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Upgrade Open Rear-End to Posi...

Discussion in 'Got gears?' started by Dan E, Jun 21, 2023.

  1. Dan E

    Dan E Well-Known Member

    I'm restoring a '72 Sun Coupe which has an open rear-end that I'd like to upgrade to a posi-unit to match the added Hp I'll be building into the motor. I assume it will require an entirely new rear-end assembly, carrier through axles. Is that correct? I imagine many have already gone down this path, any recommendations on what is the best bang for the buck?
     
    Dan E, Jun 21, 2023
    #1
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No, all you need is the carrier and gears. If your 72 has the original 10 bolt rear, it will be an 8.5 10 bolt. Some Canadian built cars came with a 12 bolt. More commonly though it is the 10 bolt. Stock axles are 28 spline. You can upgrade to 30 spine axles if you want, but the 28 spline axles are plenty strong.

    https://www.summitracing.com/search/department/transmission-drivetrain/section/differential-rear-end-components/part-type/differential-carriers?N=differential-case-design-type:gm-8-5-in

    https://www.summitracing.com/search/department/transmission-drivetrain/section/differential-rear-end-components/part-type/ring-and-pinion-gears?N=differential-case-design-type:gm-8-5-8-6-in

    First, make sure what rear you are working with.

    Monzaz, and Brian Trick here on V8 build rear ends. They may be able to save you money.
     
    LARRY70GS, Jun 21, 2023
    #2
    chiefsb30 likes this.
  3. Dan E

    Dan E Well-Known Member

    Thanks Larry, very helpful.
     
    Dan E, Jun 21, 2023
    #3
  4. BrianTrick

    BrianTrick Brian Trick

    If you like the gear ratio that is currently in there,just replace the open carrier with a posi carrier. The Tru Tracs are a nice unit for those.
     
    BrianTrick, Jun 22, 2023
    #4
  5. knucklebusted

    knucklebusted Well-Known Member

    I've got an 8.5 with a TrueTrac and love it. No special lube, no clutches to wear out and no noise going around corners.
     
    knucklebusted, Jun 27, 2023
    #5
  6. DaWildcat

    DaWildcat Platinum Level Contributor

    And Dan, as easy as it sounds, changing over from an open carrier to a posi carrier, even when using your existing ring and pinion gears, might not require pinion depth adjustment, but may require carrier side shim adjustment to ensure proper gear contact and bearing preload. Not for the faint of heart without the right measuring tools and understanding the procedure.

    Larry's advice is excellent - Brian and/or Jim (username Monzaz) are the resident go-to guys on this stuff. We're lucky to have them here on V8.

    Devon
     
    DaWildcat, Jun 27, 2023
    #6
  7. monzaz

    monzaz Jim

    yes just backlash is required to re adjust if your just changing a carrier.
    Jim
    JD Race
     
    monzaz, Jun 28, 2023
    #7

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