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Discussion Starter #1
I just tore down my gearbox after having it rebuilt, and then losing third gear within about 500 miles.

Upon teardown I discovered that the shift fork has .107" play in the hub and has machined quite a bit of itself away. 3rd gear synchro seemed to be fine, not bottomed out and still giving good braking. Thrust measurements were .021 for 3rd and .026 for 4th. What would cause this level of shifter fork wear in this little time?

I am using the car for TT so I am driving it hard, but my average shift time is still pretty high per data, and I don't drive around with my hand on the shifter. What else should I look at here before I put it back together and potentially have the same problem?
 

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A random shot in the dark: can the engine/transmission moving about on its mounts during high lateral acceleration put tension on the shift cables when in gear?
 

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Discussion Starter #3
A random shot in the dark: can the engine/transmission moving about on its mounts during high lateral acceleration put tension on the shift cables when in gear?
Probably not, the engine doesn't move very much, but that is among the things I will check for when I reassemble, along with maybe a hose resting against the counter weight. There isn't much in that area anymore though so I don't think it's the case.
 

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It sounds like it is acting as if you are leaning on the shifter in 3rd, not mashing it into gear[hulk smash!]

So maybe something is happening that gives it that effect
 

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Discussion Starter #6
Could it have been miss-aligned during the rebuild, and machining itself ever since?
It doesn't seem so, it is very positively located where it goes, it wouldn't go together any other way I don't think.

One option I want to raise is that it only wore out after the transmission stopped shifting into 3rd. But why would it have stopped shifting into 3rd if the synchro is still good and the dogs are more or less still in good shape?
 

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I just tore down my gearbox after having it rebuilt, and then losing third gear within about 500 miles.

Upon teardown I discovered that the shift fork has .107" play in the hub and has machined quite a bit of itself away. 3rd gear synchro seemed to be fine, not bottomed out and still giving good braking. Thrust measurements were .021 for 3rd and .026 for 4th. What would cause this level of shifter fork wear in this little time?

I am using the car for TT so I am driving it hard, but my average shift time is still pretty high per data, and I don't drive around with my hand on the shifter. What else should I look at here before I put it back together and potentially have the same problem?
Sounds like the shift fork definitely had pressure on it when in gear. So either the rod detent to shift fork dimension is off (unlikely), or third gear to shift fork relationship is wrong (good possibility). If I recall correctly, the assembly process for this gearbox requires shims to properly locate the gears on the shafts. Perhaps your third gear was improperly located which led to a constant thrust on the fork when you were in third gear, wearing the fork, etc.

Another possibility, albeit unlikely, is the 3-4 synchro/hub is asymmetrical and was installed backward. All speculation without an assembly manual in front of me, but things to check.

Good luck,
Bob L
 
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