MTFCA Forum

Allan wrote: ↑

Wed Jan 27, 2021 7:23 pm

The one critical area in the assembly is setting the correct crown wheel/pinion gear mesh. Often this is interpreted as setting the backlash, but there is more to it than that. Stan's adding/removing paper gaskets in the driveshaft assembly will help to achieve an alteration in backlash. He also mentions perhaps having to use a shim if removing gaskets will not close the backlash far enough.

I come at this from a slightly different angle. the first thing I do is select 4 steel thrust plates of different thicknesses, the larger the difference the better.

Then introduce the axle/diff centre assembly into the left axle housing, with any two of the steel thrust plates and one of the new bronze thrust washers.

Next, fit the right hand axle housing, thrust plates and bronze thrust washer, and using three of the bolts, lightly clamp the two halves together, leaving the assembly still free to turn. This will leave a gap between the two housings.

Measure this gap. This measurement, plus .003" to .005", will need to be machined off the bronze thrust washers. When machining the thrusts, I aim to end up with a difference of some .012" between the two. Now you have a pack of steel plates and two bronze thrust washers of differing thicknesses with which to play. I mark them with a felt pen.

The whole assembly can then be reintroduced to the housings to check that the steel plates, bronze washers and diff centre are free to move.

Remove the right hand housing and plates and the driveshaft assembly can be bolted up to check the gear mesh. When the mesh is correct, the bevel on the back of both gears will be in line. The backlash may well be out.

Prussian blue on the pinion gear will mark the ring gear to indicate the depth of mesh. It should register along the centre of the ring gear teeth. Deep on the teeth indicates the two are into a too deep mesh, high on the teeth a too shallow mesh.

Rather than readjust the mesh by shifting the pinion gear, the variable thickness stack of steel thrust plates and bronze thrust washers allows the diff centre to be adjusted sideways by swapping the plates/thrusts from side to side. That way the correct rolling fit to the gears is maintained, with correct full width contact across the gear teeth. Shifting the pinion gear in and out shifts the wear pattern in and out on the gears. Backlash becomes a byproduct of this procedure. It is what you get when you have finished the setting up.

When the right hand housing is bolted up, I check that the whole assembly is free to move in the housings. If not a paper gasket between the two housings will correct this.

Others do it differently, and it works. I've no doubt the factory didn't go to these lengths either, but it does make for quiet, long wearing components.