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How Worn is my 10 Bolt Posi?

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Old 09-04-2015, 05:53 PM   #1
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How Worn is my 10 Bolt Posi?

Hello Guys
I got a 10 bolt 7.5 posi 3.08 (my car was a rare T5/TPI and now it is auto) under my 87 Trans Am WS6. My pinion is loose and clunks when I put in Drive - Reverse - shift to 3rd. Today I opened the case and found wear on the spiders and Ring gear. What do you think judging the pics attached?


Last edited by F-body driver; 09-04-2015 at 05:59 PM.
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Old 09-04-2015, 06:46 PM   #2
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Re: How Worn is my 10 Bolt Posi?

Can't tell anything about it from the pictures. I don't see anything terrible, but that's about all.

There are a couple of tests you can do though, that will tell you a little something about it.

How much end play do the axles have? and, if you put it in 5th and try to turn one wheel, how much torque does it take?
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Old 09-04-2015, 07:17 PM   #3
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Re: How Worn is my 10 Bolt Posi?

Thank you for your reply!
Now my car is not a T5 anymore because the previous owner swapped an 89 block and a 1987 700r4 in it. Before my mech changed the pinion seal the diff. worked great. After the job (IMHO he didn't tightened the pinion nut well) the yoke has a rotative play and it clunks. Even with an expensive oil and the GM posi additive the play didn't go away. I pulled the whole axle and checked myself the clunk is from the pinion. Will the diff. hold good with street tires (even with little abuse)? I can tell the posi unit works good because during hard accelerations it did 2 wheel peel.
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Old 09-04-2015, 07:22 PM   #4
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Re: How Worn is my 10 Bolt Posi?

Crush bearing. Tighten the nut down some more.
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Old 09-04-2015, 07:39 PM   #5
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Re: How Worn is my 10 Bolt Posi?

If you have a inch pound torque wrench tighten the pinion but till it takes about 35-40 inch lbs to turn the pinion with the driveshaft and brakes removed. Also since cover off if you have a dial indicator check backlash should be .006+-.002 measured in 3 or more places around the ring gear. What you are checking I with the pinion held still you should be able to turn ring gear a total of about .006.
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Old 09-04-2015, 09:53 PM   #6
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Re: How Worn is my 10 Bolt Posi?

I have to disagree with checking the pinion bearing preload with the differential and axles installed. There is no way to get an accurate measurement like that. The only way you can check pinion bearing preload is with nothing in the rear end except the pinion.


Backlash must be checked with the pinion help perfectly still. If the pinion moves any at all then the backlash measurements are going to be completely wrong. Backlash on used gears is usually going to be about .001 or .002 more than it would be with new gears, so if you had a .008 +/- .001 tolerance then you might have as much as .010 or .011 and still be good.


If you are getting rotation at the pinion, but no up and down or in and out movement then it's probably fine. If you have any movement other than rotation then it needs to be fixed. If it's making any noise then it may have a bearing going bad or the bearing preload may be off.
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Old 10-12-2017, 01:11 PM   #7
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Re: How Worn is my 10 Bolt Posi?

i tightened my pinion nut but the rotational play wont go away.
I figured today, when doing a circle that the tire that turns slower kinda squeaked like someone tried to lock it.
The car makes donuts good with 2 wheel peel
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Old 10-12-2017, 02:11 PM   #8
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Re: How Worn is my 10 Bolt Posi?

Rotational play will not go away. That's called backlash and some amount is required. If it is excessive then I would suspect that the Auburn differential is worn. As the cones wear it will increase all of the clearances inside the differential and cause backlash and axle end play to increase.
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Old 10-13-2017, 03:16 AM   #9
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Re: How Worn is my 10 Bolt Posi?

Ok now I perfectly understand! Thank you!
I guess I have kinda 25-28 thousands of backlash
I don't know what is the safe limit for those auburns.
I am currently running 70W90 with 3/4 of a bottle of lsd additive
I can buy a 70W140 if it helps tho
Here in Italy is so expensive to rebuild a rear end (I do all the stuff on my own) so I hope it will last
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Old 10-13-2017, 06:55 AM   #10
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Re: How Worn is my 10 Bolt Posi?

70W90 should be fine. I don't see any need to run the thicker oil in a street car.
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Old 10-13-2017, 07:47 AM   #11
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Re: How Worn is my 10 Bolt Posi?

Quote:
25-28 thousands of backlash
That's WAY too much: about 4 times as much as it should have.

It sounds like whoever set that rear up, didn't do it right.

The backlash is set with shims on the carrier bearings. If you take off those 4 bolts holding the carrier in, you'll find the bearings, each with a shim (or a pack of them, depending on what they did) between the bearing and the housing. The factory shims are each a single thick piece of cast-iron, somewhere near .250" thick. Replacement ones are usually a pack of thinner ones, the most popular style being 2 that are .100" thick each with a few really thin ones stacked in between.

If you imagine what would happen inside the rear as you change their thickness, you can see that if you take shim from one side and move it to the other, you shift the carrier (along with the ring gear) from side to side within the housing, moving it toward and away from the pinion. This is how you set the backlash. If you add shim equally to both sides, you put preload on the bearing.

There MUST be at least a little backlash, because gears GROW when they get hot, which means, the backlash (gap between pinion and ring) goes away when they heat up; and while having a large gap is bad, having LESS THAN ZERO gap is even worse. REAL BAD things happen when 2 moving parts try to occupy the same volume.

