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Thread: 1980 255 to 351w Swap - Flexplate Question

  1. #1
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    Default 1980 255 to 351w Swap - Flexplate Question

    I am having a hard time finding the answer to this question. Maybe I am making this harder than it needs to be (I am good for that...). Here's the deal. My 1980 Tbird has the 255 V8. I am kicking around the idea of dropping a 351w in it. I have a 351w from a 1986 F150 sitting on a stand in the garage. The goal here is to swap this thing in as cheaply as possible. I would like to eventually put an AOD with the 351w, but for now I would like to just use the C5 that is in there. The short question is, is that possible? I know a C5 is supposedly a weak tranny, but I am not going to beat on it.

    My concern is the flexplate. From what I understand the 351w uses a 28oz flexplate. I always hear that you have to swap to a 50oz (why? I don't know). However, the 255 is neither a 28oz or a 50oz, but some other oddball weight. What does the flexplate weight have to correspond to? The transmission?

    How is it that one can put a 50oz flexplate on an engine that had a 28oz anyway? Wouldn't that screw up the engine's balance? I just don't get the whole flexplate thing. Maybe somebody can start at the beginning and explain it to me, lol.
    CoogarXR

    1986 LTD Wagon
    1985 Cougar XR-7


  2. #2
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    just get a flexplate for the new motor. otherwise you'll have bad vibrations and ruin the engine.

    the flexplate tooth count and weight are the only really important bits. get the same tooth count as the one in there so your starter will still work. any 28oz flexplate will work. I guess you could junkyard it from any 351 with an auto.
    Quote Originally Posted by jcassity
    I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
    Hooligans!
    1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
    1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
    1986 cougar.
    lilsammywasapunkrocker@yahoo.com

  3. #3
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    If the 351 uses a 28oz, use a 28oz with the same tooth count as used currently with the 255/C5.

    Also, there wouldn't be a C5 in there. What is the trans code?
    Long live the 4-eyes! - '83 Tbird Turbo

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    I have to get under it later today, so I'll see what the transmission is. I thought it would be odd for a C5 to be behind a v8, but that's what I read on Wiki (gospel, I know). I know it's not an AOD, heh.

    About the flexplate... So let me get this straight. The balance has to match the motor. I must have a 28oz flexplate period. I just need to get a 28oz flexplate with the tooth count & diameter that matches whatever is on the current setup. Is that how it goes?
    CoogarXR

    1986 LTD Wagon
    1985 Cougar XR-7


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    yes. there is also an offset to the tooth ring on the flywheel. it will be either 3/8" offset, or 3/4" offset. the starter must match the old offset to work with your old starter.

    the flexplate/flywheel is the external balancing for the motor. it must he matched.
    Quote Originally Posted by jcassity
    I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
    Hooligans!
    1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
    1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
    1986 cougar.
    lilsammywasapunkrocker@yahoo.com

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    Thanks for the help so far.

    I looked up a 1986 Ford F250 5.8 on rockauto, and it shows:
    Teeth: 164; Pitch: 12; O.D.: 14.23" (D1ZZ6375A)
    And I looked up a 1980 Thunderbird 255 V8 and it shows:
    Teeth: 164; Pitch: 12; O.D.: 14.23" (E0SZ6375A)

    Since the flexplate is already on the 351w, and the diameter, pitch, and tooth count all match (according to rock's listing), I should be ok? Unless the offset is different? If the offset is different, I suppose I could just get the starter from the f250?
    CoogarXR

    1986 LTD Wagon
    1985 Cougar XR-7


  7. #7
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    exactly right
    Quote Originally Posted by jcassity
    I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
    Hooligans!
    1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
    1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
    1986 cougar.
    lilsammywasapunkrocker@yahoo.com

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