Pinion depth setting for 10-bolt 8.5" & posi breakaway torque

Discussion in 'Transmission & Driveline Topics' started by jwhickman, Oct 21, 2003.

  1. jwhickman

    jwhickman Veteran Member

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    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. [​IMG]
    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!


    ------------------
    Jeremy
     
  2. Todd80Z28

    Todd80Z28 Moderator Staff Member Lifetime Gold Member

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    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.
     
  3. angel71rs

    angel71rs Veteran Member

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    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.
     
  4. big gear head

    big gear head Veteran Member

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    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.

    ------------------
    '69 RS/SS396 Pro Street
    427 4 speed 9"
    Byars Performance
    High Performance Drive Train Parts And Service
    www.lubedealer.com/biggearhead
     
  5. jwhickman

    jwhickman Veteran Member

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    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! [​IMG]
    Does the lim-slip rebuild include a new S-spring? And it needs shimmed too?
    Thanks for all the help!

    ------------------
    Jeremy

    [This message has been edited by jwhickman (edited October 22, 2003).]
     
  6. rscamaro73

    rscamaro73 Administrator Staff Member

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    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) [​IMG]

    Just my 2 cents plus free tip.

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  7. jwhickman

    jwhickman Veteran Member

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  8. big gear head

    big gear head Veteran Member

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    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.

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

    Zee Veteran Member

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    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.
     
  10. henry1088

    henry1088 Veteran Member

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    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?
     

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