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NP205 TH350 to TH400 conversion

Discussion in 'Transfer Case' started by D-Rock, Jan 24, 2021.

  1. D-Rock

    D-Rock Junior Member

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    First Name:
    Derrick
    Truck Year:
    1979
    Truck Model:
    K20
    Engine Size:
    350
    I currently have a TH350 with an NP205 transfer case, I am building a TH400 that I'm going to put into it. What all is needed to make my transfer case work with the TH400. I already know that my driveline lengths will be different and I'll need to move the cross member... I'm just concerned right now with what is needed to make the tcase and tranny mate up.
     
  2. nvrenuf

    nvrenuf Full Access Member

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    The correct tailshaft in the th400.

    A short 32 spline input for the 205. (this will require boring open the 205 shell to accept the bigger bearing)

    A th400 to 205 adapter.

    Driveshaft mods are the easy part.
     
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  3. D-Rock

    D-Rock Junior Member

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    Engine Size:
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    The 400 came out of a 4wd so it should have the correct tail shaft.

    Advanced adapters makes a kit that has all of the parts but it's just over $600. How much are the parts by themselves, ballpark price?
     
  4. Bextreme04

    Bextreme04 Full Access Member

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    You’d probably be time and money ahead to just find a 400/205 equipped truck and buy it complete. Then you’d get all the parts you need and can probably sell your original 350/205 setup for as much as you buy the truck for. My 80 K25 came with a 400/205 and cost me $800 as a running, driving truck. All 1980 trucks had a 205 BTW.
     
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  5. nvrenuf

    nvrenuf Full Access Member

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    Not quite... there were 2 versions of the 400/205 - pre '85 and '85+. These 2 versions use different adapters, different trans tailshafts and different versions of the 205 (adapter bolt pattern and input shaft lengths). If you decide to buy a 205 (to save cost / make the swap easier) keep this in mind so you get the correct tcase or parts.

    You probably have a 205 with a figure "8" bolt pattern on the face for the adapter. If so, the 400's tailshaft needs to stick out ~4.5" from the tail of the trans case to fit a figure "8" version 205 with adapter.

    I have no realistic idea of cost anymore. Once upon a time, the correct 205 could be bought for $400-$600 and the adapter was $150-$250 if bought separate. Nowadays, FB etc have everyone believing that even bald tires are worth $100 ea so pricing is crazy. Don't forget the cost of machining your tcase input shaft hole (which requires complete disassembly).

    Off Road Design is a great source for the parts and they also have a really good tech page about tcases.
     
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  6. D-Rock

    D-Rock Junior Member

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    Engine Size:
    350
    Ok, thank you for the info and help.

    I've tried looking around for a tcase that was with a 400 and all I can find so far is rebuilt ones for much more than I can do it myself for.

    The machine work might cost me a 6 pack or a trade for some electrical work so I'm not too concerned about that cost.
     

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