5th gear pop-out on a MK1 NA is one of those things that just about every MK1 owner will experience at some point. Basically you put it into fifth and either when trying to accelerate or lifting off the car will pop-out of fifth gear leaving you with neutral and a screaming engine! Obviously it becomes pretty annoying pretty quickly. You can hold it in gear by resting your hand lightly on the gearstick and holding it in, but this gets tiring on a long drive. Basically the hub and fifth gear wear on the teeth in which they engage. Over time you can tell if this is something you are going to see because the gearstick will 'nod' as you accelerate or more commonly when you lift off. As the wear increases it gets to the point of where it will knock itself out of gear. 

First job is to remove the gear case. All that is under here is basically 5th gear. The other gears are within the box itself.

Next job is to simply undo the nut that holds the selector fork in place. Once undone this will allow you to slide off the selector fork and the hub complete. It really is that straightforward!

Here you can see the old fork and hub assembly. Actually the fork was hardly worn though there was some wear on the dog teeth on the hub and on 5th gear itself.

New 5th gear selector fork. £20 from Toyota

Other side of the new selector fork

New 5th gear hub £50 from Toyota

Side view of new hub. This bit is where the selector fork sits.

Other side of the hub.

Installing the new fork and hub really is dead easy and takes a minute or two. Basically just slide it on altogether, taking a minute to line up the hub dog teeth with those on 5th gear it will only go one way. Then simply do up the nut that holds the fork on with a 10mm socket.

Here is a short movie showing how 5th gear operates: images/fifthgear/5th_gear.wmv

 

After this it is simply a matter of using some hermetite around the gearbox end cover and doing up the 12mm bolts that hold it in position. None of the bolts are particularly difficult to get at.

I should add that all the wear is in the hub and the dog teeth on 5th gear. Obviously here I haven't changed 5th gear so there is still wear here. But by halving the wear so to speak I hope that the 5th gear ought to be usable for another 20K miles or so. A thorough job would involve using a puller and changing 5th gear, though I wasn't exactly sure where the wear would be so I ordered the easy to change parts.

If you look closely at the dog teeth you can see that the leading edge has had metal ground away from it so the teeth are missing the top leading edge, this is where the nodding gearstick comes from when you lift off.

In summary this is a really easy fix for the sake of £75 inc. It takes an hour and a half to do and really is a very straightforward job. I am not sure of the costs of a new 5th gear but am expecting it wouldn't be overly cheap.