Heys guys,

My gearbox slipps out of gear in 3rd an 4th, like when you back off the power it pops out of gear if you dnt hold ya hand on the shifter..

any ideas what it could be?

cheers, Ben

worn or broken detents springs, or missing detent balls. Sounds like you've given it a hiding.

yeh true true there lol.. it is running 260rwhp so yeh.

ill have to check it out.. there in the for bolts on the side of the box hey?

I'm not massively familiar with datsun specific gearboxes, but have some decent experience of gearboxes in general. Assuming it's not a 6 speed with 1:1 top, then the layout internally is all pretty much similar for most 4 speeds out there.

3rd and 4th are the front pair of gears - they basically 'sit' either side of the input shaft and the main shaft in the gearbox. The 'nose' of the mainshaft sits inside the back of the input shaft, and there's usually a set of needle roller bearings (sometimes in a cage, other times not depending on manufacturer) - called a spigot bearing (similar to how the spigot bush in the crank tail locates the tip of the gearbox input shaft).

If that bearing self destructs or wears enough, then the input shaft and mainshaft can 'wobble' and any tiny deflection usually has an interesting side effect - it knocks it out of gear and back into neutral when you back off the power (very annoying for highway cruise as even a slight liftoff can do it).

That is possibly the problem.

Another common issue is that once 4th is selected, the selector itself has a detent ball so it doesn't go 'all the way' back just goes till that ball slots into the groove and holds the gear in place. The problem is that a lot of people (possibly anyone and everyone who owned that car/gearbox before you) like to drive in top gear with their hand resting on the shifter. This will push past the detent ball's slot and basically ride the selector/baulk ring (synchro ring) an over time can damage them, which allows it to wobble, which then loads up the previously mentioned spigot bearing.

Basically either one leads to the other, both make the other worse and end up with the result you now have. If you have to keep that gearbox, then plan on at least new spigot bearing and 3rd and 4th synchros. If you are going that far, might as well replace those on 1st and 2nd. And with 260 rwhp, you'd probably want to look at replacing the layshaft and layshaft/laygear bearings (usually when the bearings wear, they cut up the layshaft too. The inner 'bore' of the laygears is usually hard enough to survive, or in some cases the bearings have their own outer cage/shell so the laygear bore/sleeve itself can't wear. If it is worn, you can get them machined out and a hard sleeve pressed in to reclaim them.

Obviously this info is based on general experience and theory, and given the sheer output of the engine, you'd really have to allow for the possibility that more is in need of a freshen up when you dis-assemble the box and inspect it.