first the synchros.
with the bare gear shafts, move the synchro by hand. you should be able to slide it on fully to engage a gear and when you pull it out of gear, it should 'click' back into neutral position and the ball checker should hold/secure it's position.
next most important part that's easy to mess up is the selector gate assembly (24).
-all the synchros must be in their middle neutral state,
-slot the shorter 1st/2nd selector fork arm (23) into the selector cage (24) (obviously cos you can't fit that arm with the cage in the way).
-NOW its important to make sure when fitting the cage onto the bell housing that all 3 selector fingers in the cage are held in neutral and the selector 'striker' peg (21) is located within the 'slot' of the middle finger.
- still keeping the fingers equally in neutral, fit the middle fork (27) and the long fork (14) and the shaft fork (19) to hold it together.
- slide the reverse cog in place and insert the cog axle
- now if done right, you should be able to easily turn the rod in neutral between 1st/2nd and 3rd/4th and selecting 5th/rev should only require slight force pushing against the sprung check spring (4). if it doesn't turn easily between left/right, then the fingers weren't in neutral during fitting and is out of position.
- holding the two shafts steady, engage all the gears and it should feel and appear to engage positively as expected.
- whenever the selector goes into neutral between gear changes, the synchro is moved into neutral and the ball should hold it in position. if you see the previously selected gear synchro 'sag' and unexpectedly slip towards a cog (like the 5th gear synchro pic) while you turn the rod to select one of the other 3 selector fingers, then the synchro ball checker isn't working properly.
next, whenever you push or pull the rod to engage any gear, it is held/locked in place by the selector rods sprung ball check (20) slotting into 1 of the 3 groove valleys at the end of the rod. if a gear isn't fully engaged (whether misaligned fingers or driver error), the ball check won't be fully slotted into the valley to resist back/forth movement, hence could pop out of gear.