Powerglde to THM 350 swap

Discussion in 'The "Juice Box"' started by Rich Skylark, Sep 11, 2017.

  1. Rich Skylark

    Rich Skylark Well-Known Member

    Halfway through swapping in THM 350 in place of the original Glide & no real "glitches" yet. Anybody who has done this swap have any tips or tricks to keep this moving along? Have the old tranny out & cleaning up underside a little , waiting a few days to a couple of parts to arrive. Thanks in advance
     
  2. Rich Skylark

    Rich Skylark Well-Known Member

  3. 64 skylark mike

    64 skylark mike Well-Known Member

    You didn't mention what year your car is, but I did that swap in my '64 Skylark last summer. Physically it was a bolt in fit. I had to bend the transmission cooling lines a bit, but very little. And that could have been caused when removing the old engine and transmission. The vacuum line going down from back of manifold to transmission needed a little tweaking, but may have been partly because I was also swapping the 300 engine for a 350. The biggest problem for me has been figuring out how to make the kickdown operate. Still haven't tackled that one. If your car is newer, and has the kickdown and throttle cables going through firewall to throttle pedal, it shouldn't be too hard I would think. That type hook up is what some of the guys on this board have recommended.

    Good luck!
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    First, if it is the original transmission, it is NOT a Powerglide, it's an ST300, they are not the same, but both are 2 speed transmissions. Some ST300s are Switch Pitch transmissions. The ST300 uses an electric kick down just like a THM400. If you swap in a THM350, it uses a cable for kick down. The 64-67 cars used a throttle mounted switch to control the electric kick down. The 68 and up cars used a gas pedal mounted switch for the ST300 and 400 electric kick down. The 68-72 Buick cars with a THM350 had the kick down cable that went to the gas pedal. Chevrolet THM350 cars had the cable go to the carburetor. The kick down cable needs to reach the end of it's travel at WOT. It needs to be adjusted that way. You may be able to set it up at the carburetor like a TV cable (2004R and 700R4)
     
  5. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    Measure the length of the th-350, only some of them are the same length as your 2 speed 300 trans.
     
  6. rmstg2

    rmstg2 Gold Level Contributor

    If you have the car the transmission came out of? It should have everything you need for the swap.

    Bob H.
     
  7. Rich Skylark

    Rich Skylark Well-Known Member

    Some of
    The info on my car is as
    Follows; it is a 1968 skylark custom conv purchased from original owner 3 yrs ago. The car was assembled in Wilminton . The car is mostly stock w/ 1 repaint. The drivetrain is a 350-4bbl with a ST 300 ( sorry for that egregious error) and a 2.56 peg-leg The tranny I'm replacing it with is freshly rebuilt unit w/a shiftkit & slightly smaller converter. The core from the tranny I will be using I got from Bruno D senior . Little problem so far removing the 300, I'm doing in my garage on my back, just don't want any glitches to arise down the stretch
     
  8. Rich Skylark

    Rich Skylark Well-Known Member

    I thought all BOP THM350's (short tail ) were the same ?
     
  9. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    Yes the short tail is the same but the medium and long tails are longer.
     
  10. Rich Skylark

    Rich Skylark Well-Known Member

    Thanks Sean I was told conflicting answers as per moving the crossmember and is it the same for both trannies. What is your thoughts on the driveshaft , bolt in or need to be shortened ?
     
  11. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    I must have been wring on my recolection of the cross-member placement. The driveshaft should not need to be modified.
     
  12. 65specialconver

    65specialconver kennedy-bell MIA

    I was under the impression you only had to move the cross member when going to a TH-400?
     
  13. 64 skylark mike

    64 skylark mike Well-Known Member

    My crossmember fit the TH350 in the original location. Drive shaft bolted right up. Yoke fit also. Bell housing and dowels matched the 350 engine. If I remember right, I used my transmission mount from the ST300 because it was in like new condition, just needed cleaned up. I assume the previous owner had replaced it since the car had 126,ooo miles on it at the time. My understanding is the torque converters do not interchange from the ST300 and TH350.
     
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  14. 300sbb_overkill

    300sbb_overkill Well-Known Member

    They are interchangeable if the st300 is from a '68 car which IIRC was the last year for the st300 or was it '69, it was the last or one of the last years of the st300 when they were in transition to changing over to the TH350. Those year(s) the converters were no longer switch pitch converters and are interchangeable with the TH350.
     
  15. 64 skylark mike

    64 skylark mike Well-Known Member

    Thanks for clearing that up. I didn't know there was a ST300 that was not switch pitch.
     
  16. Rich Skylark

    Rich Skylark Well-Known Member

    I was told the linkage ( column shift ) shouldn't be an issue when going from a 2 speed to a 3 speed. Original linkage is a little loose/ sloppy, anybody know what to do to snug it up a little. Looks like the one bushing on the bottom is shot. Are there any other ones & who if anyone sells them ?
     
  17. 64 skylark mike

    64 skylark mike Well-Known Member

    The column shift linkage worked on my car. We had to adjust the linkage travel where the rod coming down from the column mates up with the arm to the transmission. Goes into park and all the gears fine. Just doesn't read correctly at the gear indicator window.
     

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