Just a thought - There are some other reasons why the sliding dog for 4th gear (or any others for that matter) may not engage deeply enough:-
Too much end float on the selector shaft
Wear on the pins of the selector fork(s)
Wear on the selector forks
Any or a combination of these could contribute to the problem - but I must say - partly for my own peace of mind - that I would expect some problems with jumping out of 4th as an early warning before the "welding" took place.
I hardly like to ask, but had the faulty boxes run out of lubricant before the failure and what lubricant had been used?
Matty
Too much end float on the selector shaft
Wear on the pins of the selector fork(s)
Wear on the selector forks
Any or a combination of these could contribute to the problem - but I must say - partly for my own peace of mind - that I would expect some problems with jumping out of 4th as an early warning before the "welding" took place.
I hardly like to ask, but had the faulty boxes run out of lubricant before the failure and what lubricant had been used?
Matty