Bold reference numbers in this section identify parts not shown in the figures alongside the text, but which can be found in the exploded view diagram.

Withdraw the selector drum (16) taking care not to lose shims (31) and (27) mounted on the shaft. Note that the positions of the shims must not be inverted.

Now you can renew the needle roller retaining ring (32) and the special needle rollers (33).

Remove gear selector forks (29) and (28).

Remove the gearbox primary (2) and secondary (25) shafts complete with gears, taking care to recover the spacers on the ends of the shafts.

If the bearing inner rings (A) are left on the shafts, slide them off the ends of the gearbox primary (2) and secondary (25) shafts (Sect. N 9.2, Overhauling the main bearings).

Remove the chain-side shim (15) and clutch-side shim (26) from the secondary shaft.

Remove the chain-side shim washer (11) and the clutch-side shim washer (1) from the primary shaft.

Check the condition of the front coupling dogs of the gears. They must be in perfect condition and with no sign of wear on the edges of the teeth.

At each overhaul, check the condition of the needle roller cage (13) and the oil seal (14) located on the end of the gearbox input shaft: remove them if necessary using a suitable puller and renew them.

Engage the gears and check that the gearchange mechanism does not stick (selector fork - gear groove, and fork pin - desmo­dromic drum groove) due to incorrect end float. Restore the correct end float by shimming the gearbox shafts and the selector drum with suitable shims.

For the total gearbox shaft and selector drum end clearance values, refer to Sect. C 1.1Gearbox.

Visually inspect the gear selector forks. Bent forks must be renewed as they may lead to difficulties in gear changing or may suddenly disengage when under load.

If the service limit has been exceeded, check whether or not it is necessary to replace the gear or the fork by referring to the limits specified for each part (Sect. C 1.1Gearbox).

If the service limit is exceeded, determine which part must be replaced by comparing these dimensions with those of new com­ponents (Sect. C 1.1, Gearbox).

Figure 1 shows all the parts to be reassembled on the gearbox primary shaft (2), with the calculated end shims (1) and (11) (Sect. N 9.2, Shimming the shafts).

Figure 2 shows all the parts to be installed on the gearbox secondary shaft (25), with calculated end shims (15) and (26) (Sect. N 9.2, Shimming the shafts).

Take particular care when installing the idler gears. The assembly of the 3rd and 4th speed gears and the relative fixing compo­nents on the secondary shaft is given as an example.

Fit the circlip (7), checking that it is fully inserted into its groove on the shaft. Push the circlip into position with a suitable size tubular drift.

To fit the needle roller cage (4) onto the shaft, first lubricate it with plenty of grease (of recommended type) and then open it slightly to make it easier to slide on to the shaft.

Fit the 3rd speed gear (22).

Fit on the gear the three-pointed washer (12), which can be distinguished from its counterpart (6) by its bigger outside diameter.

Fit the 4th speed gear (21).

To refit the gearbox components follow the procedure under Sect. N 9.2, “Reassembly of the crankcase halves“, relating to re­assembly of the engine crankcase.

As a final practical test, ensure that with the gearbox in neutral the front coupling dogs (A) of sliding gears (B) are equidistant on both sides with respect to the corresponding coupling dogs on the fixed gears (C).

Check also that there is always a small amount of clearance between fork (D) and relative groove (E) on sliding gear (B) when engaging the gears.