GPA20 differential - Adjusting the taper distance, install and shimming the pinion

Adjusting the taper distance

Check the components and replace those that are defective.
Using a press and a suitable fixture, Install the cone (30) fully onto the pinion shoulder.

Adjusting the taper distance

The taper distance must be adjusted before the pre-load shimming of the pinion bearings. The thickness of shims "E" (50) required to position the pinion is calculated according to the formula below:
E = Z + T - (X + Y)
E = 184.955 + T - (X +90.520).

Z Nominal pinion dimension = 184.955 mm
T Correction of the nominal dimension engraved on the end of the pinion; this may be a positive or a negative value.
X Dimension of the centre housing stamped on the rear right-hand side of the housing, behind the lift ram.
Y Dimension of the pinion with bearing = 90.520 mm.

Using a press and a suitable fixture, Install the bearing cone (31) fully onto the pinion shoulder.
Install the shims (50), their thickness determined by the above formula.
Lightly smear the external surface of the cup (37) with Loctite 603 or equivalent and Install it to make contact with the shims (50) using a suitable fixture. Eliminate all traces of Loctite. The cones and cups must be cleaned and lubricated after fitting. Install the pinion and the cup (33).

Preparing for pinion shimming
Determine a thickness of shims greater than the overhang dimension "a" on the cup (33) measured in relation to the housing face to obtain a provisional clearance of 0.10 mm to 0.15 mm maximum for the pre-load shimming that follows.

Screw two diametrically opposed guide studs onto the housing. Place the shims (36) determined during step and Install the thrust plate (35).
Install and tighten the screws (32) to 90 - 120 Nm after removing the studs.

Shimming the pinion

1st method: adjustment by preload
Place the dial gauge feeler pin on the end of the pinion.
Pull hard on the pinion through the front of the centre housing while turning it alternately from left to right in order to correctly "seat" the cones in the bearing cups. With the assistance of an operator, set the dial gauge to zero.
Depending on the clearance measured, select a definitive thickness of shims in order to obtain the following preload: P1 = 0.03 to 0.09 mm.
Take out the two diametrically opposed screws (32) and replace them with two guide studs. Remove the other two screws and the thrust plate (35).
Install the shims and reinstall the thrust plate.
Install the screws (32) lightly smeared with Loctite 270 or its equivalent and tighten to a torque of 90 - 120 Nm, having first removed the guide studs.

2nd method: torque adjustment
Remove or add shims (36) to obtain a pinion rotational torque of 0.80 to 2 Nm, measured using a Dial type torque wrench, with the pinion turning at approximately 1 to 5 rpm. Once the correct torque has been obtained, refit the screws (32) lightly smeared with Loctite 270 or equivalent, then tighten to a torque of 90 - 120 Nm.

Position the plug (41) in the housing, then refit the union (52) and the lubrication pipe clip nuts.

Reinstall and shimming the differential assembly

Reinstall the right-hand flange (25)
Carefully place the assembled differential in the housing. The right-hand flange (25) is assembled on the centre housing in a similar way to the left-hand flange (9). The positioning is ensured by the locating ring (45).

Reinstall the left-hand flange (9)
Remove the cup (14) and the deflector (13) from the flange (9).
Place the cup on the cone (17).
Install the tool ref. 3376847M91 on the housing, using two screws (56) tightened to a torque of 85 - 100 Nm.
Tighten the central screw to a torque of 10 Nm. Check that the ring gear is not restricted by the pinion and turn it several times to correctly "seat" the cones in the cups. Check the tightening torque of the tool central screw again.

Assemble and moderately tighten the hexagonal calibrated spacers "A" in the other two holes. Check that they are in contact with the housing.

Carry out shimming to obtain: P2 = 0 to 0.25 mm.

Place a ruler edge against the two calibrated spacers.
Using a set of shims, measure the clearance "J" between the finger of the tool and the ruler "B".

Calculate the dimension "C" between the cup (14) and the mating face of the flange (9)C = (114 + J) – 70.

On flange (9), measure dimension "Y" using a depth gauge and a ruler.
Select the thickness "E" of the deflector (13) and shim (49) (if used) to obtain: E = P2 + (C - Y).

To carry out "P2" shimming, there are nine classes of deflector, each of a different thickness. If possible, shim to the maximum tolerance.
Remove the shimming tool and cup (14).
In flange (9), place deflector (13) and shim (49) (if used) and cup (14). The shim (49) (if used) must be fitted between the flange (9) and the deflector (13). Ensure the deflector is fitted in the right direction.

Reinstall the flange (9). Install the pipe (40). Check the backlash.

Adjusting and checking the backlash

- The backlash between the ring gear and the pinion ensures crownwheel and pinion reliability and correct operation of the assembly.
- This backlash must be checked or adjusted after:
• adjusting the taper distance
• shimming the differential unit.

Remove the differential assembly.
Using a "standard" reversible flange extractor, remove the cup (27).

Prepare a thickness of shims (38) of 0.30 mm.
- Do not take into consideration the DC (ring gear offset, e.g. -0.20) engraved on the external surface of the ring gear after the pairing number.
- The pre-adjustment can be followed by an adjustment at a later stage, if the backlash is incorrect during the final check.
Install the previously prepared shim(s) in the cavity of the half-unit (29). Lubricate and Install the cup (27)Reinstall and shim the differential assembly unit.

Checking the J2 backlash
J2 = 0.15 to 0.40 mm
Place the dial gauge feeler pin half-way along a ring gear tooth.
Check that the backlash is between 0.15 and 0.40 mm. Carry out this check on three points of the ring gear, at intervals of approx. 120°.
- If the backlash is incorrect, select and Install a new thickness of shim(s).
- Then offset the ring gear (23) in the desired direction, by increasing the value of the shims (38) and by reducing by the same amount the thickness of the deflector (13) or vice versa, in order to obtain the required backlash, without modifying the differential unit preload.

Reinstall the hand brake mechanism assembly. The plate (5) is rectangular. It must be positioned horizontally, lengthways, so that the pin (6) is correctly positioned against the rod (13)Reinstall the gear (2) (if fitted) and the circlip (1).

Reinstall the left-hand cover plate. Install the hand brake control support, the proportional PTO support (if fitted on the cover plate), remove the guide studs, Install and tighten the screws to a torque of 72 - 96 Nm.
Position the control rod and adjust the hand brake mechanism.
Reinstall :
- the right-hand hydraulic cover plate.
- the intermediate shaft, the driving gear and the top link support.
- the linkage cover plate and the brake pistons.
- the left and right-hand trumpet housings.
- the stabiliser supports.
- the hitch hook.
- the wheels. Tighten to a torque of 400 - 450 Nm.

Reconnect the tractor between the gearbox and the rear axle.
Top up the oil level in the housings and check the level on the gauge located at the back of the rear axle. Remove the safety chocks. Reinstall the body (if removed). Start up. Bleed the main brakes and the trailer brake (if fitted). Carry out a road test. Check the correct operation of the controls and the electrical circuits. Check the tightness of the trumpet housing mating faces on the housing, the cover plates and the hydraulic unions.