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TH400 rebuild

Discussion in 'The Hokey Ass Message Board' started by Zumo, Jun 6, 2008.

  1. Zumo
    Joined: Aug 30, 2004
    Posts: 1,389


    Is there anything I need to pay close attention to when rebuilding a transmission? I have never done by myself or even touched once since high school(15 or so years ago)

    I was going to order a rebuild kit and have at it. Just tear it appart, clean and vat it and then put it back together the way I took it appart. It there a tech manual on the th400 anywhere I could reference from?
  2. Zumo
    Joined: Aug 30, 2004
    Posts: 1,389


  3. It's been several years since I did one.
    I have a video somewhere that was REALLY useful in IDing parts, and their orientation when reassembling Best way to go IMHO.

    IIRC I needed to turn down to OD on a socket to get the center support bolt out, and I did all the drums with C clamps.
  4. wally bell
    Joined: Apr 15, 2008
    Posts: 418

    wally bell
    from VA.

    Send it to a pro.

  5. JB 472
    Joined: Aug 22, 2006
    Posts: 23

    JB 472
    from Maine


    I will offer my opinion, for what it's worth. The THM-400 is a complicated beast. You will be surprised how much stuff is inside that case. You have to do some homework first!

    Search this site and the internet for information on turbohydramatics. You will find that how you rebuild it will depend on what year it is and what you plan to do with it.

    JB 472
  6. Oh, bloody hell. The TH400 is one of the easiest to rebuild, I oughta know, I've done many.
    Get yourself ANY GM manual for ANY car that has/had a TH400. Follow the rebuild intructions therein.
    Or, order the TH400 manual by Ron Sessions from HP Books.
    Follow the rebuild intructions therein.
    THe most complicated thing in the trans is the valve body, so be careful how you take that apart, if you do (you prolly should disassemble to clean it thoroughly).
    The most difficult thing is the piston seals for the clutch drums. Patience and a feeler gauge are your friends here. And you WILL be frustrated by this. But it CAN be done.
    Two 'C' clamps will suffice for removing the clutch springs.
    Measure the freeplay at the input shaft BEFORE disassembly, you'll have a better reference for reassembly. This is simple: place a straightedge across the bellhousing and pull the input shaft out as far as you can. Measure from straightedge to shaft. Push shaft in. Remeasure. Subtract. Compare to specs. Done.
    Remember, you do NOT need to know how the damn thing works to rebuild it. You DO need to know how it works to modify it, unless you follow the oft-beaten path taken by so many before you. The Session's book is good here.

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