gm 8.5" posi unit rebuild or replace?

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

  1. 71RS6SPD

    71RS6SPD Veteran Member

    116
    4
    Nov 14, 2016
    My 71 8.5" 10 bolt posi rear end has loose spider gears and is whining at speed under slight acceleration. The bearings seem good so I believe it's a ring/pinion alignment issue. 4.10 Gears only have 1k miles on them and were completely quiet until the past 10 miles.

    After a little inspection I see that the spider gears move up and down about 3/16 but look to be in decent condition. Can I get thicker spacers for them to take up the slack or must I replace the spiders? Not sure about the clutches. A little research shows new spiders and clutches are 3/4 the cost of a new chuck.

    I am fine with a 10 bolt and do not want to start looking for a 12.. What have you guys done with your 10 bolts?
     
  2. SRGN

    SRGN Veteran Member

    181
    49
    Feb 20, 2009
    Central NJ
    I've run 8.5 rears down to the low 10 s without issue, Eaton posi with 30 spline aftermarket axles is pretty bulletproof.
     
  3. Zspoiler

    Zspoiler Veteran Member

    I have an 8.5 10 bolt in my 1979 Z-28 RS Pro Touring car. with a 500 HP supercharged small block..The 8,5 is just as strong as a 12 bolt.Thats the reason why GM quit building the 12 bolt.
     

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  4. badazz81z28

    badazz81z28 Veteran Member

    13,960
    91
    May 4, 2001
    Las Vegas, NV
    I upgraded to a 30 spline Eaton with moser 30 spline direct fit axles. Rebuilding the stock diff is not cost or time effective in my opinion.
     
  5. badazz81z28

    badazz81z28 Veteran Member

    13,960
    91
    May 4, 2001
    Las Vegas, NV
    Really? I figured because the horsepower the cars produced after 1970 didn't justify it.
     
  6. SRGN

    SRGN Veteran Member

    181
    49
    Feb 20, 2009
    Central NJ
    12 bolt has a 8.875 ring gear, and the increase in tooth contact is negligible vs a 8.5 10 bolt. A Ford 9 with a Daytona style pinion support will hold more than a 12 bolt, with slightly more drag since tooth contact is much higher. Under 700 HP it won't make much difference anyway.
     
  7. 71RS6SPD

    71RS6SPD Veteran Member

    116
    4
    Nov 14, 2016
    Yep want to keep the 10 bolt. Just asking about the posi unit rebuild. Loose spider gears but in good condition. I read that thicker washers are available for the small spiders to make up for wear. Anyone heard this? Further research: 310 for new Eaton spiders and (steel) clutches. 500 for a new unit with carbon clutches. 4.10 6spd 450 hp.
     
  8. SRGN

    SRGN Veteran Member

    181
    49
    Feb 20, 2009
    Central NJ
    I'd buy a new unit and never look back.
     
  9. czizza

    czizza Veteran Member

    779
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    Jan 8, 2004
    N. Massapequa, NY, USA
    From what I know the 10 bolt 8.5 vs the 8.875 12 bolt are almost the same in power ... based on basic math you are looking at a 4.23% increase in size ... I think the weak spot is the axles and this has been documented all over this site ... if you search for c-clip eliminators people have written that the axles break but not at the c-clip but in the middle of the axle and people who race change the axles to 30 splines and stronger metal.

    I have seen them built up to 800 hp; I think they can go to about 1200 but once you go beyond that you should go with a 9inch (about 6% bigger in size) which we all know can go 2000hp (prostock) but remember they run really short shafts so maybe it is not so much the size of the pumpkin but the strength of the axles.

    Both the 10 bolt and the 12 bolt and have the same pinion shaft size of 1.625.

    http://www.superchevy.com/how-to/48518-posi-10-bolt-upgrade-install/
     
    Last edited: Oct 10, 2017
  10. 71RS6SPD

    71RS6SPD Veteran Member

    116
    4
    Nov 14, 2016
    I'm going to check the axles when I pull it apart tomorrow. If the splines are worn. I will go up to the 30 spline spiders and axles.

    As stated, when they were making 12 bolts, the 10's were smaller than these from 71 and up. 7.5", 8.2". The 8.5" 'corporate 10 bolts share many internals with the 12 bolt and the ring gear is only 1/8" smaller.
     

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