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MG Midget and Sprite Technical - Service Tool 18G 191 A - Differential Bearings

Hi All.

Looking at rebuilding my differential. I've read and understood (I think!) the instructions in my manual (not Haynes, the proper one).

Only thing that is a bit annoying is that the measurements to take into account diff bearing size variations are quoted as +/- from a thickness (Block A on Service tool 18G 191) - and I can't find that thickness anywhere.

Is it OK to measure existing (possibly worn) bearings and shim (either + or -) the new bearings to match the thickness of the old ones? We're talking 1 or 2 thou so how much wear does a diff bearing see? I'm concerned wear in the bearing would result in this method being wrong.

I'm fitting an LSD so I don't have stamped C and D variations on the cage; I'll be measuring these off the machined LSD case.

Thanks!

Rob Armstrong

Hi Rob, if the diff is quiet, when fitting a slippy I proceed as follows:
If leaving the pinion in place (when the bearings feel good) measure the backlash between CW&P before dismantling then adjust shims with slippy to achieve that same backlash. You need to have sufficent shiming to get the required bearing pre-load at the same time.

If changing the pinion bearings, measure the pinion head setting. This can be done relative to the carrier bearing bores, but i have found that a flat plate bolted between housing and bearing caps with a hole in the right place allows you to measure the setting with the tail of a digital caliper. As the resolution of either method is significant in terms of the variations you need to measure, take at leat half a dozen readings, discard the outliers and take an average. That then ensures the pinion head is in the same position as originally, and so restoring the backlash will see the gears eshing as before.

Following this procedure has always resulted in a quiet diff.

Adjusting shims to compensate for the differences between old and new bearings is a worthwile initial step, it will get you close to where you want to be but always go for maitaining the pinion head and backlash settings before dismantling if the diff was quet to start with.
If it was noisy, that's another story and a lot more work ...
Paul Walbran

thanks Paul.

I don't know if it's noisy as it hasn't been in yet...

But, the teeth look good so I think I'll be following your first bit and leaving the pinion alone, and setting the diff up to be the same subject to an acceptable level of backlash when measured.

I have 2 3.9s with blue teeth that I can have a go at if I feel it's necessary!
Rob Armstrong

If the teeth look good, it should be OK to leave the pinion and set up the LSD with the same backlash.

What I forgot above is that you will also need to do is compensate for any difference in width of the cage measure across the shoulders against which the bearings and shims mount. This can be a tricky measurement to take as the crownwheel mounting gets in the way, but can be made easier by doing so with the crownwheel removed. I use a set of deep jawed digital calipers, again taking the average of seeveral measurements.
Paul Walbran

Thanks again Paul. I don't think I have a set of suitable calipers to measure round the crown wheel base so I'm going to have to employ some brain energy to get that measurement.
Rob Armstrong

That's do-able. What is worth getting is a ground flat plate to use as a reference. I have one with a hole in it that allows the carrier bearing spigot to protrude through so the shoulder the bearing presses up against is in contact with the plate. That removes a significant source of uncertainty.
Paul Walbran

Worked it all out in a spreadsheet. Measured everything at least 3 times. New bearings in. Aiming at 7 thou backlash as that's what is stamped on the crownwheel. I've got 7.4 thou. I had calculated the extra 0.4 and chose 7.4 over 6.8 as I haven't got any shims less than a thou.

Putting it in tomorrow, hope it works!
Rob Armstrong

And is it quiet, or are you too busy lauching and enjoying yourself to notice?
Paul Walbran

It is quiet! Very tiny noise on overrun with the hard top on but that's expected as it's not a new crownwheel/pinion set. I've gone down to a 4.2, with the 5 speed it's 3600 rpm at 70 so still sensible. In gear acceleration is somewhat brisk....
Rob Armstrong

Cool. LSD might feel a bit odd at first, but once used to it you'll never look back!
Paul Walbran

This thread was discussed between 21/11/2016 and 29/11/2016

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