Billman250

Shifts smooth with no clutch (pinpoint timing), but grinds 5th when clutch is in use (timing is off which activities syncro) is a burred 4-5 collar or a failing 5th syncro assy. You could try rowing, the same DIY used to fix a track car stuck in fifth.

Car Analogy

Is 'rowing' the term used to describe driver action used to deburr a syncro, wherein one uses the cars momentum to hammer away burrs by repeatedly, and to some degree violently, oscillating between on throttle, off throttle, as abruptly as possible, while car is driving at high rpm in the gear being deburred, in attempt to create maximum accel, decel, accel, decel momentum?

Billman250

That is one way yes

By rowing I mean 5th and 6th back and fourth 100 times with car off.

Car Analogy

Oh, feel stupid now. Rowing through the gears. Of course.

Is there a term for that other process? 'Throttling the syncro'? 'Momentum deburring'?

Saki GT

I'd also add, what happens at higher rpm shifts vs lower, since we all know that the synchros tend to work more smoothly above 4k rpm. Maybe OP is shifting at low rpm too quickly? Does shifting above 4k rpm into 5th help anything?

flanders

I got 4th to 5th crunch on high rpm but it only seems to be happening after really hard driving.
Last time it happened 5th gear felt really weird and it took a lot of force to get it in and out 5th but all other gears felt normal.
I did the ~100x rowing and it seems to have gotten a lot better.
I'm thinking the root cause maybe is clutch related?