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How to tell the difference between a 5700lb or 7500lb axle

Discussion in 'Differential & Driveline' started by pduffyd, Jun 9, 2020.

  1. pduffyd

    pduffyd Supporting Member Supporting Member

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    Ok so how do you tell what rear axle you have on your 1975 C20 with out looking at build sheets, (mine are gone). It came with 5700 or 7500lb on the rear, so how do you know what you have?
    I know mine is 10.5" 14 bolt ff with 4:10 gears but i dont know if its a 5700 or 7500 rear.
     
  2. Big Chip

    Big Chip Full Access Member

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    I always thought the difference was the springs and not the actual axle?
     
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  3. pduffyd

    pduffyd Supporting Member Supporting Member

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    Hi Big Chip.
    Yeah I just cant find that info.
    If thats true great!
    My old truck was a cab chassis, 131.5" WB, just cant find an answer for sure. Thought it may have had a code stamped on the axle or something. Mine is an 8200 lb GVW, 2850lb G51 springs on the back with 13" x 2.5" drums.
     
  4. Big Chip

    Big Chip Full Access Member

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    2850x2=5700. I'm not sure if my thinking is correct but I'm sure someone will chime in soon.
     
  5. bucket

    bucket Super Moderator Staff Member

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    I believe that's right.
     
  6. Craig 85

    Craig 85 Full Access Member

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    I could be the size of the rear brakes too. Some of the 73-75 had 11" drums on the rear. I know the 14 bolt FF I put in my K5 back in the 90's did. I checked on Rock auto and on a 75 C20 11's or 13's are available.
     
  7. pduffyd

    pduffyd Supporting Member Supporting Member

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    Hi Craig85, mine are the 13 x 2.5 drums, and the 2850 lb springs are the biggest for the 3/4 ton.
    There were 3 different spring I think 2000, 2600 and 2850, although they were all available for the 5700 axle?
    I just found a spec sheet online for C20 cab, extra cab, and crew cab. Also K20 and C30

    Screenshot_20200610-135230_Gallery.jpg

    Looks like the C/K20 shared the 5700 the C20 extra and crew Cab share a 7500 also found in the C30.
    I think I found my answer.
    Thanks
     
  8. 75gmck25

    75gmck25 Full Access Member

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    I always assumed that anything higher than the 8400 GVW would have required a dual tire rear axle.

    Max load rating for a 10 ply (E load rating) tire is about 3000 lbs, so with single rear tires you only get about 6000 lb max total weight on the rear axle. This is a good match for the 5700 lb rated axle. If you use a 7500 lbs rated rear axle you can’t load it to that level with single rear tires.

    Bruce
     
  9. Bextreme04

    Bextreme04 Full Access Member

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    Max load on my BFGoodrich All-Terrain T/A KO2 in 315/75R16 is 3850lbs. That means max load for the tires on the rear is 7700lbs, which is an aweful lot.

    Looking at the GM Heritage brochure: https://www.gmheritagecenter.com/do...evrolet-trucks/1975-Chevrolet-Truck-PCK-1.pdf, on page 9, the only difference is the spring rating between the crew cab and regular cab. The capacity rating listed isn't for the rear diff, it is for the rear suspension in general. The crew cab is listed as having the heavier springs as standard and therefore the higher standard rating.

    You should keep in mind that none of this will tell you what your truck actually has. If you want to know what it was originally equipped with from the factory, you can order the invoice and original RPO sheet from the GM heritage museum for $50. Since it is 55 years old now though, there is no telling what is has now. As far as I know, all 14B FF should be rated for the same load, regardless of original equipage. Its the frame and springs that are different.
     
  10. highdesertrange

    highdesertrange Full Access Member

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    same rear axle. springs and brakes are different. highdesertranger
     
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  11. pduffyd

    pduffyd Supporting Member Supporting Member

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    Well thanks for the info. I tried the heritage center, but because mine is a RHD export model there is no record, damn it! I tried Holden over here, but back then there was no real worry about tracking RPO codes on a work truck, cab chassis. So I guess im stuffed as far as getting the exact info.
    The ID tags do say its a 3720KG GVW, which is 8200LB, with 7.50 x 16.5-10 tyres
    The ID tags have this info stamped C20903, and the vin tag 8CT235 Jxxxxxx
    So the C20903 suggest its C20 cab chassis with a 131.5" WB, drive train was a L25 292 with a CH465/M20 and the rear end has 4.10 gears, with 13" x 2.5" brakes, which is what it has, the engine and tranny have been removed for a 454 rat and a more modern 4L80e, because of that the front springs have been changed to the 1900Lb set from the 1750 std.
    The rear springs are an 8+1 spring pack which I did measure up and compared to the 75 shop manuals which suggested the HD 2850lb set.
    This is where I could not find the info to say if you have this combo of parts its a 7500 rear end, or its still a 5700. Hence this thread.

    Still I include this for interest of a RHD C20 in Australia
    1973-1980-chevy-gmc-truck-chassis-vin-decoder (1).jpg

    So Vin Number 1st digit is 8 = GM Holden (Australia)
    everything else is pretty straight forward. 8CT235 Jxxxxxx
    Holden
    2WD
    292-6
    C20
    Cab Chassis
    1975
    assembled in Janesville (disassembled ? as its a CKD for export).

    I dont think Im going to worry too much about the rear end, its a 14 bolt FF after all, and seems to have all the HD stuff mounted to it. I was more into what I could haul off the back end, it looks as though a class V hitch would be in order, or an HD Class IV, 14000lb MAX, though I wont be hauling anything THAT big behind once Ive finished my rig. Its more of a nice to know, or be able to point to as proof.
    Thanks
     
  12. pduffyd

    pduffyd Supporting Member Supporting Member

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    Found this

    GVW.jpg

    You are right its the setup of the brakes, suspension and tyres, need them duals to get that 7500lb rating.
     
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