Mechanical reverse shuttle GBA20 gearbox - Selector rails


On the gearbox
The selector rail (7) holding the different forks is located on the right-hand side of the gearbox, on the selector cover plate side. It passes through the gearbox housing and the Hare and Tortoise range control piston (13). It is held in position by a set screw (8). A cup plug (9) closes off the bore at the front and a cover plate (15) closes the rear end. The 1st - 2nd (5) and 3rd - 4th (6) forks are not adjustable. The creeper gear fork (28) is adjustable by mechanical pressure. Two adjustments are necessary to obtain the correct functioning of the fork (20) in both ranges (Hare / Tortoise):
- adjustment of the space between the fork pads and the synchroniser sliding coupler;
- adjustment of the stop (18), in relation to the position of the indexing rod fitted on the selector cover plate. A creeper gearbox may be fitted as an option. A super-creeper gearbox may also be fitted as an option. In this case, the end-of-travel pin (29) is not fitted.

On the input unit 
The mechanical reverse shuttle synchroniser is controlled by:
- a selector;
- a selector rail;
- a fork.
The selector and the fork are adjustable.


Parts list 
(1) Plug (2) Spring (3) Detent plunger (5) 1st - 2nd Fork (6) 3rd - 4th Fork (7) Selector rail (8) Set screw (9) Cup plug (10) Cylinder (11) "O" ring (12) "O" ring (13) Hare and Tortoise range piston (14) "O" ring (15) Cover plate (16) Screw (17) Screw (18) Locking stop (19) Adjustment screw(20) Hare/Tortoise fork (26) Nut (27) Locking screw (28) Creeper gear fork (29) End-of-travel pin

Maintenance guide

This guide describes a succession of operations to be followed depending on the mechanical component to be worked on.

Creeper gear fork
- Remove the selector cover plate.
- Remove the right-hand hydraulic cover plate.
- Remove the creeper gear fork

Hare / Tortoise fork
- Remove the selector cover plate.
- Remove the right-hand hydraulic cover plate.
- Remove the creeper gear fork (if fitted).
- Move the selector rail forward.
- Disassemble the hydraulic components (ram) of the Hare / Tortoise mechanism.
- Take out the range fork.

1st-2nd and 3rd-4th gear forks
- Remove the selector cover plate.
- Remove the right-hand hydraulic cover plate.
- Remove the creeper gear fork (if fitted).
- Move the selector rail forward.
- Disassemble the hydraulic components (ram) of the Hare / Tortoise mechanism.
- Take out the range fork.
- Move the selector rail backwards.
- Take out the 1st-2nd and 3rd-4th gear forks.

Main selector rail
- Remove the selector cover plate.
- Remove the right-hand hydraulic cover plate.
- Remove the creeper gear fork (if fitted).
- Move the selector rail forward.
- Disassemble the hydraulic components of the Hare / Tortoise mechanism.
- Take out the range fork.
- Take out the 1st-2nd and 3rd-4th gear forks.
- Move the selector rail backwards as far as it will go and disengage it through the selector cover plate opening.

Tractors with no creeper unit
The selector rail can also be removed through the front on tractors not fitted with a creeper unit. To do this it is necessary to:
- disconnect the tractor between the engine and the gearbox;
- remove the gearbox spacer (only on tractors fitted with 6-cylinder engine).

Reverse shuttle selector
- Remove the selector cover plate.
- Remove the reverse shuttle selector.

Shuttle fork
- Disconnect the tractor between the engine and the gearbox.
- Remove:
- the selector cover plate;
- the reverse shuttle selector;
- the input unit;
- the shuttle fork.

Shuttle selector rail
The shuttle selector rail is removed using the same procedure as for the fork.



Mechanical reverse shuttle selector - Fork and selector rail

Reverse shuttle selector


Remove the selector cover plate.
Remove the screws (1). Remove the selector (2).
Clean the thread of the screws and the tapped holes in the selector. Check that the screws turn freely in the tapped holes.
Slide the selector onto the selector rail and adjust it.
Engage forward and reverse positions, and vice versa, using the selector rail to position the synchroniser components.
Next, position the selector rail (3) in neutral.
Fit the tool 3378054M1 to the gearbox. Secure it in place using two locally
obtained screws.

Adjusting the selector 


Lightly smear the thread of the screws (1) with Loctite 242 or equivalent.
Bring face "F" of the selector (2) flush with the tool, using the screws (1).
Alternately and gradually tighten the screws (1) to a torque 35 Nm without modifying the selector
position. Install the selector cover plate.

Fork and selector rail 
Disconnect the tractor between the engine and the gearbox.
Remove the selector cover plate.
Remove the input unit.
Remove the plug (7), the spring (8) and the detent plunger (5). The detent plunger (5) for the shuttle
fork (6) has a different shape from those fitted on the 1st / 2nd and 3rd / 4th forks of the gearbox.
Take out the screws (3) from the fork (6). Remove the rail (4) and the fork. Clean the thread of the screws and the tapped holes in the fork. Check that the screws turn freely in the tapped holes.
Slide the fork into the synchroniser sliding coupler.
Slide the selector rail (4) into the fork (6). Adjust the fork.

Adjusting the fork 

The positioning of the fork (6) is obtained by the difference between the centre line "Y" of the holes and the centre line "Z" of the indents on the selector rail (5). A different fork movement is obtained either by adjusting the front screw or by adjusting the rear screw (3) according to the adjustment required.
Install the detent plunger (5), the spring (8) and the plug (7). Tighten the plug to a torque of 50 - 70 Nm. When disassembling all detent plungers (gearbox and input unit), the positioning of the reverse shuttle selector rail detent plunger must be respected.
Lightly smear the thread of the fork screws with Loctite 242 or equivalent. Fit the screws.
Position the locked fork towards the front. Hold the synchroniser sliding coupler up against the gear.
Pre-adjust the fork using the above-mentioned principle.
Check that the front pads of the fork are not driven hard against the synchroniser sliding coupler.
Repeat the operation (locked fork towards the rear). Check that the rear pads are not driven hard against the synchroniser sliding coupler. The correct adjustment is obtained when clearance between the fork pads (front and rear) and the synchroniser sliding coupler is nearly equal, in both the forward and reverse positions.


Alternately and gradually tighten the screws (3) to a torque 35 Nm without modifying the fork adjustment.
Install the input unit.
Adjust the selector.
Install the selector cover plate.
Reconnect the tractor between the engine and the gearbox.
Top up the oil level in the housings. Check it using the gauge at the rear of the centre housing.
Bleed the hydraulic mechanism of the engine clutch.
Carry out a road test of the reverse shuttle (forward and reverse positions) and the control assembly.
Check the tightness of the mating faces and hydraulic unions.