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85 GMC C20 rear diff

Discussion in 'Tech Discussion' started by ShortNwide, Mar 2, 2021.

  1. ShortNwide

    ShortNwide Junior Member

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    Gene
    Truck Year:
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    Engine Size:
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    Hey folks, I sure appreciate all the help you have given me!
    Now I have another question....is my rear differential posi or limited slip? And if not, is there a kit I can buy to make it a posi?
    85 GMC C20. 8 lug 16" wheels.
     
  2. SirRobyn0

    SirRobyn0 Full Access Member

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    With the limited info you have given, is it's a factory setup it's either an open diff, or a govloc, a govloc is form of limited slip.

    You can look up your RPO codes to find out what was inside the differential from the factory, or pop the cover and take a look. Those are my thoughts at the moment.
     
  3. nvrenuf

    nvrenuf Full Access Member

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    Identifying the axle will also help - is it a full or semi floater?
     
  4. 75gmck25

    75gmck25 Full Access Member

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    If you look at the option code sticker in your glove compartment, the G80 option is the Gov-Lock rear end. However, its a fairly simple posi and its not particularly durable if abused.

    If you don't have the G80 option, its very simple to swap in a limited slip, but its not cheap. Price out the Grizzly, Detroit Locker, Auburn, Yukon, etc. and you will see what I mean.

    Depending on the GVW of your truck it might be a 14 bolt Full Floating axle, or it might be a 14 bolt semi-floating axle (much more likely in '85 on a C20). If you have hubs sticking out of your rear axle, its a FF. If the end of the axle is flush with the wheel, is a semi-floating axle.

    Bruce
     
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  5. ShortNwide

    ShortNwide Junior Member

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    The hubs are even with the wheel. Thanks for the info everyone!
     
  6. SquareRoot

    SquareRoot Full Access Member

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    Semi
     
  7. SirRobyn0

    SirRobyn0 Full Access Member

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    To someone more knowledgeable than me on these rearends, what the govloc available on the semi float?
     
  8. crpntr78

    crpntr78 Full Access Member

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    Yes.
     
  9. SirRobyn0

    SirRobyn0 Full Access Member

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    @crpntr78 thank you.

    @ShortNwide so in my way of thinking if I were you I'd want to know if it currently has a govloc, look for that RPO code I think is was G80, then you'll know what's inside the diff. If you do have the govloc and it is functional I would just run it. Some folks don't really like the govloc, but on the other hand if it's working you might just opt to keep it vs spending big dollars for an aftermarket posi.
     
  10. 75gmck25

    75gmck25 Full Access Member

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    It also depends on why you want a limited slip.

    A functioning Gov Lock should be fine if you just want something that gets you out of the mud if you are stuck. Just make sure you don’t spin it up real fast, since the sudden engagement of the locking mechanism is what can wreck it.

    If you’re planning to do some street racing or heavy duty off-roading, you need something better than a Gov Lock.

    Bruce
     
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  11. dsteelejr

    dsteelejr Full Access Member

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    Posi is a broad term that just means positive traction, where both wheels are powered and turning in the same direction. There are lots of types of units, including limited slip, that fall under the umbrella of Posi. The original Posi was a clutch pack style limited slip that is still made by Eaton and you can buy and install.

    Another factory option, other than the G80 gov lock, was the G86 limited slip differential. My '73 C20 has it and it's the clutch pack style limited slip.

    It's just my opinion, but I'd stay away from the G80. My uncle is a retired mechanic and he says he's seen too many G80 gov locks blow up to have a favorable opinion of them. They seem to be long lasting only if they are engaged at low speeds over the course of their service life. If you have a G80 in there already and it works I'd just use it until it blows up unless you're willing to spend a good amount of money to upgrade.

    The popular limited slip these days is the torsen style, which is all gears and no clutches. Eaton makes one called the TrueTrac and I installed one in the front differential of my '80 K25. I'm pretty happy with. It's transparent to the operation of the vehicle and can send 3.5x more power to the wheel with traction than the wheel without. Since its all gears and no clutches it's always engaged and working and power is transferred to the wheel with traction very smoothly. You don't have the shock load of a G80 or even a clutch pack style limited slip. It has a lot of good reviews. TrueTrac is great for a daily driver and some off-roading. You're looking at $500-$800 depending on the differential size. It's a full carrier replacement.

    An option for more severe Posi are lockers, which mechanically locks both wheels when throttle is applied. It's hard on tires when going around turns, but great for off road. You have to learn to take your foot off the gas when making turns. I have a Detroit locker in my '80 K25. It's not for a daily driver or if you want a silky smooth ride. The lockers sometimes will bang in and out of lock up.
     
    Last edited: Mar 8, 2021
  12. rt66paul

    rt66paul Full Access Member

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    A friend's G80 exploded because of tight U-turns. He had to do it every time he left his house. With the windows down, you could hear one tire dragging 3/4 of the way through the U-turn. He had good size tires and those U-turns were too much.
     
  13. Craig Nedrow

    Craig Nedrow Full Access Member

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    start at post #34 here: https://www.gmsquarebody.com/threads/1973-k20-350-to-454-swap.29840/page-3. Has been excellent, quiet, positive, easy to set up. Been using Strange and Yukon stuff for years with very good results. Lunchbox lockers also do not require any pinion removal, so all that needs to be done is set the backlash. I have a 454 mild build, but now no one wheel peel

    IMG_2993~photo.JPG
     

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