Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Gear ratio question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MyK-5, Jul 26, 2000.

  1. MyK-5

    MyK-5 Registered Member

    Joined:
    Jun 18, 2000
    Posts:
    50
    Likes Received:
    0
    Location:
    Phoenix
    Ok I got a 1981 Blazer. I think it has a 350(atleast thats what the guy said when I bought it), But the vin says that it a 305. Its got a Th350 with a Np 208. The Rear end is a 12 bolt. Question is what would the Rear end gear ratio be, and is it limited slip.

    thanks

    81 Beater
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

    Joined:
    Jul 23, 2000
    Posts:
    36,170
    Likes Received:
    1,377
    Location:
    E-town baby!
    I have an 81 Jimmy (looks like a beater) My vin also said 305, turned out to be a 4 bolt 350, year unknown. I had to replace my rear main seal about a year after I got the truck, so when I got the pan off and saw the 4 bolt mains I knew I had the 350. I guess someone swapped it in. Mine had the 4 spd manual with the 208 case and 12 bolt rear. The gear ratio was an awful 2.76:1 (open diff) common ratios were 2.76's,and 3.08's. You can check if it is posi by jacking and blocking the rear end, then spinning one tire by hand, if both tires spin the same direction its probably a posi, if one tire spins the opposite direction its open.
    Rene
    81 K5 Jimmy,350, SM465, NP205, FF 14 bolt, HD Dana 44, 3.73's Body in progress
     
  3. Executioner

    Executioner 1/2 ton status

    Joined:
    Feb 22, 2000
    Posts:
    1,530
    Likes Received:
    0
    Location:
    Temp. Ft. Walton Beach Fl. home=Tucson, Az.
    Neat test for a possi, but I don't think it is very accuraten under most circumstances !
    Will it always work if the clutch parts are wore out ?
    Will it work on an Eaton Gov-lock ?
    Will it work on a True-track(made by Detroit, uses worn gears) ?
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

    Joined:
    Jul 23, 2000
    Posts:
    36,170
    Likes Received:
    1,377
    Location:
    E-town baby!
    The test will only tell you for sure if it is an open diff. It will not work on the gov-lok, because it requires some centrifugal force to lock it up. I had one in an S-10, what a worthless waste of time (IMHO). Even a worn posi will spin the tires the same direction. If you want to check how worn, have a buddy try and hold one tire from spinning, that will give you an idea of tight or worn it is (a good tight posi should'nt break away) I'm not sure about the Detroit all gear posi.
    Rene
     
  5. NYC K5

    NYC K5 1/2 ton status

    Joined:
    Apr 3, 2000
    Posts:
    185
    Likes Received:
    0
    Location:
    Staten Island NY/da bronx
    all the info on your truck should be on a sticker in the glove box. as for the motor being swaped youll have to run the #'s. to check the gear ratio jack up one end mark the tire with chalk turn the driveshat until the chalk mark returns to its original spot 2 and 3/4 turns of the driveshat means a 2.75 rear (2.73 actual thats what was common)
     
  6. 6.2Blazer

    6.2Blazer 1/2 ton status

    Joined:
    Feb 24, 2000
    Posts:
    4,675
    Likes Received:
    26
    Location:
    Ohio
    This test is not always accurate, depending on what type of differential is installed. The only thing this will tell you for sure is if both tires spin the SAME direction, then you DO NOT have an open diff.
    For a Detroit Tru-Trac (I have one in the rear of my Blazer), the tires will spin in opposite directions...just like if you had an open diff.
    The typical clutch type limited slip will still spin both tires in the same direction even if very worn (it doesn't require that much force to spin the other side). However, if a person can hold one side still while another person spins the other side, then the limited slip is severly worn and will be almost completely ineffective as a person can't generate nearly as much torque on the tire as the engine and gearing can.
    I'm not sure about the Gov-Lock.
     

Share This Page