I am rebuilding my 79 Camaro's 2.41 open to a 2.73 posi (used). Its a fast street car...if I ever get it on the road again that is.
I see 'big gear head' is the resident expert, so I'll say before you do that I'm aware of the tools and tolerances required, etc. http://www.nastyz28.com/ubb/smile.gif
1. I'm all ready to start w/ the pinion depth setup; I don't have a 'pinion depth tool' but its seems if I knew the distance from the pinion bearing seat to the case centerline then I'd be set. I saw website at http://www.tavia.com/05001_instructions.html
that showed a measurement of 4.260" for a GM 8.5" 10-bolt. Is this the measurement I'm looking for, or is my logic incorrect?
I saw an old post about measuring the pinion from bearing seat to rear face, and then subtracting half the case race width, but that doesn't seem to make sense, w/o knowing the distance from the pinion bearing to the case centerline. Anyone still following me here?
So my reasoned measurement would be...
[pinion bearing face (from new bearing placed in race) to the front of the case race distance] + [radius of case race] - [4.260 or whatever the distance should be?] = [pinion shim required]
Does the pinion gear height matter (from the pinion bearing seat to the rear face of the pinion) which is used in the other method (this minus the case race radius) at http://www.nastyz28.com/ubb/Forum12/HTML/003882-2.html ?
2. The GM manual says the posi breakaway torque should not be less than 35ftlbs. Mine is about 28ftlbs (measured out of the case @ room temp; ie - not in fluid (but not dry either)), so should I definitely rebuild it; and is a dealer the place to get a kit? What is a typical breakaway torque for a stock posi in good shape?

Thanks for any help!


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Jeremy

I'm curious about the posi kit, too, as mine is currently measuring just over 20ft-lbs to breakaway. I'm not spinning one tire, though.

I don't worry about those pinion depth tools. Even when you are installing new gears, the numbers etched on the pinion are only the opinion of the person that lapped the gears in on a Gleason machine. Might be right, might be wrong.

I go by the pattern. I like to start with the same thickness shim as the current pinion has on it. Then I check the pattern. The original thickness shim will usually get you close, then you adjust from there depending on how the pattern looks, if necessary.

But with used gears, you are probably going to have a tough time getting a proper pattern.

The way that the pinion depth tool works is by measuring from the center line of the differential bearing to the face of the pinion gear. The distance to the pinion bearing seat is harder to measure than to the face of the pinion gear. I have a pinion depth tool, but almost never use it. The best way to set the pinion depth is by reading the contact pattern. Start with the pinion shim that was on the original pinion and work from there. The original shim will usually be within about .005 inch. If you are installing used gears then the pattern will hard to read. You should try reading the pattern on the coast side of used gears because this side gets less wear. The backlash should be set at .008 for new gears and around .012 for used gears. If this didn't answer your question then let me know and I'll try again.

The clutches for the 8.5 posi are available at the GM dealer, but be sure that you find a parts man who knows the difference between your Traction Lock and a Eaton posi. It seems like every time I try to get 8.5 clutches from the dealer they always try to get me Eaton clutches. Shiming the clutches can be a challenge because you can't buy a shim kit for it like you can for the Eaton.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

Does trying the original pinion shim matter if I went from a <=2.56 case to a >=2.73 case? Should I reuse the original shim, or try a new one of the same thickness (or one or two thousandths thicker?)? How exactly does one measure the pinion depth then? What should the distance from the pinion face to the case centerline be? But you're saying don't worry about measuring it, just try the orig thickness shim, and if setting the right backlash doesn't get me the correct pattern, then adjust the pinion shim from there? Does reusing the orig thickness usually get you right pattern?
So is the one w/ the two plates and 4 coil springs the Eaton, the S-spring is a 'Traction-Lock' (by GM or what?), and the cone ones are obviously Auburn?
You think I should definitely rebuild my posi - mine is at least better than Todd's! http://www.nastyz28.com/ubb/smile.gif
Does the lim-slip rebuild include a new S-spring? And it needs shimmed too?
Thanks for all the help!

