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MG MGB Technical - Can't change gear with engine running


(Bit of a mechanical novice so please excuse any didgy terminology!)

Have got a '72 4 speed synchro gearbox which won't allow me to change gear with the engine running. Will go through the gears very nicely with the engine switched off but I don't find it all that useful! Does anyone know of a definitive fault for this?

Many thanks,

V Smith

When did this problem first occur? Did it come on gradually or suddenly? MGBs use a hydraulic clutch system instead of a mechanical system. Thus, if the hydraulic system is low on fluid (small reservoir inboard of the large brake master cylinder), the clutch won't sufficiently travel to allow engaging the gears. Do you hear anything when attempting to engage while engine running? Any grinding? The quick way to check is to turn the engine off, engage 2nd gear and be on a bit of a slope. See if engaging the clutch lets the car roll. The hydraulic system is in no way affected by the engine running or not. You might get by with topping off the clutch reservoir only or might have to rebuild the reservoir and the clutch slave cylinder. It's not a very difficult job at all. I would also recommend looking beneath the car for fluid leaks under the clutch slave cylinder. It could also be the hose to the slave cylinder. The slave cylinder is on the passenger side of the transmission just ahead of the passenger door.
Rick Penland

Vince, sounds like your clutch throwout bearing may be at fault (worn out), the steel ring inside of the clutch pressure plate maybe broken, or your clutch disc is rusted to the flywheel. Of course the clutch disc rusting to the flywheel only occurs if the car has been sitting for sometime in a moist environment (but then the UK is a moist environment isn't it?)

Easier first thing to check is to look under the car at the clutch master cylinder located on the driverside of your car and is mounted to the bellhousing of the transmission just underneath the starter area. Have someone depress the clutch and see if you have about an inch or so of travel of the clutch slave cylinder rod. If it doesn't move, then the slave cylinder or master cylinder for the clutch may be the problem...I would start here. You can also look at the carbon bearing face of the throw out bearing and check it for wear. Providing you have the original carbon type bearing instead of a modded roller bearing of course. Pull the rubber boot back from the bellhousing where the clutch fork passes into the trannie. This is what the clutch slave cylinder attaches to on the transmission. Using a torch take a look up into the bellhousing and look at the other end of that clutch fork. That round fiddly thing is the throwout bearing. It should have a black carbon surface on the motor side. New it would be about a half inch thick. If you just barely see any carbon or no carbon, then there is your problem. I have also seen these carbon bearings broken into little chunks laying in the bottom of the bellhousing.

Good luck.

Thank you for your ideas. I've been running OK all winter but suddenly, mid-drive today, the ability to change gear disappeared. I'll check out your suggestions when I am back at my car.

Once again, many thanks.
V Smith

Have you completely lost the ability to change gear, or does it just grind when changing gears? If it's just the slave cylinder, you should still be able to shift by matching revs. I've had to do it in my B, when I lost the clutch slave on a trip last year. I'll bet Mark's got the answer with the throwout bearing.
Greg Bowman

Vince, two years ago my wife called to inform me that she could no longer change gears in her 80B. One moment it shifted fine then the next nothing. I did what Greg Bowman said and managed to get the car back home by matching revs to shift gears. Being a guy that knows what I am doing and all (chauvenistic here).

Anyway, pulled the drivetrain and all looked normal until I looked inside the pressure plate. The ring that runs around inside the fingers of the unit had snapped in half on one side. This unit is expensive and doesn't always get replaced on a clutch replace and the previous owner never replaced the clutch so it was old and fatigued.

New clutch, engine rebuild, radiator replaced, transmission overhauled and she's calling me nice things again.

Sounds like the clutch is dragging. Does it grind when trying to go into reverse and baulk at the others? You should have 1/2" to 5/8" of travel of the slave piston. If less then a problem in the hydraulics is indicated, if OK then a problem in the bellhousing.
Paul Hunt 2

I'll try checking the hydraulics this weekend and let you know if I find anything - glad that they've forecast rain for the next few days!!!
V Smith

It seems to be a hydraulic problem - if you pump the clutch pedal a few times it will then go into gear. Will try bleeding system and then investigate slave cylinder if that doesn't fix things.
V Smith

Bleeding is almost universally claimed to be a real pain on the MGB, so much so that I didn't even bother trying. I had replaced the master, slave and flex hose and filled the system using a gunson's EeziBleed connected to the *slave* nipple with very low pressure. Looked into the master until I could see the fluid rising, switched off the Gunsons, topped off as normal, and immediately had the full travel. If trying this way I'd strongly recommend opening the slave nipple first and pumping the pedal until the fluid runs clear, you will be amazed at the black gunge that comes out if it hasn't been touched for years, which most of them haven't.
Paul Hunt 2

I have an associated problem. I just rebuilt my brake master cylinder and had the complete assembly out including the clutch master. Bleading the brakes went fine but I have not been sucessfull getting the clutch to work. It worked fine the last time I drove it this past summer.

I have tried bleading it for a few hours and my wifes leg was getting sore from pumping. I do have some travel now but not mutch. I had about give up thinking I needed to rebuild the clutch master but then read the above.

Why are they so hard to bleed? There is only one line and it is not that long.

Where does the air get trapped. Is it down at the transmition in the slave? If so I would think that after a while it would seek a higher point and bleading again may push it out.

What is this Ezibleed? Is it expensive? It sounds like it may make a mess if I already have the can full and most of the line.

