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Thread: ST300 to TH350 Swap

  1. #1
    Join Date
    Apr 2015
    Posts
    3

    Default ST300 to TH350 Swap

    Hi guys, relative newcomer here & I am doing some some long awaited upgrades to my 68 custom convertible . I am losing the 2.56 peg leg in favor a 3.08 posi(8.2 chevy) and retiring the ST 300 in favor of a TH 350 with a shift kit & a slightly looser converter. These 2 upgrades should add a little more zip to the step. M questions are 1) do I need to change my crossmember ? 2) Any ideas on how to swap linkage from my 2 spd to a 3 spd ? 3) Do my tranny coolant lines need to be swapped as I was told the different trannys flow is opposite ? Thanks in advance for your help

  2. #2
    Join Date
    Apr 2002
    Location
    connecticut
    Posts
    1,913

    Default

    300 and 350 are the same length. the 300 is prndl, the 350 is prndll, so no l1. the trans lines don't care where they flow. I don't know about the rear change. might be the same which would use the same driveshaft. you need to check.
    "help support commerical viability of bovine aviation"

  3. #3
    Join Date
    Sep 2004
    Location
    Cincinnati, Ohio
    Posts
    160

    Default

    Cross member stays as is, 350 should bolt up the same.
    I do not know about the 8.2 Chev rear for sure - strong possibility there is no change but measure twice.
    Linkage - will bolt right up, you will not see a low position but it is still there. I've got 2 cars with this 2 sp to 3 sp conversion and they both will go into low no prblem.
    You did not ask about the kickdown - on the older cars that is a challenge to hook up, totally do-able but takes some thinking.
    Ted Nagel
    WILDCATS 'R' US

  4. #4
    Join Date
    Apr 2015
    Posts
    3

    Default

    I'm told I need a bracket that mounts on the manifold and a cable. Also going to need new speedo gear, anybody know who sells one?

  5. #5
    Join Date
    Jun 2002
    Location
    Oakland Gardens, N.Y.
    Posts
    1,004

    Default

    Quote Originally Posted by Skylarkman View Post
    I'm told I need a bracket that mounts on the manifold and a cable. Also going to need new speedo gear, anybody know who sells one?
    The THM350 uses a kick down cable Buick used a cable that went from the gas pedal to the transmission. Chevy used a cable from the carburetor to the transmission. You haven't mentioned what car you are working on.

    You can get speedo gears from a number of sources. You'll need to know the rear gear (3.08), the rear tire size (?), and the number of teeth on the transmission DRIVE gear. Then use the formula to figure out what DRIVEN gear you need.

    http://www.tciauto.com/tc/speedometer-gear-calculator/
    Larry
    1970 GS Stage1
    TSP 470, 602HP/589TQ
    Best ET, 11.54@115.00 MPH, Best MPH, 11.58@115.89 MPH
    1998 SC3800 Riviera
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    Larrymta@verizon.net
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  6. #6
    Join Date
    Apr 2015
    Posts
    3

    Default

    Larry, it is going into my 68 Skylark Custom convertible .

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