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3/4 semi floating axle to full floater swap

Discussion in 'Differential & Driveline' started by SirRobyn0, Jun 3, 2020.

  1. SirRobyn0

    SirRobyn0 Full Access Member

    Posts:
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    Joined:
    Dec 6, 2019
    Location:
    In the woods in Western Washington
    First Name:
    Rob
    Truck Year:
    1984
    Truck Model:
    C20
    Engine Size:
    350
    So my 1984 C-20 350 W- 700R4 has the semi floating rear axle and I'm looking to swap it to the full floater we are talking the factory full floating diff, not some other diff. I believe that it is very likely the current axle is 3.08 unit, but I've not removed the cover to count. RPO code indicates that it is the "standard axle" with no reference to axle ratio, and really I should and will find out my true axle ratio before proceeding.

    My main questions, I "think" that the axle housing is the same size so I "think" I won't need to shorten my drive line, but I could be wrong, if someone knows that info that would be great.

    I know some of you are going to get whinny about this, but fuel economy is a concern, for me. Look I know most of these trucks aren't mileage makers, but I don't want mine to be any lower than it is. I spend a good amount of time loaded, but also have some long freeway drives to get some of those loads. One of my concerns is that most of the stock full floating diffs I've seen seem to typically have axle ratio of 3.73 or 4.10

    I'm thinking if I look for one with a 3.73 ratio that should be ok, with the 4.10 probably being a bit lower than I'd want to go, really I'm just looking for a second opinion / other thoughts from ya'll.
     
  2. Bextreme04

    Bextreme04 Full Access Member

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    Joined:
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    Location:
    Salem, OR
    First Name:
    Eric
    Truck Year:
    1980
    Truck Model:
    K25
    Engine Size:
    350-4bbl
    A 1984 C-20 would not have come with a 3.08 as standard with any power team according to the GM Heritage booklet. What are the Actual RPO codes for your truck? An LT9 with Trans Code MX0 And Axle Code GQ1 would be a 3.73. For the same engine/trans code but GT5 axle code you would have a 4.10. If you have Trans Code MX1, then GQ1 is a 4.10 and there is no GT5. You can look up all the standard power teams and options from the 84 trucks for yourself here: https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1984-Chevrolet-Truck.pdf
     
  3. SirRobyn0

    SirRobyn0 Full Access Member

    Posts:
    429
    Likes Received:
    370
    Joined:
    Dec 6, 2019
    Location:
    In the woods in Western Washington
    First Name:
    Rob
    Truck Year:
    1984
    Truck Model:
    C20
    Engine Size:
    350
    I appreciate that. As your probably aware I'm mechanic, but that doesn't mean that I'm up on specs at all. So unfortunately my RPO code sticker is in pretty bad shape, but I was able to make out the axle code of GQ1, and I just looked my trans code is MX0, so that would make my axle a 3.73 as it is or was from factory. I sure appreciate you setting me straight on that. And that being the case I'll have to consider if I should look for FF that is 3.73 or 4.10, I didn't want to take to big of a jump, but a slight increase to help with the heavy loads might be a plus. I'll have to think on it a bit.
     
    Bextreme04 likes this.
  4. Curt

    Curt Junior Member

    Posts:
    9
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    2
    Joined:
    Sep 11, 2017
    Location:
    Carlsbad
    First Name:
    Curtis
    Truck Year:
    1984
    Truck Model:
    K-20
    Engine Size:
    383
    I just swapped the full float 14 bolt for the semi float 14 bolt.I had to have my driveshaft lengthened 2”.

    The semi float has a pretty long nose.Standard axle on this 84’ model K20 was 4.10 gears.

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