Ideally, you should adjust the shim packs to as thick as you can make them and still easily put the carrier with bearings into the housing; then shift the thickness from side to side, leaving the sum of the 2 sides the same, until the backlash measures around .008" - .010" for used gears.

Get a "gear installation kit". https://www.summitracing.com/search/...6%2B4294945459 It comes with everything you'll need, and more besides; but as far as I know it costs more to buy just the parts you need, than the whole kit.

Take the carrier out, being very careful to keep the shims that come out, associated with the bearing they were behind. Measure their thickness with a micrometer, which you already know is wrong, but at least it gives you somewhere to start, once you get to the point of selecting the right ones. Remove the old carrier bearings and press new ones in the kit onto the carrier. If you can't do this yourself, any competent machine shop will be able to do it for you.

With the carrier out, tighten the pinion nut until the preload on the pinion bearings is around 25 inch-pounds (the actual preload should be a little less than that, but the grease seal adds a certain amount of drag, which you just kinda have to guess at). Just like you set front wheel bearings: as you tighten the nut the play in them goes away, then once the play is gone, you tighten them a small amount more, to preload the bearings. You can easily make a tool yourself to hold the pinion still, then use a socket and a breaker bar with a VERY long handle to tighten the nut. Here's the tool I made for this. It's just a piece of metal about " thick, 1" wide, and 4' long, from the hardware store, with 2 holes drilled along the edge, to bolt it to the yoke.



Then once the pinion bearing preload is set right, put just a little bit of wheel bearing grease in the new bearings (don't "pack" them, it only takes a little to lube them enough for assembly), put the carrier back in with the new outer races, and using 2 of the thick shims in the kit on each side with some thinner ones stacked in between, make up packs the same thickness as the ones you took out. By trial and error, adjust the thickness of the 2 packs until their total thickness is to the point that you can just barely get the whole setup back in, and the backlash is in the .008" - .010" range as described above. Then take it back out one last time, use 2 large C-clamps on the outer bearing races to compress and preload the bearings, and add an equal amount of shim to both packs, to establish bearing preload (just like you would do for front wheel bearings except there's no nut you can tighten for that); you should be easily able to add .005" to each, and probably as much as .010", and still be able to get it back in. Then final-assemble it: set the shims in the housing and slide the carrier in with the clamps on it as far as it will go that way, take the clamps off, and slide it in the rest of the way by tapping GENTLY on the bearing races with a hammer. Be careful not to bend the shims or otherwise damage anything: if you find that it's real easy to put it back together, take it back out and add a couple of .001"s more shims to each side, but once it gets to the point that it's hard to get it back in, you've reached adequate preload. Don't get do greedy with adding preload that the bearing races catch on the shims and distort them.

I totally agree with BGH's oil recommendation. Thick oil is not a good idea. I prefer 75W-90 Mobil1 gear lube with either a single bottle of the factory posi additive or 2 bottles/tubes/whatever of the aftermarket stuff. Put in all the additive first, before beginning to add the oil. It takes a bit less than 2 quarts (liters) of lube to fill it: don't overfill it, fill it only until fluid just begins to dribble out of the fill hole, with the housing in the orientation it normally sits in. If you fill it too full it will just all leak out the axle seals and get all over your brakes.

Last edited by sofakingdom; 10-13-2017 at 07:54 AM.
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Old 10-13-2017, 08:34 AM   #12
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Re: How Worn is my 10 Bolt Posi?

That step by step guide is impressive! Thank you A LOT
I will surely rebuild it!
If you see the pic I attached 2 years ago you can see my spider gears: They seem worn (the car has been driven with very low fluid in the past).
Since the carrier itself is quite expensive what do you seggest to do?
A simple rebuild (bearings,shims, etc) or a full one?
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Old 10-13-2017, 01:54 PM   #13
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Re: How Worn is my 10 Bolt Posi?

When you said that you have .025 to .028 inch backlash, were you measuring this at the ring gear or at the yoke? IF you hold the pinion gear perfectly still then you should have about .008 to .011 backlash at the ring gear with a sued gear. If you check the backlash at the yoke then you are going to measure much more backlash because of the size difference between the pinion head and the ring gear diameter. Backlash should never be measured at the yoke, or pinion gear. Also if the pinion moves even the very slightest amount when you measure backlash at the ring gear it will throw your backlash measurement way off.
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Old 10-14-2017, 11:33 AM   #14
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Re: How Worn is my 10 Bolt Posi?

at yoke
It makes a clunk when I put in gear (like a bang) and makes the same clunk when I downshift and brake to a stop
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Old 10-14-2017, 12:23 PM   #15
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Re: How Worn is my 10 Bolt Posi?

Tried new universal joints?
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Old 10-14-2017, 02:36 PM   #16
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Re: How Worn is my 10 Bolt Posi?

Bought em but i need time to install em
They seems good tho
The strange thing is my crossmember mount
that's the strange mount. just ordered a poly one from energy susp
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Old 10-14-2017, 08:30 PM   #17
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Re: How Worn is my 10 Bolt Posi?

If you have about .025 backlash at the yoke then the backlash at the ring gear is probably just fine. It wouldn't hurt to check it if you are able.
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