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Jeremy

[This message has been edited by jwhickman (edited October 22, 2003).]

After watching Jim Moser do one on TV this past weekend (2 guys garage on SPIKE tv), it seems ALOT easier than I had expected, and would follow the methods listed. They used a trick of heating up all the bearings on an electric griddle so the inner section expanded - no more pressing them on (with a hammer) http://www.nastyz28.com/ubb/wink.gif

Just my 2 cents plus free tip.

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" Come to the Dark Side....the BBC force is STRONG within us all...."

73 LT 454 and a small touch of giggles....hee hee hee. More good parts are lining up the shelves.....
mailto:rscamaro73@nastyz28.comrscamaro73@nastyz28.com</A>

http://www.cardomain.com/id/rscamaro73

84 4x4 Suburban 'rust bucket'

Is this a good method to verify pinion shim thickness...(see the very bottom patterns)

http://www.differentials.com/install.html

Start with the original pinion shim, regardless of what ratio you are going to use. Some after market gears have the pinion depth marked on the pinion head, but I have found this to be off by around .003 most of the time. Just set the pinion depth by the contact pattern. The instructions that are on the websight that you posted are pretty good. Set the pinion depth so that you get the pattern between the root and tip of the ring gear tooth.

The Eaton does have the 4 springs and the Traction Lock has the S spring.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

I saw the two guys garage segment and got a good laugh out of it. Not to dismiss the entire segment. The bearing tricks seem like the way to go. Especially buying another pinion bearing and honing the center out and checking your pattern that.

Anyway, I agree with the posts above. I did it for the first time and played with the shims until I got an acceptable pattern. Turned out about .002 more than the stock shim was perfect.

By far the hardest part was setting the pre-load on the pinion. A task which the two guys garage covienantly does over a break and retuns to have them adding an aluminum cover. They say they did it with an impact but I don't see how that would work as I snuck up on it and once you get close, small movements make a pretty big difference in preload.

<font face="Arial,Verdana" size="2">Originally posted by jwhickman:
Is this a good method to verify pinion shim thickness...(see the very bottom patterns)

http://www.differentials.com/install.html</font>

I've read else where that when you're checking the pattern to stick a pry bar between the carrier and housing and apply pressure while doing the pattern.....but that site doesn't say to do that...so whats the right way?

You must set the bearing preload each time you change the pinion shim, but don't install the crush spacer until the final assembly. If you have the pinion bearing preload set to 20 inch pounds this will be enough pressure to give you a good pattern. Put the marking compound on 3 teeth in a row and turn the differential with a wrench on one of the ring gear bolts. Run the marking compound past the pinion several times and check the pattern.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

Does changing the pinion depth by 0.001" make that much of a difference? ie - if I need to adjust it, in what increments should I step?
I presume the old nut should be used (w/o the crush collar) when setting up the gear teeth pattern? Then add the new nut and crush collar when ready to go.

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Jeremy

[This message has been edited by jwhickman (edited October 27, 2003).]

The factory manual says the case bearing preload is set by reaching near-zero clearance then adding 0.004" (I think) to each side - is that still the way to go? That seems like a lot.

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Jeremy

[This message has been edited by jwhickman (edited November 02, 2003).]

I adjust the pinion by .002. You need to get it within +/- .001 if possible. Yes, you should use the old pinion nut until you get the pinion depth set right, and don't use the crush spacer until the final assembly. The differential shims should be put in as tight as you can get them without damaging the shims or the housing. It should be tight enough that you can't pull the differential out by hand.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

I think the smallest shim that came with my kit was .005"

I machined a new pinion bearing to quick-change on the pinion, and was able to measure its installed height as .001 taller than the new one I'm installing. I did not however, keep the old front bearing.
Should I freeze the pinion and heat the new front pinion bearing to seat it, and then use a hammer & block of wood to remove it after checking the pinion depth(s), or just get another bearing and machine it out too for that?


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Jeremy