ST300 to TH400 Swap

Discussion in 'The "Juice Box"' started by Christos, Feb 8, 2013.

  1. Christos

    Christos Member

    Hello all, I currently am gathering parts to install a 69 th400 short shaft (From a Pontiac I believe, trans has PW on serial tag) to replace my st300 which is leaking fluid all over the place. My questions are, will the st300 cross member work with the th400? Will i have to cut my drive shaft even though i have a short tail trans? I am told to get a th400 yolk from a 73-77 Grand Prix. what kind of speedo gear should i get for the stock 10 bolt 2.73 rear end? I also want to install a floor shifter. I am looking at the Hurst Pro matic 2 but can't decide to get the regular one or the Longer truck version due to the fact that i have bench seats and might be farther away from the shifter. (Car right now is a Column shift)

    Fire away.

    ---------- Post added at 11:09 PM ---------- Previous post was at 11:08 PM ----------

    Can I also use my existing trans lines?

    BRUCE ROE Well-Known Member

    I did that swap in my 68 Cutlass sedan, using a 68 Pontiac TH400. Using the shortest TH400 yoke, I
    managed to use the same drive shaft for another 50,000 miles. Pretty sure I used the same cross
    member, just slid it back 6 3/8". Can't remember if I changed the shifter shaft, left the 300 dash
    indicator in place. The speedo gears need to have the same ratio as what is in your 300, assuming
    it is accurate now. Not accurate, determine the error and make a correction in the conversion. The
    lines can usually be bent to work, though some early TH400s had 3/8" tubing fittings. These can
    easily be swapped for the common later 5/16" tubing fittings, probably from the 300.

    Oh yea, some early TH400s had a smaller outer dia yoke than latter. Shaft is the same, but different
    from the 300. Use a matching yoke, or change the tail piece for to match your yoke.
    Bruce Roe
  3. Christos

    Christos Member

    Does the drive shaft need a 27 spline yoke or a 32? i read that the th400 is a 32 but i cant seem to find out what spline the drive shaft is that was for the st300.
  4. nekkidhillbilly

    nekkidhillbilly escaped mental patient

    a th350 would be a direct bolt wouldnt it? just a thought i have one im going to sell.

    BRUCE ROE Well-Known Member

    I don't remember how many splines a TH400 output has, but its a unique, FAR BIGGER shaft than all those
    other light duty transmissions. Bruce Roe
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If you have the 400 Transmission, pull the speedo gear sleeve, and count the number of teeth on the DRIVE gear. The DRIVE gear is on the output shaft, and you can see it through the sleeve hole. Once you have that number, the rear ratio, and rear tire size, calculating which DRIVEN gear you need is easy.

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