GBA20 gearbox – Layshaft


The layshaft (69) is mounted on two tapered roller bearings (1) (17) with cups (2) (16), resting in the two lower bearings of the main gearbox. Each cup has a root face giving it optimum contact with the housing.
At the front it receives the input gear (67). Between the two lower bearings, it supports the 1st (7) and the 2nd (14) idle mounted driving gears, as well as the 1st and 2nd gear synchroniser assembly (11), the hub of which is splined. At the rear, it holds the 4th driving gear (18) and the 3rd and 4th gear synchroniser assembly (23). The 1st, 2nd (11) and 3rd, 4th (23) synchronisers are of the single cone type. The rear bore contains a needle roller bearing that holds the front end of the output shaft. The turning parts are lubricated by a central channel and radial bores. The tapered roller bearings are fitted with preload obtained by shim(s) (3) inserted between the splined washers (6) positioned against the input gear (67) and the ring (8). A washer (77) with flat sections acts as a stop for the 1st driving gear (7).


Parts list 
(1) Bearing cone (2) Bearing cup (3) Shim(s) (6) Splined washers (7) 1st driving gear (8) Ring (11) 1st, 2nd single cone synchronizer (14) 2nd driving gear (16) Bearing cup (17) Bearing cone (18) 4th driving gear (23) 3rd, 4th single cone synchronizer (26) Circlip (33) 4th synchroniser cone
(67) Input gear (68) Nut (69) Layshaft (70) Snap ring (71) Lubrication pipe (72) Spring (73) 1st synchroniser cone (74) 2nd synchroniser cone (77) Washer with flat sections (78) Tab washer


Split the tractor between the gearbox and the rear axle. Split the gearbox from the engine. Remove the selector cover plate. Remove the input unit assembly. Remove the intermediate shaft. Remove the selector rail and the forks. Remove the output shaft. Remove the layshaft.

Disassembling and reassembling the shaft

If work is only carried out on the drive gear (67), hold the gear and 1st, 2nd synchroniser assembly in position using a screwdriver so as not to displace the washer with flat sections (77).
Remove the circlip (26).
Remove the 3rd - 4th synchroniser (23) and the 4th gear ring. Mark the assembly order. Remove the cone (33) and the 4th gear (18).
Position locking tool 3378082M1 and the makeshift supporting sleeve for the shaft (69).
Remove the lubrication pipe (71) and the spring (72).
Unlock the nut (68) and loosen it using socket 3378010M1.
Remove the input gear (67) with the cone (1).
Remove the washers (6) and the shim(s) (3).
Remove the tool and sleeve.

Take the shaft out through the rear of the housing, while supporting the 1st, 2nd gear assembly.
Remove the 1st, 2nd gear assembly and the synchroniser (11) with the ring (8). Remove the cups (16) and (2). Pair up the cups and the cones if they are to be reused.
If the input gear (67), the shaft (69) or the tapered roller bearings (1) (2) and (16) (17) are to be replaced:
- remove the washer with flat sections (77);
- extract the bearings cones (1) and (17);
- remove snap ring (70).
Clean the housing and the mating faces.
With a jet of compressed air, check that no channels in the gearbox or in the shaft are blocked.
Clean and check all components. Replace those that are defective.
Lubricate the cones, cups, bearing bores and the ring.
If removed:
- fit the bearing cone (17) onto the shaft (69) using a press and a suitable fixture;
- refit the washer with flat sections (77), with the lubricating grooves facing towards the 1st gear (7);
- refit the snap ring (70).

Assemble the cups (2) and (16) inside the housing.
Install the 1st, 2nd gear assembly, the synchronizer (11), the cones (73) (74) and the ring (8) into the housing.
Install the shaft from the rear of the housing while supporting the gear/synchroniser assembly, and ensuring that the ring (8) is correctly fitted in the gear (14).
Install the locking tool 3378082M1 and the makeshift supporting sleeve.
If it has been removed, fit the cone (1) onto the gear (67), using a press and a suitable fixture.
Install the washers (6) separated by a number of shims (3) sufficiently thick to obtain a temporary clearance of 0.10 to 0.15 mm approx. in preparation for shimming with preload.
Install the gear (67) onto the shaft (69).
Install the nut (68) using socket 3378010M1 and tighten to 130 - 170 Nm.
Remove the tool and sleeve.
Shim the shaft.
Place the dial gauge feeler pin at the end of the shaft.
From the front of the housing, pull hard on the shaft, turning alternately from left to right to correctly seat the cones in their bearing cups.
Set the dial gauge to zero.

Depending on the clearance measured, select the thickness of final shims required to obtain a preload of P1 = 0.14 to 0.20 mm. If possible, shim to the maximum tolerance.
Install the locking tool and the supporting sleeve.
Unlock the nut (68).
Remove the gear (67).
Install the gear.
Degrease the thread of the shaft with a solvent.
Lightly smear the new nut (68) with Loctite 270 or equivalent then tighten to a torque of 130 - 170 Nm.
Using a suitable pin punch, lock the nut in place by bending the collar into the grooves of the shaft without breaking it.
Insert the spring (72) and the lubrication pipe (71) in the shaft.

Remove the tool and the supporting sleeve. Fit:
- the gear (18);
- the 4th synchroniser cone (33);
- the 4th gear ring and the 3rd - 4th synchronizer (23). The lubricating grooves of the synchronizer should be turned towards the 4th gear (18).
Install a new circlip (26), opening it as little as possible. After assembly, check that it is not bent.
Manually check:
- the axial clearance of the gears;
- the rotation of the shaft and its gears.
Check that the 1st, 2nd synchroniser operates properly.
Install and shim the output shaft.
Install the intermediate shaft.
Install the input unit assembly.
Shim the intermediate shaft.
Install the selector rail and the forks.
Install the selector cover plate.

Reconnect the gearbox to the engine.
Reconnect the gearbox and the rear axle.
Check operation of electric circuits.
Carry out a road test of the controls.
Check the tightness of the unions and mating faces (selector cover plate, gearbox on rear axle).