What is needed to get a POSI?

Discussion in 'Transmission & Driveline Topics' started by SPG, Oct 26, 2018.

  1. SPG

    SPG Bumblebee Builder

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    Hello, I'm currently putting the finishing touches on my build sheet for the 75 camaro and the final thing I need (I feel like I keep saying that) is to turn my differential into a posi. At first i thought it was because my sister said both wheels were spinning during a burnout, but I looked at the numbers on the back, and checked the skid marks...I most certainly don't have a posi.

    Anyways, I plan on putting a 5th gen lt1 into the car and supercharging it later down the road, so I need to upgrade this a little. I'm told these axles are solid so i don't need to 100% replace them. What do I need to convert this to a posi and get a 30 spline axle? (preferably more since I'll be looking at 600+ hp)

    I looked at moser's site and I'm honestly very confused at what I'm looking at to even start to accomplish this.

    If it matters at all, I plan on putting in 15x10 rims in the back so the wheel diameter would end up being around 28"

    edit: Meant to post this in Transmission & Driveline Topics, sorry about that
     
    Last edited: Oct 26, 2018
  2. camaro71/holland

    camaro71/holland Veteran Member

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    Just get a posi :)
    At the same time you'll probably want to change the gearing as well.
    To convert to 30 spline, you need a posi with 30 spline spiders, bearings and axles.
    You can't get more splines in a 10 bolt without going to a spool but you should be ok @ 600 hp.
     
  3. big gear head

    big gear head Veteran Member

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    If you are going to drive this on the street and a little drag racing then get the Eaton Posi. If you are going to drive it and do some auto cross then get the Eaton Truetrac. Both are available with 30 splines. You will need the HD installation kit with either of these. Moser makes direct fit 30 spline axles for the 8.5 Camaro rear end, so no modifications are needed with them.
     
  4. SPG

    SPG Bumblebee Builder

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  5. big gear head

    big gear head Veteran Member

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    The Truetrac is correct. Moser makes 30 spline axles for the '71 to '81 Camaro 8.5 that fit without modifications. I don't know if Moser made a mistake when they made that page, but the description at the top says 10 bolt but when you read farther down it says 12 bolt. You can not use 12 bolt axles in the 8.5 without modifying them. I would contact them before ordering anything. The installation kit that you picked out has junk shims. I would use a kit from Tom's Differentials, Yukon or USA Standard. Be sure that you get the HD kit that works with the Truetrac. The gears that you picked will probably be fine if you have overdrive and a tire around 28 inches tall.
     
    Last edited: Oct 26, 2018
  6. SPG

    SPG Bumblebee Builder

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    Would one of these work?
    https://www.amazon.com/Yukon-BK-GM8...F8&qid=1540580588&sr=1-1&keywords=BK+GM8.5-HD
    https://www.amazon.com/USA-Standard...=1540580588&sr=1-1-fkmr0&keywords=BK+GM8.5-HD

    And I would have OD and 28" wheels so we're fine there.

    I will reach out to moser to see if that's an error on their page.
    Edit: I looked up the part number, looks like it's just a typo https://www.summitracing.com/parts/msr-a103003
     
  7. big gear head

    big gear head Veteran Member

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    The Yukon kit that you listed doesn't have any shims in it. I've never seen that USA Standard kit priced that low, but it will work. The difference between the Yukon and USA Standard kit is the bearings. The Yukon kit has Timken and the other kit has Koyo. I haven't had any problems with the Koyo bearings except in the '04 to '06 GTO rear ends. The one thing that I don't like about the USA Standard kit is the pinion nut. I would rather use a OEM style nut instead of the generic nut that comes in the kit.

    Some kits for the 8.5 have ring gear bolts with a 3/4 inch head and some have bolts with a 5/8 head. Do not use the bolts with the 5/8 head. I would rather use the original bolts with the 3/4 inch head than use the other bolts. You are not supposed to reuse the ring gear bolts, but used bolts with the 3/4 inch head are much better than new bolts with the 5/8 inch head.
     
  8. SPG

    SPG Bumblebee Builder

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    Alright thank you for all the info, it was very helpful! This was one of those things where looking up all this information was very confusing since I didn't know what I even needed!
     
    big gear head likes this.

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