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MG MGB Technical - clutch??

73 mgb replaced slave cylinder still wont shift if engine is running hold brake and start engine in first or second gear napoleon moves around pretty good try to shift grinds does not go into gear?????? before buying a 73 mgb i had a 73 lemans three speed stick on the column owned godzilla for 27 years now im 43 and learning to drive a stick on the floor did i miss some steps changing the slave cylinder clutch pedal has zero pressure
cc perrine

If your pedal has no pressure then it's probably not operating your clutch.

If we start at the bit you've just changed -
. did you have the bleed nipple at the top
. all connections tight
. any leaks or fluid drop at reservoir
. did you fully bleed all of the air out of the system
. did you replace anything else (flexi-hose).

BTW - when using the car only have your foot on the clutch pedal when pulling off or changing gear, no other times like starting the car or with the car at a stop.
Nigel Atkins

As Nigel noted-
When the new slave cylinders are packaged they fit the bleeder into the wrong hole so that it fits the package --when you fit the cylinder to the car the hose goes in the bottom fitting and the bleeder in the upper position----then you will be able to bleed the air out-

William Revit

I had some problems when i changed the slave cylinder on my 75 MGB which i added to my blog with the solution i found together with other info i have found about the same at
Hope this helps.
steve livesley

In practice the bleed nipple position makes very little difference, I've reverse filled clutch systems and had a good clutch straight away.

They can be a beggar to bleed, the best approach if normal i.e. top-down bleeding doesn't work is to wedge the pedal fully down overnight, then release it steadily - i.e. don't just knock the wedge away and let it come back with a bang - next morning.

Unlike the brakes it will help if the rear of the car is raised a little as well.

Nah. the bleeder needs to go in the top

Also there are some dodgey replacement cylinders about that haven't had the bleeder drilling drilled far enough into the housing meaning when the cylinder is bolted onto the g/box and lays at an angle the drilling doesn't reach the very top of the cavity in the cylinder meaning the only way to get the air out is to unbolt the cylinder and roll it over straight to bleed it
I hope yours isn't one of those--made in Ch*** ones

And a pic of a properly drilled one showing how far the bleeder hole goes into the housing (well past half way)

William Revit

No one is saying the bleeder should be anything other than in the side and the hose in the end.

I've no idea where my replacement was manufactured, but the old one has the drilling right across the top corner, further back than you show. That has the circular flat Twist mentions so almost certainly OE, and quite probably original as they made things to last in those days, not like now. A replaced V8 slave also has the hole in the top corner.

Some say they have to unbolt the cylinder and let it dangle, or press the piston right back into the cylinder to get all the air out, so those are also things to try in the event of problems.

Twist mentions the slave push-rod and clevis pin, yes they do wear, as does the corresponding hole in the release arm, its pivots, and maybe even the interface to the release bearing, but they have no effect on clutch operation unlike wear at the pedal and master end. The hydraulic system compensates for wear at the slave end in exactly the same way it does for wear in the rest of the mechanical components including the graphite release bearing, which probably accounts for the vast majority of mechanical wear given the leverage.


ETA: as often happens I was typing my post whilst Paul posted his - can't believe advocating all this wear in the system, but of course if you carry spare slave push-rod and clevis pin along with all the other spare parts you could have the potential to enjoy another fabulous roadside repair 'adventure' rather than preventing and sorting it in a more comfortable and timely environment.

Great post by Willy, I'd forgotten that JT vid.

But I was thinking of this JT vid -
How to Not Destroy Your Clutch the Easy Way -
Nigel Atkins

Thinking about it I'm going to add a condition to renewing the worn parts with new, if the new parts are so piss-poor as many are and thee worn parts are severely worn then you may be better sticking with the existing parts as their useful life after this point might be longer than the new parts, so change next time or as required.
Nigel Atkins

just responding to your post---that's all

04 July 2020 at 15:18:37
No one is saying the bleeder should be anything other than in the side and the hose in the end. "

earlier---04 July 2020 at 09:32:05
"In practice the bleed nipple position makes very little difference,"

just a bit confusing
William Revit

Willy - the thread is about bleeding. Maybe I should have made it clear that the bleed nipple and hose were in the correct positions when I reverse-filled a new system, and I had full clutch operation straight away. In other words the fluid was going in at the top of the cylinder and and out from the middle.

Nothing wrong with changing worn clevis pin and push-rod as part of other work, but that is very unlikely to change clutch operation, which is why I doubt anyone has ever had wear there that has caused a roadside problem.

Similarly Twist's push-rod length variation. In practice the piston and hence push-rod move about 1/2", in a cylinder bore that is more than double that, so you have quite a bit of leeway either side of the middle. Having said that if the push-rod is too long then the piston can bottom in the cylinder and the clutch won't be fully engaged, with excess pressure on the release bearing. And if it's too short then the piston can start coming out of the end of the cylinder. But between those two extremes quite a bit of variation is no problem.

This thread was discussed between 03/07/2020 and 06/07/2020

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