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Thread: Gear Box FUBAR

  1. #1
    Lighting Specialist jaudette3's Avatar
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    Gear Box FUBAR

    I just had my transmission rebuilt by a local shop. It was making a lot of noise and we suspected the ring and pinion, which turned out to be the culprits. They were replaced, along with bearings and synchros. I was advised that my original gears (A-F-M-S-X) were in great shape and we could go ahead and use them.

    Well, the car is quiet. And shifts smoothly. But....

    I have a very good feel for this car as I have driven it in a number of high performance events. The car feels exactly the same in first and second as it did before, but a shift to third feels like going into overdrive. There is a 50% drop in RPMs from 2nd to 3rd - and then only a 10% drop from 3rd to 4th. It's just about impossible to keep things in the power band when going from 2nd to 3rd. The transmission shop insists that they replaced the same gears that they removed, but of course that is impossible.

    I know there are some gear experts on the list. Can you tell from the information I have provided what is going on? My sense is that the original "M" 3rd gear has been replaced with something like an "O" or a "R".

    Thanks for your help.

    John Audette
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  2. #2
    Did they flip third gear? Is that even possible?
    1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
    Early 911S Registry Member #425

  3. #3
    Restoration newbie.
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    You can flip 3rd but then it fits in either 4th or 5th position.
    Perhaps they flipped both 3 and 5 and then transposed them
    giving you something like A-F-P?-S-ZA.

    Does your 5th feel taller?

    andy
    67S in pieces
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  4. #4
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    It is pretty difficult to flip a gear if you watch what you are doing. When done properly there is a progression of sizes and the angle of the gears on a shaft are all the same. A flipped gear just stands out like a sore thumb. Third can be flipped into 4th or 5th or vice versa.
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  5. #5
    Lighting Specialist jaudette3's Avatar
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    The ratios are about right for flipped gears. We're going to remove the gear box and open it up at my shop next week. I'll report back what we find.

    Thanks for the input.

    JohnA
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  6. #6
    Senior Member Grady Clay's Avatar
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    Quote Originally Posted by jaudette3 View Post
    The ratios are about right for flipped gears. We're going to remove the gear box and open it up at my shop next week. I'll report back what we find.

    Thanks for the input.

    JohnA
    If the gears didn’t get in the correct position, I would want to confirm that the differential bearing pre-load is correct, the pinion position is correct and the backlash correct.

    Please post an image of the gear cluster as it is removed. That should be interesting (in a comical sort of way).

    Best,
    Grady

  7. #7
    Lighting Specialist jaudette3's Avatar
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    Took the gear box apart this morning and here's what happened.

    3rd Gear
    --------
    "S" 4th gear, 25:26, installed in 3rd gear position: 1.040; correct "M" 3rd, 22:29, is 1.318

    4th Gear
    --------
    "X" 5th gear, 28:24, installed in 4th position, and also reversed to 24:28: .857; correct "S" 4th, 25:26, is 1.040

    5th Gear
    --------
    "M" 3rd gear, 22:29, installed in 5th position, and also reversed to 29:22: .759; correct "X" 5th, 28:24, is .857

    With the correct "F" 2nd gear of 1.889 and the incorrect "S" in the 3rd gear spot, the drop from 2nd to 3rd was 45%. I had estimated it at 50% judging by the RPM drop. The correct drop, with correct "F" and "M" gears in place is 30%.

    With the incorrect "S" gear of 1.040 in the 3rd gear spot and the incorrect "X" gear of .926 in the 4th gear spot, the drop from 3rd to 4th was 11%. I had estimated it at 10% judging by the RPM drop. The correct drop, with correct "M" and "S" gears in place is 21%.

    With the incorrect "X" gear of .926 in the 4th gear spot and the incorrect (and reversed) "M" gear of .759 in the 5th gear spot, the drop from 4th to 5th is 18%. The correct drop, with correct "S" and "X" gears in place is 17.6% (even a blind dog finds a bone once in a while I guess).

    Kind of like having a three speed: first, second and 3-4-5 overdrive. Probably good on gas mileage, but a little boring to drive.

    FUBAR was right. I can't for the life of me figure out how someone who does this for a living can screw up so badly. The shop is backing its work and will pay for the engine/trans R&R in addition to setting the gears correctly. Back on the road by mid-week.

    Cheers,
    JohnA
    Lighting Resources for Hardcore Air-Cooled Porsche Enthusiasts”
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  8. #8
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    Unbelievable !

    Glad you're getting it sorted John
    Andy

    Early 911S Reg #753
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  9. #9
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    John,

    Glad it is getting sorted out and that the shop is standing up for it's mistake.
    Harry

    Member #789
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  10. #10
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    Um, FUBAR is a very apt description for this configuration.
    I glad it is a simple mistake to correct.
    Porsche Historian, contact for Kardex & CoA-type Reports
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