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MG Midget and Sprite Technical - Diff. Sun/Planet gears - thrust washers

Not wanting to confuse Graemes "Diff Pinion Seal" thread, this one's about the differential (sun and planet) gears, and the thrust washers behind them.---Note, not to be confused with the Pinion or Crown gear thrust washers and shims.

Sussex spares list just one size of thrust washers for the sun and planet gears. But Moss lists 5 sizes for the sun gear thrust washers, and 2 sizes for the planet gear thrust washers.

Sussex list.
ATA7039 Sun gear thrust washer, and 2A7062 planet gear washer. Both are 0.035" which are the original sizes.
Moss list.
ATA7039 -- Sun gear thrust washers, 0.035" 0.040" 0.042" 0.046" 0.048"
2A7062 -- Planet gear thrust washers, 0.035" 0.054".

I rang Moss, and they told me they supply the additional sizes to accommodate wear in the sun and planet gears. I haven't found anything in the official BMC workshop manuals that describes setting up the differential(planet and sun)gears, other than how to remove and replace them. There is no mention of different thicknesses for the washers, or how to measure for them.

Both the thrust washers wear out (get thinner), and Moss are selling additional thicknesses, because the gear bearing surfaces also wear out.

My questions are:
1) Should you buy new planet and sun gears AND new standard thickness thrust washers together?

2) If keeping the existing sun/planet gears, how do you measure which sizes of thrust washers you need? How much free play should there be?

3)I've read in some threads, that the SUN gear thrust washers are fibre, and often disintegrate. However, I've just stripped a diff, and the SUN thrust washers are copper/bronze, or the like. Does anyone know if fibre replaced copper/bronze?

Lawrence Slater

Lawrence, I thought both sun & planet gears were now NLA.
All the sun washers I've seen recently have been fibre; I guess Moss are listing thicker washers/shims to allow re-use of worn gears as there are no new ones about?
David Smith

Lawrence-
TW were originally bronze; side gear ones later changed to fibre, which sucks. Don't think fibre would live a week on the planet gears, and never saw a fibre one there. That is for MGB; I assume same for Spridget.

It is not very critical, and there is no spec for clearance, but there should be a bit of play = absolutely NOT TIGHT. A min .005 feeler behind any one thrust face should not create any tightness or drag as you rotate the gears, .010/.015 or so is probably what they had new. The outside back faces of the gears should be approximately flush (visual) with each other, to ensure correct mesh - that is the design standard for such gearsets. If the backsides are significantly out of line, then the gears will be excessively loaded on the tooth tips; this is exactly the situation with worn TW or mating surfaces.
You can measure what to order by shimming all gears in some combination until snug, then order slightly thinner than your shims.

It is VERY welcome news that OS TW are available - they never were and are often desired - the dealers used to look oddly at me when I asked. But, those are pretty strange steps in thickness - I'm thinking at least the sun TW are from some other application. Hope they start supplying them for B series axles as well.

FRM
FR Millmore

I've replaced the thrust washers in quite a few diffs and don't recall finding anything other than fibre washers on the sun gears.

The replacements that I bought from Rover/Unipart back in the '90s were fibre, also.

I also wasn't aware of any oversizes.
Dave O'Neill2

I've taken a few diffs apart and come across steel, bronze, and the remains of fibre washers from behind the sun gears. I think the fibre ones are used in the later diffs and don't stand up well although probably lasted long enough that nothing failed in warranty. I still have some bronze sheet to punch some new ones out of but like many things it's a round tuit to make the punches, well one anyway as I got a whole range of fly press punches off ebay so the inner diameter is already sorted.
David Billington

Thanks chaps.

David, Moss list both the gears as NLA, but Sussex have the diff pin and planet wheels made by the same company. You're right though, they don't list the sun gears, I thought they did.

Thanks FR, now I have some idea of what I'm looking for.

Just for completeness, I'll post a pic of the large flat bronze sun gear thrust washers tomorrow. From what FR says, it may be that they aren't worn that much at all. The planet "cup" thrust washers are very worn though.

There's another reason I asked about replacing the planet gears.

The pin that supports them is worn. Don't the gears wear at this point too? Even when you move them along to an unworn section of the pin, they feel sloppy. How "snugly" should they fit on the pin? should they rock a little?
Lawrence Slater

The pin ought to be softer than the gears themselves (don't have any data on this, but they "ought" to)

When I found a worn pin, I found that a new one fit nice and snug, supporting my, above referenced wishful thinking!

However, your test of sliding it to an unworn section of the pin makes me wonder about yours. Maybe the best answer to your questions is to buy them all, and return what doesn't work out (that's what I did with the shim thickness question).


Norm

Norm Kerr

The planet gears do wear on the pin, but it is usually not much of an issue. Most of the wear will be on the pin, and any left just gives a bit of slop on reversing the drive. The exception would be scored bores, resulting from severe abuse/no oil. The gears themselves actually float a bit, which lets them equalize the loads on the teeth, such that it is shared by both gears. Some very complex gearboxes rely on this to function at all, like the Spicer twin countershaft Roadranger, on which it seems like all the many large gears are going to simply fall out.
Often, what appears to be TW wear is more wear of the mating surfaces in the carrier, especially the planet pockets - that's why OS TW are nice. It is important to check that new TW are not sitting on wear shoulders at either location. Same caution for measuring.

FRM
FR Millmore

This thread was discussed on 28/10/2012

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