BW78 Pinion Machining for Truetrac Diff Centre

[Grimes]

I'm in the process of overhauling a BW78 differential with a new 3.7 gear set and Truetrac centre to put under my ute.

I've dummy fitted the new pinion into the housing and taken clearance measurements to work out how much I need to machine off the pinion head following the logic below: -

1.8mm - measured pinion interference with new truetrac housing.
1.0mm - Allowance for pinion shimming (dummy fit was with no shim installed).
0.3mm - Thermal growth of diff centre and pinion head assuming 100degC delta T. (It was 8degC ambient here today)
0.15mm - 5% margin of error allowance.
__________________________________________________________
3.25mm - Total calculated machining required off pinion.

The old and new pinions were stamped '0' so I expect I'll be able to reduce the shimming allowance a bit to give a total machining requirement of 2.8mm off a 40mm thick pinion.

This seems to be quite a lot; enough to materially increase the tooth loading.

Is 2.8-3.0 common for pinion machining to fit a Truetrac to a BW78?

Thanks in advance.

 

[losh1971]

Are you running the M78 gears in a stock VS M75 housing? If yes then machining is normally required.

 

[Grimes]

Housing is a 78. The Truetrac has a larger diameter planet gear cluster housing than the standard open centre so the pinion has to be trimmed. ~3mm seems a lot which is why I was asking the question.

 

[Grimes]

Sorted this after putting it aside to think about it for a while. I forgot to account for the pinion offset. The clearance created by the offset is 1.1mm so the pinion only needs 2.15mm taken off. If the shim for the old pinion ends up being the same as the new one needs it will only need 1.7mm. Much better.

 

[Grimes]

Another question before I spend a weekend shimming and meshing myself in circles for hose with first hand diff experience. I have an 0578 stamped diff housing that used to have a 0575 crown gear and pinion. I'm installing a new set of 0578 stamped gears into the 0578 housing.

The old 0575 pinion only had 0.5mm of shim under the inner carrier bearing. Will the new pinion carrier bearing seat need to be machined down 0.12" or will it go in and simply need the normal shimming work to mesh it up?

(and yes, I've searched and read pretty much every thread here on the topic, but haven't come across this scenario.)

Cheers.