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Differential gear 'slop'....how much is too much?

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guzzul

guzzul

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I have the differential out of my '78 Spitfire. It was whining at speed, so i thought the pinion/ring gears may be worn. As it turns out, one of the output shafts does have a growly bearing, so I'm thinking I need to replace that.

On checking the ring and pinion by just moving it back and forth, there doesn't seem to be too much 'wiggle', and they don't look overly worn (althoug I don't really know what I'm looking for here...). But the sun and planet gears do seem to have a fair amount of play in them, primarily the sun gears (i.e. the gears with the splined centers that the output shafts slide into).

In fact, I can move the sun gears laterally about 1/16"-1/8". I'm not sure how much of the lateral 'slop' is supposed to be there, or whether these gears and the planet gears are supposed to be pretty tight together. There is wear on the teeth, for sure, this is an old diff. But how much is too much? https://www.britishcarforum.com/ubbthreads/images/icons/confused.gif

Anybody have any thoughts?
 
Get a manual ,do the best you can to follow the instructions.Be ready for it to still whine when you're done.This is a job for a pro that knows diff's (hard to find nowdays).You would be well advised to just find a good swapmeet unit to put seals into and go back-together...
MD(mad dog)
 
[ QUOTE ]
This is a job for a pro that knows diff's (hard to find nowdays).

[/ QUOTE ]

He's correct. I was a corporate technical trainer, and our differential class was dropped in the early '90s, as there was not enough failure anymore to justify that skill.

Note item #4 in the Moss catalog is a collapsible spacer. If you've taken the diff apart, it will have to be replaced and the correct procedure followed to collapse it to get the proper pinion depth. If not, you only need to check the wear pattern. You will see wear where the teeth mesh. That should look like a little oval right in the center of the ring gear teeth. Once set up properly, the pinion depth will not change with time.

Here is a beautiful method I learned from an old master years ago. Take a sheet of paper and run it through the gears one time. If the diff is set up properly, the paper will be cut as though with scissors. If too loose, the paper will be bent. If too tight, it will be mangled.
 
I will try and find a differential manual. I have the Leyland Repair Operation Manual, which does a good job of describing tear-down and rebuild, but does not describe 'diagnostic' procedures.

I would have taken it to a pro, but I can't find one. All the shops just swap in a rebuild. Somebody must be rebuilding them somewhere, but not here I guess.

Thnx.
 
Thanks Paul. I have only craked the case open, and taken out the drive shafts. I was careful to count the turns on the pinion nut, and I do not need to take out the pinion shaft, just replace the seal. I think I can get it assembled ok again.

I was surprised that the ring and pinion seemed pretty good. But the sun-planet gears are the ones that concern me.

Thanks for the tip on gear spacing. I was wondering whether I could get a feeler guage in there somehow. Paper sounds brilliant.
 
guzzul,
You might want to give John Esposito at Quantum Mechanics a call. He's rebuilding them now and does an excellant job. I just got mine back (a TR6 diff to be put into my TR250). I thought it would need a total rebuild and was ready to part with some serious cash to have it done. I couldn't have been more pleased. John inspected it and said all I would need was a couple of bearing carriers and some shims. The estimates was much better than hoped for. If there was a drawback, it took him a lot longer to rebuild it; it kept getting pushed back because of other work he had going. But I didn't need it as I was still using the original one in the 250. I have no financial interest in this; just want to pass along my good experience with a fellow enthusiast.
 
guzzul,
I have a factory mrk4 manual. Quoting Instruction #34 from the final drive section, "By selection from the above range of pinion shims, reduce the end float to give ZERO (their capital letters not mine) backlash. Note that at zero backlash the assembly will be tight and difficult to rotate. Lubricate prior to final assembly"
I've done this on my threes and is a piece of cake. The only hassle is making a guesstamite as to the thickness of the spherically shaped bronze thrust washers. My suggestion is check the current slop with a dial gauge or feeler gage and divide by 2. Take that number and add it to the thickness those thrust washers in your unit. Then order three sets (about $6 total) one thickness greater and one thickness less than the thickness number you came up with and start assembling/disassembling. Don't think I've ever gotten one on the first try. As far as the crown wheel and pinion If there's just a bit too much backlash (maybe 3 or 4 thousandths) you can cheat just a bit by adjusting the carrier shims to move the crown wheel over 001 or .002 closer to the pinion without getting the contact point too far off center thereby quieting the assembly even more. Sure is nice though when you get the slop out of them.
Tom Lains
TS6851
TS58107
 
Yes Tom, but his is a later model which does not use shims. It has the crush spacer instead. Different set up.
 
Thanks Tom. I think the pinion you mean is the pinion that the planet gears mount on, and not the main pinion gear. The sun-planet pinion is the same setup in my 1500 I expect as in your Mk4, with shims between the gears and the side of the carrier.

Your approach makes sense. And you have answered my question, mainly that there should be zero backlash in the sun-planet gears. I'm going to try and check it more accurately with a feeler guage tomorrow, but it is way more than 'zero' for sure.

I was trying to avoid pulling the carrier out of the case, but maybe that has to be done to fix this (not by me, hopefuly I can find a shop).

You suggestion on moving the crown wheel over a tad would work to to take a little slack out of the main crown-pinion gear. That's actually not too bad in my diff. Although I have read in another source that the overall backlash there should be in the range of .005", and I probably have more than that too.

Appreciate your checking this out.
 
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