ST300 Convertor in a TH400

Discussion in 'The "Juice Box"' started by avmechanic, Jan 19, 2022.

  1. avmechanic

    avmechanic Well-Known Member

    Hi folks.
    I am swapping out a 300V8 with St300 transmission in my wife's 67' Skylark for a 69' 430 and a 70' TH400 that I have. I don't know much about the Th400 I have but it came in another purchase I made a while back. I am going to have a quick look inside and stuff a shift kit in it before I drop it in. I was told it worked fine and the fluid looks good so it is worth a try for our budget big block swap. I was thinking of looking for a convertor on the local marketplace for a little higher stall to go with the cam I am using. I have not seen much come up in the last couple months. I have a custom convertor in my 66' with a 455/TH400 combo. I will likely do something similar with this car eventually once we iron out the rest of the car. I was thinking it might not be a bad idea to use the ST300 convertor in the TH400. My guess is it has a slightly higher stall and smaller diameter than the convertor that was with the TH400. This Th400 is from a Riviera. It is a 13" convertor. I am pretty sure the ST300 should be a 12". The ST300 is a non switch pitch. It seems to work fine so I can't see why not to give it a try. Any extra stall speed would be useful. Anybody see any issues?
  2. LARRY70GS

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

    No issues, that was a popular swap back in the day, for precisely the same reason.
  3. wkillgs

    wkillgs Gold Level Contributor

    The ST300 converter from your wife's '67 would be a switch-pitch.
    My 66's have two different bolt patterns on the flex plate, one for the ST300 converter and the other for the larger diameter ST400 converter.
    I've ran both ST400 converters and ST300 converters on a ST400.... definitely prefer the smaller one. It stalls at around 2000 low/3000 high. The 400 converter, IIRC, was about 500 rpm lower.
  4. ragtops

    ragtops Gold Level Contributor

    Did the '70 model year Buicks use a switch pitch 400 transmission? I'm thinking no. Which means the 300 converter is not going to fit.
    SpecialWagon65 likes this.
  5. LARRY70GS

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

    1965-67 Buicks used the switch pitch transmission. That mean a 1967 transmission was originally a SP, but Buick used the ST300 transmission in 1968 and 69, and it was not a SP in those years. Of course 55 years later, all bets are off. SP transmissions were sometimes converted to fixed pitch.
    TrunkMonkey likes this.
  6. avmechanic

    avmechanic Well-Known Member

    Since we just bought this car a couple months back and I have not pulled the transmission yet I have yet to confirm whether the St300 we have is a switch pitch unit or not. There is no kickdown or switch pitch wiring on it at all. It is tough to tell what the previous owners have done to the car so it is possible they just removed the wiring for some reason. I am familiar with the differences and what to look for. My 66' Skylark (that I have owned since the 80s) was originally a 340 4B switch pitch 2 speed car. The previous owner (a family friend) removed the 340 when he blew it up and put a 300 in along with a non switch pitch ST300 2 speed. I had a parts car 300 switch pitch 2 speed and was going to swap it in but never did. I ended up swapping a 455 TH400 in instead. Unfortunately I threw away the 300 ST 300 switch pitch. I tossed lots of **** in those days as I had no place to keep it. This was 30+ years ago, I was an apprentice mechanic and just payed for room and board for a place to live. I Kick my ass now but you can only hold on to so much ****. I suspect my wifes car is a non switch pitch ST300 and will see when we pull it. If it happens to be a switch pitch convertor I will send it to Jim W. for a core. I will likely toss or give away the 300 engine and transmission otherwise. I am aware that you can not mix switch pitch stuff and non switch pitch stuff. I do have a stock big car convertor with the TH400 I have so we will be fine either way. I just wanted to confirm that I can install a non switch pitch convertor in our TH400 and get an idea if there is a little stall increase in doing so. To the best of my knowledge I was pretty sure this could be done and was just confirming. I am an experienced mechanic and have been working on Buicks and other hot rods for over 30 years. I always research my plans anyways to be sure. Thanks for the concern and advice guys.
  7. TrunkMonkey

    TrunkMonkey Totally bananas

    Small and large switch pitch converters interchange with both 300 and 400 switch pitch trans.

    And small and large non-switch pitch will interchange with 300 and 400 non-switch pitch.

    In case anyone ever needs to use a 400 converter in a 300, to get a car on the road.
  8. LARRY70GS

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

    All else being equal, a smaller diameter converter will stall higher than a larger diameter converter behind the same engine.
    avmechanic and TrunkMonkey like this.
  9. Super Bald Menace

    Super Bald Menace Frame off oil changes

    The st300 converter being lighter will also let the engine rev quicker
    avmechanic likes this.

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