First TH350 rebuild, what did I do wrong?

Discussion in 'Transmission & Driveline Topics' started by mtngun, Jul 26, 2019.

  1. Peck

    Peck Veteran Member

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    This is turning out to be an incredibly informative, picture rich thread on the th350 transmission. I’m appreciate the time spent writing this up with such detail.
     
    mtngun likes this.
  2. Da_Raabi

    Da_Raabi Veteran Member

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    This all sounds very familiar. I rebuilt my TH350 last year following all the same steps as you, but I used the ATSG manual. Also included all of the same modifications as you.

    I used the simple Transgo SK kit, as I did not need the full 1-2 shift kit that direct feeds with the gasket - I did it myself like you. Also, make sure you use all of the check balls and modulators. They do not hurt performance (when built right) and help protect the trans. I did this and my trans shifts/performs awesome behind my now defunct 454.

    I had a huge shim problem as well, and ended up needing to do some serious build-up at the back of the case and at the mid-point (struggling with the correct terms here - its been a while).

    If interested take a read through my build log here: https://www.gmsquarebody.com/threads/i-knew-this-trans-was-dead-th350-rebuild.21928/ I've got some good info that may help you.

    Good luck, you will get it done! Once you put it together/take it apart 3-4 times its a breeze!
     
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  3. mtngun

    mtngun Member

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    Thanks for the link to your build, Da_Raabi. I'll read through it tonight.
     
  4. mtngun

    mtngun Member

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    Parts arrived, so I got to disassemble the tranny for the gazillionth time. The 0.020" rear shim goes on the output shaft like so.
    [​IMG]

    The rear shim should have no affect on the low-reverse clutch clearance, but just to be sure I did the dime test. This particular dime was 0.054" thick and the center support and its snap ring were still able to fit with the dime in place.
    [​IMG]

    It's a royal pain to wrestle the center support out of the case when the case is standing upright on the work bench and there is nothing to grab hold of, so I whipped up these simple hooks to grab the edges of the center support and lift it up.
    [​IMG]

    The 0.089" intermediate steel actually measured 0.090, about 0.020" thicker than a standard 0.070" steel. The formerly sloppy 0.082" clearance was tightened up to 0.064" with the usual caveat that there is a lot of "feel" when measuring with a feeler gage.

    I used a small screwdriver to gently hold open a space to insert the feeler gage. You don't want to forcefully pry with the screwdriver because that might damage a friction, or compress the wavy steel.
    [​IMG]

    Aaaand .... final endplay was 0.022". With the current setup all air checks pass and my clearances are golden, yea! :cool:
    [​IMG]

    Lessons learned so far:
    -- check all clutch clearances.
    -- the forward clutch must be air-checked with your chosen pump shims in place. If you change pump shims, then you must redo the air check because shimming the pump can prevent the forward clutch from sealing on the stator.
    -- rear shims are a thing, and allow you to dial in endplay without causing problems for the forward clutch.
    -- check the modulator supplied with your kit with a vacuum tester. You would assume that a brand new modulator would be good, but it was just my luck to receive a defective modulator.

    On to the valve body ....
     
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  5. mtngun

    mtngun Member

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    Looking for modifications to the valve body, or valves that might be jammed up or gunked up. I did not find anything amiss with the valves.
    [​IMG]

    Accumulator inspected and cleaned.
    [​IMG]

    I used a straight edge to check the valve body for warping, but did not find any.
     
  6. mtngun

    mtngun Member

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    I did not find anything amiss with the valve body, so it goes back together same as my failed rebuild. Servo piston & spring.
    [​IMG]

    P.204 of Ron Sessions' book has an error, showing only 3 check balls for the TH350, even though his text says 4 balls.
    [​IMG]

    The four check balls in their standard positions.
    [​IMG]

    On the failed rebuild, I was confused about which valve body gaskets went where since I did not have a stock valve body setup to compare to (both my old tranny and my spare parts tranny had shift kits), but in hindsight I did get the gaskets right.

    Here's the gasket that goes between the case and the spacer plate. The spacer plate was drilled 0.125" as marked, as per Jake's suggestion.
    [​IMG]

    Here's the gasket that goes between the spacer plate and the valve body.
    [​IMG]

    This is far as I got today. I will carry on tomorrow.

    Since nothing was found wrong with the valve body, my thinking at the moment is that my first rebuild failed because 1) the two pump shims prevented the stator from sealing inside the forward clutch drum, so the forward clutch was only halfway functioning and 2) the modulator included with the rebuild kit was defective. But as usual, holler if you think I missed something.
    [​IMG]
     
  7. camaro71/holland

    camaro71/holland Veteran Member

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    Most gaskets have a V or VB and a C stamped into them telling you which one goes against the valve body or the case.

    You could be right about the stator rings causing part of the problem since you had faulty air checks in the clutches that failed after your first rebuild.

    If your modulator was faulty too, then it could affect the automatic downshifts but also the upshifts if it was stuck in a position.
     

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