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Thread: Gearbox: Worn synchro hub and slider (Experts chime in please)

  1. #1
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    Gearbox: Worn synchro hub and slider (Experts chime in please)

    I am currently rebuilding a R34 GTT gearbox that has a problem getting stuck in 4th gear and unable to down shift from 5th to 3rd even if you rev match (block out condition). Sometimes you have to jerk the shifter to get it to pop out of 4th, but funny enough, all the gears goes in like butter even on full song.

    All the gears can be selected in/out with the car running, clutch-in whilst on the brakes (i.e. the main shaft not rotating) indicating that it is not clutch drag related.

    Information on previous rebuild:
    • New bearings and synchros
    • 3rd/4th synchro key popped out, one was found in the drain plug


    Inspections on tear down during current rebuild:
    • Input shaft end play was checked and within specs
    • Countershaft end play was checked and within specs
    • Synchro gap and grab was checked and within specs (all gears)
    • Synchro teeth condition was good (all gears)
    • 1st/2nd and 3rd/4th synchro keys and spring was in place
    • All bearing condition was good
    • All shift forks had slight wear on where the brass lining were but as expected. No cracks.
    • It was found that 3rd/4th synchro hub and the slider felt loose and was able to rock when assembled compared to the 1st/2nd synchro hub


    As all the issues are apparent in the 3rd/4th cluster, an educated assumption is that when the 3rd/4th synchro key popped out as per previous rebuild due to play in the hub and slider, more kilometres gave it additional wear that made it so severe that the engaging the slider caused it to be twisted off centre and jam into place, unable to get out of 4th. Is the above a reasonable conclusion to the problem?

    TIA
    [IN PROGRESS....]

  2. #2
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    Excessive side play with the slider and hub could be binding up as you go to pull it out of gear. Can you measure the id of the slider teeth the od of the hub teeth and compare it what it should be brand new?
    A transmission place that has the assembly in stock might be able to give you the measurements. Or you could at least compare it with the 1st/2nd assembly.

    Or is that what you already measured as the syncro grab and gap?

    Other alternative is a key is potentially getting jammed on the slider, they don't usually just pop out unless there's a big enough gap for it to escape the slider holding it in place.

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  4. #3
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    I've got a new hub, slider and forks coming from streeter. Highly appreciate your input, will check against the new one when the arrive.
    [IN PROGRESS....]

  5. #4
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    No prob. It will be interesting to see what sort of wear there is on the old assembly compared to new.

  6. #5
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    So parts came in from Amayama (3.5 months later, fine as nissan has to source some parts). The primary cause were the 3rd/4th fork pads was so worn that the slider sits cockeyed. It was night and day difference when compared to the new ones.

    The wear was so bad that it was on the piss where the slider made contact to the aluminium.

    Fitted new ones of those along with all new bearings and synchros and forks. Car's off the hoist, good bye.
    [IN PROGRESS....]

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