Jumping out of 3rd gear on over-run

A gearbox that jumps out of gear can be a little annoying, and my box has been sliding out of 3rd gear while Over running (engine braking) for a while.. Third Gear remains fully engaged during acceleration, only jumps out when the load comes off..
It's a Series 2a Suffix C Gearbox with lower 1st and rev ratios. This box is coupled to a suffix C transfer box (with the larger dia intermediate Shaft) but with a 2.888: 1, Low transfer box ratio, (Thanks to a new Intermediate gear and suffix A internals)
This gives a really useful low ratio reduction..
However, I now need to resolve the main box jumping out of 3rd gear.. Do you think new main shaft bearings are needed, or something else?

Cheers

Does this sound similar

Detent ball or spring.

My son's did this for all the years he had it, ball and spring did not fix it, worth a try though.

Mainshaft nut loose?

mine jumps out going up hill in third- always has done....

I think the box will need of new set of main shaft and lay shaft bearings. There also appears a bronze bush that carries the 3rd
gear and syncro.. Basically replace all the wearing parts should help a great deal.. In the mean time I will continue to look up the parts..

I had this and found it was the gearbox mounting rubbers. Even though they looked ok, once i took them out to replace one was snapped at the bolts and moved the gearbox about 2 inches left and right.

Been perfect ever since

Rich

Interesting you found that to be the cause. My gearbox mountings are sound. If your box could moved the extent you said
was it physically moving the gear selector?
My box isn't jumping out of 3rd gear under load, only dropping out of gear on the over-run.
The 3rd gear wheel, with the Syncro clutch teeth attached appears to run on a Bronze Bush, so any tapper or wear here,
added to any wear in the mainshaft bearings I think are the most likely culprit. I seem to have a bit of gearbox bearing noise on the over Run too. I've ensured it's all topped up with gear oil, but I think The box will needs to come out for new bearings, bushes and seals..

I've invested in a full set of mainshaft and layshaft bearings for the gearbox, along with the Bronze 2nd/3rd gear carrier/ Distance piece, Part no; 571218, together with a all the Thrust washers and seals.
The difference in price between OEM and pattern parts is not surprising.

Hi Tony, yes it did move the gearstick by about an inch when the mount was knackerd

Rich