Any ideas- Healey gearbox problem

[Frexit]

Would welcome any ideas on what could be the cause of my Healey 3000 BJ8 jumping out of 4th
Usually it was when I would be on full load , foot to the floor but occasionally it was whilst cruising.
I've got the gearbox out and have it waiting on the garage floor of a mechanic who I'm waiting to
look at it, but any anecdotal stories / ideas would be welcome.
I'm hoping it's a simple case of ordering a component and all will be well as it has not shown other
signs of fatigue.
Thanks for any input.

[Dogwatch]

Possibly worn synchromesh.

[Flying Phil]

Another suggestion is that the detent springs, which push ball bearings into notches on the selector rods, may be broken or clogged with debris.

[john2443]

They are held in gear by ballbearings with a spring behind.

Sounds like spring/s weak or broken.

The good news is that the springs are only £1.60 each and the balls 80p.

The bad news is that it's a major dismantling job and you may as well do a complete rebuild while you're at it!

https://www.ahspares.co.uk/big-healey/Gearbox-4-Sp...

[john2443]

Flying Phil said:

Another suggestion is that the detent springs, which push ball bearings into notches on the selector rods, may be broken or clogged with debris.

I think these just keep the selectors and gearlever in place, they don't actually hold the gears in that's done by the ones inside the sychro hub.

[V8 FOU]

john2443 said:

I think these just keep the selectors and gearlever in place, they don't actually hold the gears in that's done by the ones inside the sychro hub.

More likely. These plus the undercut of the gears keeps it there. Safest bet will be to replace the whole 3/4th hub.
BTW, earlier post, synchromesh cones are only for engagement, nothing to to do with holding in gear.
Another possibility is a very worn selector fork. Very common in 4th as people tend to rest their hand on the lever while driving which wears the edge away on the fork.

[Frexit]

Thanks to all for the ideas. I wil hopefully have some feedback from the guy who's repairing it and then
on the phone to AH spares

[Frexit]

Well V8 Fou is spot on, worn 3rd and 4th gear synchro hub. My mechanic has given me the following shopping list:

3 x SPRING-SELECTOR
OIL SEAL-rear(o/d)
OIL SEAL-1st.motion shaft
GASKET SET gearbox
BEARING-rear mainshaft
BEARING-1st.motion shaft
BAULK RING-3rd.&4th.gear
SYNCHRO HUB-3rd.&4th.gear

whilst I'm not mechanically incompetent I've never rebuilt engines or gearboxes so I'm happy to let him do it.
This list was given to me on the back of a fag packet so if i've missed anything which would be also worthwhile replacing please chip in

Here is the old girl

Edited by Frexit on Monday 9th January 22:01