1. With the aid of a bearing extractor (or BMC. tool 18G2) pull-off the two bearings from the right and left hand gear shafts.
  2. Mark the differential case and the drive gear so that they can be reassembled in their original positions.
  3. Knock back the tabs of the three lock-washers and unscrew the six set bolts which hold the drive gear to the differential case.
  4. Remove the drive gear complete with the left gear shaft. Pull the drive gear off the shaft together with the thrust washer.
  5. Gently tap out the tapered peg which holds the centre pin in place.
  6. Remove the centre pin and the component parts of the differential case. The differential case can now be removed from the right gear shaft. Check the bearings for side play and the rollers and races for general wear. Examine the centre pin, the thrust block and the thrust washers for score marks, and pitting, and renew these components as necessary.
  7. Examine the teeth of the drive gear for pitting, score marks, chipping and general wear. If a new drive gear is required a mated drive gear and drive pinion must be fitted. It is asking for trouble to renew one without the other. Exa.mine the oil seals in the end flanges and renew them if worn.
  8. Reassembly is a straight reversal of the above sequence. NOTE. When replacing the differential gear thrust washers (13) ensure the slightly chamfered-bores rest against the machined faces of the differential gears. The six drive gear bolts should be tightened to a torque of 60 lb/ft each.

Differential Replacement

  1. Place the differential assembly in the transmission casing with the drive gear furthest away from, but with a bias towards, the clutch end.
  2. Ensure the differential casing flanges are clean and free from all traces of the old gasket, fit a new gasket, and then carefully lower the casing over the studs.
  3. Refit the seven nuts and washers and tighten them down lightly so that, although the drive gear bearings are firmly held, they can still be moved slightly together with the differential unit.
  4. Clean the end cover flanges, fit a new right hand end cover gasket and replace the right hand end cover. Ensure the holes in the cover flange and the tapped hole in the transmission casing and differential housing are correctly aligned.
  5. Screw in the five bolts and spring washers an equal amount so that the inner face of the cover bears evenly on the differential bearing outer race. As the bolts are tightened a little at a time the differential assembly will move away from the flywheel end of the casing, so centralising the assembly in the differential housing.
  6. Now fit the left hand end cover but omit the gasket. Tighten the bolts so that the cover register just seats the bearing outer race. Do not over-tighten the bolts or the end cover flange will be distorted.
  7. Measure the gap between the end cover and the differential casing in several places to ensure the end cover is seating squarely on the differential assembly. Variations in measurement indicate that the bolts have been tightened un- evenly so pulling the differential assembly out of alignment. Alternatively the end cover flange may be distorted. Adjust the tension on the bolts accordingly.
  8. The correct gap necessary to give the required preload to the bearings without the gasket fitted is .008 to.009 in. As the gasket when compressed is .007 in. thick, the correct gap with the gasket fitted is .001 in. to .002 in. before the bolts are finally tightened down.
  9. Any deviation from .008 to .009 in. must be rectified by fitting appropriate shims between the register on the inside of the end cover and the bearing outer race.
  10. Remove the end cover. Fit the necessary shims. Replace the cover complete with gasket, and tighten down the securing bolts evenly to a torque of 18 1b/ft
  11. Tighten the differential housing nuts; refit the driving flanges (2.’8); replace the driving flange washers and nuts and tighten to 85 lb/ft. (Turn the slotted nut as necessary to align with the next split pin hole. Insert new split pins.
  12. Refit the remote control shaft, the shaft lever and the extension cover plate. Replace the engine / transmission unit in the car.
Measured gap (No gasket)Shim thickness
000 to .001 in.008 in
.001 to.002 in.006 to .007 in
.002 to .003 in.005 to .006 in
.003 to .004in.004 to .005 in
.004 to .005 in.003 to .004 in
.006 to .007 in.002 to .003 in
.007 to .008 in.001 to .002 in
.008 to .009 inNone necessary

Final Drive Pinion Removal & Replacement

  1. Remove the differential unit, separate the engine from the transmission casing.
  2. Undo the plug on the forward facing side of the transmission casing. This plug holds the change speed reverse detent plunger in place. Remove the plug, washer, spring and plunger.
  3. Undo the clamp screw from the selector lever and the woodruff key from the end of the gear change operating shaft.
  4. Pull the gear change operating shaft up out of the transmission casing.
  5. Undo the speedometer pinion housing bolt, re- move the housing and extract the pinion. Undo the two bolts which hold the speedometer gear retaining plate to the transmission case front cover, remove the plate and then the speedometer spindle and gear.
  6. Knock back the securing tag of the lock-washer which holds the drive pinion securing nut in place , and undo and remove the nut.
  7. The final drive pinion can then be pulled off the splines on the mainshaft.
  8. Replacement is a straight forward reversal of the removal sequence. Use a new pinion lock- washer and tighten the pinion securing nut to 90 lb/ft. (150 1b/ft. on mini Cooper ‘s’ models).