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MG MGB Technical - Jumps out of gear

What is the most likely problem with a B gearbox that jumps out of third? I'm thinking worn selectors but haven't been inside a g'box since my Triumph 2000 days so I'm not sure what to look for. Will the wear be obvious? Is changing selectors relatively easy?
Apart from this the box runs well with a reasonably low level of noise. Not sure of its age as it was a retrofit to provide overdrive. My guess is less than 50k miles.
Richard Coombs

It could be worn selectors not fully engaging the gear or the detent ball springs not applying enough pressure on the shafts to maintain the gear but my feeling is that it is wear in the third and fourth gear synchronising hub that is throwing the car out of gear. If this is the problem then it is a major dismantling job to replace the hub and cones. Just be sure that it is not something simple like the rubber boot around the gear lever applying gentle pressure to the gearlever and moving it out of gear.
Iain MacKintosh

Richard. Iain has got a good point. On my old ZA Magnette I found out, after a lot of investigation, that the engine and gearbox mounts were shot which allowed the assembly to move far enough to pop out of third.

FWIW

Tony
Tony Oliver

I had a problem with a small crunch into second gear and occasionally jumping out of third when changing down. Dismantled the gearbox and replaced syncro's and gearbox is now operating fine. The gearbox was quite easy to dismantle/replace following a workshop manual and it was the first time I had done a box
The car is a 79 model and we did it without removing the motor
Have a go
Jeff Jorgensen

The best way to fix the problem of 4 syncro 'Bs' coming out of 3rd gear is to fit a detent selector spring from a 3 syncro car. Part No 22A75. It can easily be done from under the car. Undo the 9/16 AF nut (it is the one nearest the centre of the car)holding down the spring & use a piece of wire with a small hook on it to remove the spring. It is easier with a short ring spanner. On cars with a side fill gearbox it is necessary to remove the plunger first. You will need a strong magnet held against the side of a short piece of 1/4" dia steel to lift it out. Some times it takes a few goes.
Regards.
Garth
Garth Bagnall

Hi again - I have tried to change my email address without success. It is now - bagger05@xtra.co.nz
Garth
Garth Bagnall

Richard-
Let's do the simple checks first. Inspect both the bushing on the end of the gearlever and the bushing in the top of the selector lever. If either of these bushings is worn and you fail to replace it, then you will not attain the correct amount of engagement travel of the sliding synchronizer hub, which can in turn result in the car popping out of gear.
Steve S.

Thanks for the suggestions Steve.
The box will get a good looking at when I've finished reshaping the bodywork for the respray.
Richard Coombs

Thanks Garth. I had been wondering about the detant springs from something else I had read. Why replace with a 3 synchro part?
Richard Coombs

I bought a 70 MGB in 1972 with the same problem. The MGB dealer told me it was likely a weak synchronizer. The car had about 18K on it when I bought it. While accelerating in 3rd gear it never jumped out, only when coasting in 3rd would it jump out and only after coasting a ways. I got used to hesitating a bit on shifting into 3rd. It would grind just a little if you upshifted too swiftly into 3rd. I decided to live with it until it became a problem. I drove the car daily until 1987, rebuilt the engine twice, replaced worn clutchs and throwout bearings a couple of times at least, but never touched the transmission. It never got worse.
Rick Penland

I have the same problem on my 3 synch box. I overhauled it a few years ago, with new bearings, synchro rings, springs, laygear, synchro hubs etc. As Rich says, it only happens when coasting in 3rd, usually when going downhill at some speed, and only after quite some time. Everything else is perfect, and has been so for the 11.000 miles driven after the overhaul.

The problem was there from day one. This gearbox was not in then car when I bought it, so I can not tell if it was there before. I have been thinking that the reason may be that some of the new parts are not made to exact tolerances?

Tore
Tore

Hi Richard,
The 3 syncro springs are a little longer & a bit heavier. I usually grind about 1/16" off the length before fitting but they can be fitted as is. Inhn the 20 years I had my workshop I did this dozens of times.
Garth
Garth Bagnall

Garth,
Sounds good to me. I'll certainly give it a try if I can't see anything else more obvious.
Regards
Richard
Richard Coombs

Hi Tore,
My 66 GT also comes out of 3rd gear on coast - but only since I replaced the 3/4th syncro hub with another out of my box of spares during the last rebuild. I have put up with it for about 4 years. A while ago (after I had done mine) I read an article somewhere that the early Austin A50/55 had this problem & some mod was made by the factory. Unfortunately I can't remember where I read it.
I have been fixing BMC cars for 40 years so have plenty of spares but don't know what my hub came out of. It is due for another rebuild as 2nd gear syncro has gone AWOL. It will have to wait until my 70 B is back on the road. When I have stripped it I will compare what I have with other spare hubs. Incidentally I replace the rear laygear needles with the 1st motion needles. If you wanrt details contact me.
Regards,
Garth
Garth Bagnall

This thread was discussed between 09/04/2009 and 01/05/2009

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