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Thread: ID and value question: 7:31 915 trans

  1. 11-10-2014, 03:21 PM #1
    Eminence Gris
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    ID and value question: 7:31 915 trans

    From a frankenstein car, I sold off the 3.2 engine and was left with a mag case trans with minimal identification. It has casting numbers, of course, but the boss on the bottom is unreadable. Based on how the car drove, I believe it to be a 7:31, and it does have the cable speedo drive. But the '75 also was mag case, speedo drive.

    Does anyone have a good method for calculating r & p by counting mainshaft/drive flange turns, or do I need to pull off the side cover?
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  2. 11-10-2014, 04:34 PM #2
    Flieger
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    To count turns you'd need to also know what the gear ratio was that you were using.
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  3. 11-10-2014, 06:22 PM #3
    gled49
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    Post pix of the inside of the bell housing, underside and find a date code somewhere and we can make an educated guess. Gled
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  4. 11-10-2014, 06:42 PM #4
    Jon B
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    Quote Originally Posted by Eminence Gris View Post
    Does anyone have a good method for calculating r & p by counting mainshaft/drive flange turns, or do I need to pull off the side cover?
    The standard 1st gear for a 70's 915 was 11:35.

    With a 7:31 ring & pinion, 14.1 turns of the input shaft will rotate the differential once, in 1st gear.
    (35÷11) x (31÷7) =14.09
    With an 8:31 ring & pinion, 12.3 turns of the input shaft will rotate the differential once, in 1st gear.
    (35÷11) x (31÷8) = 12.33

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  5. 11-11-2014, 09:09 AM #5
    Eminence Gris
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    Quote Originally Posted by Jon B View Post
    The standard 1st gear for a 70's 915 was 11:35.

    With a 7:31 ring & pinion, 14.1 turns of the input shaft will rotate the differential once, in 1st gear.
    (35÷11) x (31÷7) =14.09
    With an 8:31 ring & pinion, 12.3 turns of the input shaft will rotate the differential once, in 1st gear.
    (35÷11) x (31÷8) = 12.33

    Jon B.
    Vista, CA
    Thank you John!
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