gm 8.5" posi unit rebuild or replace?

Discussion in 'Camaro Questions' started by 71RS6SPD, Oct 10, 2017.

  1. 71RS6SPD

    71RS6SPD Veteran Member

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    Nov 14, 2016
    I thought the 12 bolt ring is 8.625 no?
     
  2. ggtsvnv

    ggtsvnv Veteran Member

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    Jun 20, 2002
    Reno NV,
    I believe the ring on a 12 bolt is 8.875 for cars. Trucks are smaller but I'm not sure what size they are.
     
  3. 71RS6SPD

    71RS6SPD Veteran Member

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    Nov 14, 2016
    badazz81 - I may go the same route.. 30 spline Eaton with moser 30 spline direct fit axles. Ive been looking at part numbers.. could you verify these?
    axles: A103003
    diff 19559-010
    moser studs: 8000

     
  4. Zspoiler

    Zspoiler Veteran Member

    On my 8.5 10 bolt I run a Moroso "Brute Strength" posi with 30 spline Mosier axles. And mine 1979 Z-28 RS Camaro with a 500 HP supercharged small block .Super T-10 4 speed. 130044932860518017_26257430996_o (1).jpg
     
  5. 71RS6SPD

    71RS6SPD Veteran Member

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    Nov 14, 2016
    found this on Moroso:
    The Moroso unit is a heavy duty Eaton Posi. They were made for Moroso by Eaton. All Eaton Posi parts fit in the Moroso unit. The heavy duty Eaton (Moroso) had forged 10-17 gears instead of the standard 10-18 gears, 22 disc clutch set instead of the 18 disc set and 800 pound preload springs instead of the 200 pound springs. You can take a standard Eaton Posi and swap the parts out and build exactly the same thing as the Morosounit. The heavy duty Eaton Posi is also what GM used in the '69 COPO cars.
     
  6. tom3

    tom3 Veteran Member

    10,630
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    Aug 1, 1999
    ohio
    Do those spider gears look ok? No pitting/galling and good full teeth? How about that cross shaft? Seems a shame to ditch the whole punkin just to get rid of the slack in those small gears if the rest is fine.
     
  7. 71RS6SPD

    71RS6SPD Veteran Member

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    Nov 14, 2016
    You're right, lots of reading taught me the unit is a good one and it does all look pretty good. Going to set it up again and keep it for now. All the parts actually look great. Even the steel clutches seem fine. I tried to look for a thickness range but could find any data to measure them against. I also could not find shims to help take up the slack. I'm going to run it for now.
     
  8. jthomas

    jthomas Veteran Member Lifetime Gold Member

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    Dec 5, 2007
    NorthCentral PA
    The spider gears get loose as the clutches wear. There are shims available to rebuild the internals to tighten the spiders up, but if you have gear noise, that would be caused by ring and pinion not being setup correctly or having loosened up.

    Here are two great videos by Tom's Differentials showing how to Tune a Posi which includes shimming the spiders so they are tight:

    Part 1:
    Part 2:
     
  9. 71RS6SPD

    71RS6SPD Veteran Member

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    Nov 14, 2016
    Thanks those vids are great! I have not been able to locate any clutch shims for purchase. Also - I have the old style square tab carrier/clutches. I suspect that the shims for the newer style may work in the style I have?

    Thanks
     
  10. jthomas

    jthomas Veteran Member Lifetime Gold Member

    1,493
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    Dec 5, 2007
    NorthCentral PA
    The shims should work for old and new. They have no tabs and are just round.

    You can get the clutch/spider shim kit from Toms Differentials or Randy's Ring and Pinion. Double check with them when you inquire about the shim kit to be sure they will in fact work with the stock posi. Keep in mind, If your clutches have any wear they should be replaced at the same time. There is a guy on this forum who reproduces the older clutch packs for the stock setup: Camaro-Chevelle. Also, in the video, Tom does not reinstall the springs. Big Gear Head (this forums diff. expert) reinstalls the springs so I follow his lead and do the same. Will work both ways.
     
    Last edited: Oct 17, 2017

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