C635 6 Speed manual transmission

Construction specifications

The C635 manual gearbox is designed to be installed in front wheel drive systems with the engine mounted transversely. It has a classic three-shaft layout and can transmit a max. torque of 350 Nm.

  1. Gear casing
  2. Clutch casing
  3. Gear selection/engagement assembly
  4. Reversing light switch
  5. Reverse gear mounting fixing screw

The main specifications are as follows:

  • Manual gearbox with fixed idler (or cascade gear) with 6 transmission ratios plus reverse;
  • All gears synchronised including reverse;
  • Idler wheel for reverse engagement;
  • Toothed wheels with helical teeth;
  • One input shaft plus two layshafts (upper and lower);
  • Activation by hydraulic clutch;
  • Designed for matching with twin mass flywheel;
  • Casing split into two parts.
  1. Upper layshaft
  2. Lower layshaft
  3. Main shaft
  4. Reverse gear mounting
  5. Differential

Composition of the Gearbox

Inside the gearbox each transmission ratio is produced by means of a pair of gears that mesh constantly with one idling on the relevant shaft. Reverse gear is an exception, because it is made up of a set of three gears to reverse the direction of movement.

1 Main shaft 11 Reverse synchroniser on upper layshaft 21 Sixth speed gear on lower layshaft
2 Lower layshaft 12 Third speed gear on upper layshaft 22 Lower layshaft scraper ring
3 Upper layshaft 13 Third/fourth speed synchroniser on upper layshaft 23 Gear selection/engagement kinematic component
4 First speed gear on main shaft 14 Fourth speed gear on upper layshaft 24 Clutch control coaxial actuator
5 Second speed gear on main shaft 15 Upper layshaft scraper ring 25 Reversing light switch
6 Third speed gear on main shaft 16 First speed gear on lower layshaft 26 Differential ring gear
7 Fifth speed gear on main shaft 17 First/second speed synchroniser on lower layshaft 27 Differential casing
8 Fourth/sixth speed gear on main shaft 18 Second speed gear on lower layshaft 28 Upper layshaft fork
9 Main shaft scraper ring 19 Fifth speed gear on lower layshaft 29 Lower layshaft fork
10 Reverse gear on upper layshaft 20 Fifth/sixth speed synchroniser on lower layshaft 30 Differential scraper ring

The main shaft receives the drive from the crankshaft; when a gear is selected, the torque is transferred, through the corresponding gear pair, to the layshaft and, from there, to the fixed idler connected to the differential.

Structure of the manual transmission

The following diagrams show the flow of power in the individual gears.

Location of components in the C635 transmission

Toothed Wheels

Materials and Machining - The toothed wheels are subjected to variable loads over time which produce stresses and fatigue; for these reasons nickel-chrome steel alloys are used which afford good mechanical resistance. After machining, the toothed wheels firstly undergo a finishing process (grinding and fettling) and afterwards a heat treatment that improves their resistance properties to fatigue and wear (binding and tempering).

Specification - The toothed wheels used have helical teeth.

This allows the transmission of higher torques compared with those of the same size with straight teeth; in addition, toothed wheels with helical teeth operate more quietly. Toothing with a high covering factor keeps more teeth meshed, with the advantage of smooth transmission.


Main Shaft

The gearbox main shaft has all toothed wheels integrated: in particular the toothed wheels for first, second and reverse are realised directly on the shaft through mechanical machining processes.

The third, fifth and fourth/sixth speeds are fitted by means of splining on the shaft itself and are an interference fit with the shaft.

The main shaft is supported at both ends by two tapered roller bearings.

  1. Axle shaft seal ring
  2. Second Speed Gear
  3. Third Speed Gear
  4. Fourth / Sixth Speed Gear
  5. Fifth Speed Gear
  6. Bearings

Upper layshaft

The upper layshaft supports the toothed wheels that produce third and fourth forward speeds and reverse; these toothed wheels are all fitted in neutral on needle bearings.

There are two synchronisers: one to form one piece between the toothed wheels for third or fourth speed with the shaft, the other for reverse.

The fourth speed and reverse synchronisers are two-cone type, while the synchroniser for third speed is three-cone type.

On the front end of the shaft there is a toothed wheel that transfers the power to the differential ring gear. The upper layshaft is supported at both ends by two tapered roller bearings.

  1. Third Speed Gear
  2. Fourth Speed Gear
  3. Bearings

Lower layshaft

The first, second, fifth and sixth speed toothed wheels are fitted on the lower layshaft. The lower layshaft is supported at both ends by two tapered roller bearings.

  1. First Speed Gear
  2. Second Speed Gear
  3. Fifth Speed Gear
  4. Sixth Speed Gear
  5. Bearings


The C635 gearbox uses two synchroniser types:

- two-cone for the fourth, fifth and sixth speed and reverse

- three-cone for the first, second and third speed

Two-cone synchronisers are formed by a set of two tapered clutches, that distribute the load that derives from the synchronisation of the toothed wheel and shaft speeds onto two friction surfaces. In this way, gear engagement is easier.

Three-cone synchronisers are formed by a set of three tapered clutches, that distribute the load that derives from the synchronisation of the toothed wheel and shaft speeds onto three friction surfaces.

In this way, the gear engagement movement is easier because it is used to improve continuity and progressiveness of torque transfer among the shafts.

Gear Selection / Engagement Assembly

The gear selection/engagement assembly comprises a kinematic component connected, by means of flexible cables to the control lever in the passenger compartment.

The kinematic arrangement has two possible movements:

- vertical travel selects the synchroniser

- rotational movement activates the fork corresponding to the synchroniser

The gear selection/engagement system is connected to the selector dog which, in its turn, operates the sliding rods housing the synchroniser sleeve control forks.

A safety device prevents unintentional reverse engagement.

Each synchroniser, like the rods, is used to engage two speeds.

The selector dog is kept in the engagement position by the action of spring-loaded balls which engage in the relevant indentations on the synchroniser sleeve; a further indentation is used to keep the dog in neutral position.


This is the mechanical device which performs two fundamental actions to drive the vehicle:

- it divides drive torque equally on the two driveshafts;

- it allows the drive wheels to assume different rotation speeds.

  1. Ring Gear
  2. Differential Casing
  3. Bearing