Gearbox pops out of 5th.

[Michael G]

I recently put my 5 speed back in the car after the big nut on the end of the gearbox shaft came undone and let the 5 gear assembly slide back = no 5th gear.

I replaced (all new parts): the mainshaft retaining nut, secondary shaft retaining nut, complete 5th gear and the large circlip on 5th gear synchro assembly.

I took the car out for it's first post-op test drive and all seemed OK while I was ambling around the local side streets (taking it easy - you know how it is after major car surgery).

OK, time to hit the highway and make sure cruising at speed is all fine.

Oh dear. Happily sitting on 100km/h speed limit and it pops out of 5th - just like the original problem!

When I say 'pops', a more realistic description would be 'bangs' as it is a very forceful motion.

Drivetrain was under only slight load at the time, not accelerating, not on overun.

The car is back in my garage and I'm about to pull the 'box out again but I'm seeking advice on a possible cause. Any ideas?

Anything I should check or test before I pull it all out again?

Help!

Michael