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10 to 12 bolt rear

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 86blazerk5, Aug 9, 2000.

  1. 86blazerk5

    86blazerk5 1/2 ton status

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    Ok, I have an 86 with a 10 bolt rear, it has a gov. lock diff. some people call the gernade locks, and i think mine is about to blow. I was told that all i have to do is take it to a shop and have the governor removed, however, i have a shot at a 12 bolt rear witch would be much nicer anyway. The problem is, i am verry short for cahs, and i dont know what kind of shape the 12 is in, if it needs rebuilt, i cant affords it. My question is are the gear ratios the same for my 10 as it is for the 12, I was told if not, it would screw up my t-case, so i must make sure, i would be pulling this 12 out of a jucked 75 blazer. I also have a shot at a Np205 t-case from the same truck, How is it a direct swap or do i need to change stuff. Also what is the likey hood i need noth or either rebuilt for them to be usable. Help please, i am new to this stuff.
    Ike

    Ike
     
  2. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>I see your in Maryland. I'm in northeastern NJ and I'm converting to 3/4 ton in the near future. My 12 has 3.08s (i know a guy that has a set of hardly used 3.73 and I can install them for you if you want) and all new brakes and seals (dec 98, 10K on them brakes). The only thing is the spider gears clank a little at low speeds (the only reason I can feel it is because I have a stick and everything is transfered through the drivetrain). I never broke it, and have wheeled it through some tough trails. Email me and we can talk more</font color=blue>

    http://www.chrisd71k5.coloradok5.com <font color=blue></font color=blue>
     
  3. 4GUNZ4X4Z

    4GUNZ4X4Z 1/2 ton status

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    Post deleted by 4GUNZ4X4Z
     
  4. LittlePig

    LittlePig 1/2 ton status

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    You have the transfer cases backwards. The 205 is the gear-driven part-time case. The 203 is the fulltime (which is not as weak as you might think, and not that expensive to convert).

    Anyway, to address the original question, if you are getting a 205 from a '75 truck, it was probably a manual transmission truck. The Manual 205 has a 10-spline male input. To get it to work with your 25(?) spline transmission, you will either need an adapter kit, or find a new input gear that matches the tranny you have. The TH350/700R4 use a male to male connector, which is basically a sleeve that goes on the two input shafts. Otherwise, it won't be worth it. Since you say you wouldn't haev the cash to rebuild an axle, it is a good bet you also can't afford to swap transfer cases anyway. However, don't give up. At least count the splines on the input shaft of that 205. If it is a male shaft with anyting otehr than 10 splines, you can always hang on to it and swap it later. If, by some chance, it has a female input (a hole instead of a shaft sticking out) grab it. Even if you never use it, that is the rare 205 that works with the TH400 tranny and even if you never use it, you can sell it to someone doing a TH400 conversion for big bucks that you can then invest in your own truck. But to clarify again, the 205 is the good part-time case, and the 203 is the not-quite-as-good (but still much stronger than the 208 and later cases) fulltime case. As an identifier if the numbers have confused you, the fulltime case has two "boxes", one verticalone, which is the range box, and one diagonal one with the front output shaft, which is the chain drive box. The Part time case is just one diagonal box, with all input and output shafts on the same unit. The 205 (good) case also has a circular plate in the center of the back side, held on with three bolts.
     
  5. 6.2Blazer

    6.2Blazer 1/2 ton status

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    The only sure way to get the correct ratios is to actually look at the ring and pinion. You can either count the number of teeth on both the ring and the pinion and divide the two (i.e..41 ring gear teeth / 11 pinion gear teeth = 3.73, etc...) or these numbers will be stamped on the ring gear. Your '86 might also have the sticker in the glove box which would tell the ratio's (if you know for sure it is stock). GM offered gear ratios of 2.73, 3.08, 3.42, 3.73, and 4.10 over the years so there is really no sure way to find the ratio just by saying it's an '75 Blazer.
     
  6. Boss

    Boss 1/2 ton status Author

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    Hey 86,

    Pretty much everyone is right about how to check the gears. Don't think b/c its a 75' it will automatically be 3.08 gears. I would assume it has a better chance of being 3.73. Anyways, obviously the 205 is the best t-case out there: cast iron, gear driven parttime case. Can't beat that despite the high gear ratio. The only way you will ruin your transfercase if the rear gears don't match the front is if you put it in 4WD. If you swap the 12 in and don't put it in 4wd for now and save money to get the gear changed, that could be an option since your rear is blown you said right.
    Boss
     
  7. 4GUNZ4X4Z

    4GUNZ4X4Z 1/2 ton status

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    Yes I would like to make a correction to my statement the 203 is the full time 4 wheel t case and the 205 is not. The 208 being aluminum are subject to breakage. Sorry for the confusion. Thanx for pointing that out LittlePig. I know better but the lack of sleep has been take'n over,been up all nite play'n with this site.

    Professional JEEP EATER, it's a population control thing
     
  8. 4GUNZ4X4Z

    4GUNZ4X4Z 1/2 ton status

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    I think I should go to sleep now. Disregard my comments my condition is unstable at this time.

    Professional JEEP EATER, it's a population control thing
     
  9. Boss

    Boss 1/2 ton status Author

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    4Gunz,
    Still wrong though bro. 205 is cast iron, 203 is pure steel (almost 100% positive on the steel part). But anyway, you're right that the 208 is aluminum. But the 205 is the ultimate BULLET PROOF case that came stock.
    Boss
     
  10. 86blazerk5

    86blazerk5 1/2 ton status

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    well thanks guys on all your advice, I think i will forget about changing t-cases. Its to much hassle and i will probably never get in too deep anyway, However, I am still interesed in changing to a 12 bolt rear. No, my rear axle is not yet blown, in order to save all i have to do is have the governor removed but The thought of have a 12 bolt is real nice. I aklready know if it needs rebuilt, i cant afford it. However, I need to know if it is even possible. I know both my truck and thye scrap 75 have stock gears, however, i dont know what they are. If anyone knows what the stock gear ratio for an 86 front corp 10 bolt, and for a 75 rear corp 12 bolt, Plaese let me know what they are.
    Thanks
    Ike

    Ike
     

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