Morris Minor Owners Club

My 1098cc gearbox jumps out of 3rd gear on deceleration, apart from that its perfectly ok. I had just read in a magazine that a wrongly adjusted clutch can cause jumping out of gear, it didn't give details. Knackered gearbox or badly adjusted clutch, what are your thoughts please,

I don't really see how badly adjusted clutch could cause the box to jump out of gear? are there any other symptoms like slipping or juddering?

My suspiscion is knack'd box I'm afraid

Worn box i'm afraid - nothing to do with clutch. Just possible I suppose that the engine/box may be moving about enough to knock the lever out of gear. Do NOT be tempted to hold it in gear with the lever - that just wears away the selector fork and fills the box with brass filings !!

I've done that before, the box died very very quickly (it got stuck in reverse)

thanks both, the article was in Classics Monthly June edition. Is it usual to jump out of gear on deceleration

Mine does that on 3rd, and has done for the last year! You learn a technique to avoid it, but it's not ideal. If you search the board theres a thread on it.

Is it usual to jump out of gear on deceleration

It's fairly common - I've had 2 Minors do this on 2nd gear.
Ideally get hold of a (known) good second hand gearbox which would be cheaper and maybe more reliable than a rebuilt one.
For a DIY repair, prepare to smell of used gearbox oil (similar to cat wee) for a long time. It could just need worn selector forks replacing, but you may as well replace bearings etc at the same time. The prop fits in a plain bush at the rear which needs to be reamed to size after fitting...

good second hand gearbox

I have another box, it came with the engine and back axle I have fitted and are good. I have had a look through the side plate hole and it looks very good inside, no marks on the teeth what so ever, I'll take a chance and fit it. Luckily its only a days job to do the swap.

Selector forks are only for engaging the gears - they then take no part in 'holding' the gearbox in gear. This is done by the little indent balls and springs, but the problem arises when the syncro gear teeth become badly worn (due to crunching when changing gear - caused by worn syncro rings !!) and take on a 'tapered' form - and this along with various bits of wear in the bearings allows the gears to move around enough to be thrown out of gear. Certainly on Minis this mostly happens on 2nd gear - because it is used most frequently in hurried downshifts and rarely on 3rd gear. Obviously you can live with it for quite a while - although if it progresses to the point where it jumps out under power then that is quite alarming - and likely to be seriously damaging!! If you have another box - certainly worth trying it.

Selector forks are only for engaging the gears - they then take no part in 'holding' the gearbox in gear.

if the selector forks are worn then it is possible that the gear is not pushed far enough to engage the detents, but that's not the biggest issue. The detents themselves offer very little holding in gear.
The geometry of the dogs hooks the synchro hub assembly so that when torque is applied you can't pull it out (or on decelleration the axial load from the helix gears can't push it out). When the synchro hub is not pushed far enough the 'hook' effect isn't there.

Certainly true - but if not pushed into gear properly it would jump out almost immediately under power. The detent balls etc do a good job - just try shifting a good box without the lever - it's pretty firmly held in each gear!