I agree with Paul Hunt on his technique, I don't have that ezibleed, but I do have a very large syringe that I use. It is cheap and found at hospitals. Use the same principle to push the fluid up to the master cylinder.

I also am successful bleeding by not pumping the pedal as you cannot overcome that pressure plate. That's why it won't bleed. I bleed it from the bottom by opening the bleed screw and take a large screwdriver and using something to pry against, push the plunger back into the cylinder as far as it will go, then close
the bleed nipple, pump the pedal to push the fluid down into the cylinder, open the nipple again, compress the cylinder again, close the nipple again, repeat until the unit is bled. Only takes a couple of minutes and you can do it by yourself.

The curve in the hose at the slave cylinder often makes it difficult to get the air out. This will take 2 people, but drop the slave cylinder so it hangs by the hose. Open the bleed screw a turn or so, attach a hose and place it in a jar of brake fluid so you can see the air bubbles as you apply the clutch. I've done this also with a 6 inch or so clamp gently applied to keep the plunger and piston from flying out when brake fluid starts to actuate the slave cylinder. Once you have the air out, reattach the slave cylinder and pump it up. Watch the main master cylinder for the clutch, the volume isn't much and you can start pumping air into it easily.
Rick Penland

Thank you very much for the tips. Pushing in the plunger did the trick. I now have a clutch that works. Makes perfect sense once you think about it and have an idea of what is happening.

Thanks again,

Good to hear a B is back on the road, Safety Fast!

Another way of reverse filling/bleeding without an EeziBleed is to connect a brake caliper nipple to the clutch slave nipple, open both, and use the brake pedal to fill/bleed the clutch system. Don't let the brake master run down though!
Paul Hunt 2

Glad your issue was solved so easily... but the fluid is going somewhere! Best to find it before you get stuck again.

On the issue of bleeding I have a theory... The tube and hose is much larger diameter than the brakes but the MC is smaller. So, you can't reliably exceed the tendancy for bubbles to rise in the system by pumping the pedal.

I tried for two haours to bleed my clutch once. Finally gave up for the night. Next day I went out, topped up the reservoir for another go and... Viola! full pedal.

I never bleed anymore, just wait overnight!
M.J. Anderson

"Another way of reverse filling/bleeding without an EeziBleed is to connect a brake caliper nipple to the clutch slave nipple, open both, and use the brake pedal to fill/bleed the clutch system. Don't let the brake master run down though!"

Now that is one slick tip, Paul. I really like that one!
Carl Floyd

Only passing it on, I'm not claiming credit for inventing it.
Paul Hunt 2

One way I have been able to bleed the clutch with normal hand tools is to open the bleeder then push the slave rod back into the slave cylinder with a pry bar. Close the valve and then the force of gravity will push the slave piston out to where the throw out bearing contacts the pressure plate. Repeat if necessary, but make sure you keep an eye on the fluid level.

I have also hear of people using a Speed bleeder valve. I don't know if they are readly available in the UK, but here is their web site I have never use them on the clutch but I do use them on my race car's brakes.

No matter what, I believe that one of the main problems is the lack of travel unless you push the piston back into the slave cylinder.
Glen Popejoy

Right, I've cleaned and renewed the rubber parts on the clutch slave but now when the hysraulic pressure gets high enough fluid pours from the end boot (which is held on by a metal ring). This seal seems very weak to be held by just the metal ring - is there something I've missed? The ring is a bit coroded and I don't know if I can get a new ring or does it mean a complete new slave cylinder? Thanks in advance.
V Smith

The boot & ring are just a dust seal.

The hydraulic seal is in behind the steel piston.

If you have fluid getting to the dust boot then there is something wrong in either the cylinder or the clutch/throwout bearing is so worn as to let the piston and seal reach the end of the slave cylinder.
Chris at Octarine Services

Does anyone know off the top of their head which way round the rubbed disc goes in the cylinder? Flat side towards piston or concave side? Many thanks.
V Smith

Are you getting any pressure from the pedal? If not and 2 or 3 pumps causes fluid to flood out (and incidentally the piston to cock in the cylinder) then it is probably a failure in the mechanical bits from the slave piston through to the pressure plate. If you can feel normal pressure from the clutch and some fluid seeps out with each push of the pedal then it is the slave seal that is the problem. Don't fiddle about with it, I'd just replace the slave complete and the flex hose at the same time. If doing that flush out the bottom part of the hydraulics by emptying the master through the slave bleed nipple (although it sounds like effectively you have done that already). Going back through your previous posts if pumping the clutch seemed to help you engage gears then that sounds like air in the system, which is more likely to be seals than anything mechanical in the bell-housing (you should be glad to hear).
Paul Hunt 2

Flat side to flat side of piston!
Chris at Octarine Services

Finally got the clutch back to normal operation and the car back on the road. Ended up replacing the slave cylinder and renewing the master cylinder seals. Eventually got the clutch bled after trying several of the different methods mentioned above but in the end found that closing the bleed screw, pumping till pressure obtained and then holding the pedal down while an assistant opened and closed the bleed screw letting the air out worked. Repeated half a dozen of times until the pedal was stiff. Took a lot of pumps per operation (40 or more). Thanks to all those who provided such useful information.
V Smith

Good to hear you have solved the problem, thanks for letting us know.
Clifton Gordon

This thread was discussed between 23/03/2006 and 08/04/2006

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