Clutch
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It COULD have been triggered by you changing the gear oil.
EDIT:
Or possibly the old plates had already worn enough to somehow make the issue less prominent... Why did you change the clutch in the first place? Could you give the old bits a try just to confirm it's not the assembly that's incorrect? -
@Ricky That's also consistent with a notched basket if I'm not mistaken.. Maybe your issue the whole time hasn't been the plates, but rather the basket. If the notches aren't too deep, try to file them down and see if it performs any better.
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It not going into gear could be a variety of things. Worn selector forks, bearings, clutch assembly, clutch springs.
Sounds to me like the clutch isn't disengaging.
Again what I said earlier will tell you it is clutch disengagement problem.
Never heard of a worn basket on a dt before though. Maybe if it ran with no oil. My brother ran his dt gearbox on coolant and oil with no problems. So they are pretty well engineered.