350/203 to 400/208 Swap

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Beast, Feb 6, 2001.

  1. Beast

    Beast Registered Member

    Oct 26, 2000
    Likes Received:
    Southern California
    My TH350 rear housing mounting flange is broken and rather than build a new one (replace the case) I was curious about the issues in installing a 86 TH400 and NP208 assembly. I have a line on a complete setup but was concerned if it is a bolt in to my 73 K5. Does anyone know if there are different mounting and driveshaft length requirements? I was told that the TH400 is longer but the 208 makes up for it resulting in identical driveshaft requirements. There are enough posts that deal with similar issues that I was wondering if anyone has ever put together a comprehensive parts swap/dimension/description list.
  2. pcorssmit

    pcorssmit 1/2 ton status

    Jan 2, 2001
    Likes Received:
    Denver, CO
    Your stock drive shafts will not work.

    208s (with the exception of '81's, supposedly) use a slip yoke rear driveshaft with a long tailhousing. Some (maybe all) 1/2 ton 208's use a different u-joint for the rear shaft than older 1/2 tons. A conversion joint is available. Not sure which joint the 3/4 ton 208s used.

    GM switched from a Spicer CV to a Saginaw CV around '76. They are not interchangeable. The Saginaw attached to the t-case with flat, round 4 bolt flange (similar to a Toyota). You MIGHT be able to use the front shaft from the donor truck. If it is too short, you can buy a 1" spacer for it.

    The 208 shifter mounts to the body, not the t-case. Shifter linkage adjustment is critical, if it's not right you will burn out the range fork in the t-case, leaving you stuck in low range. If you use your truck hard, I'd try to fab a shifter mounted to the t-case to avoid problems. Oh yeah, mine likes to pop out of low. A lot.

    As for the TH400, you will have to put in an electrical kickdown. Easy to do.

    Have you considered a 700R4? Its about the same length as the TH400 and would give you the overdrive as well as a better first gear (3.06 vs. 2.48).

    Good luck,


    '83 K5, 350 TBI (ex 6.2), Dana 60/14 bolt, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd

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