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  • Clutch problem

    I have at last completed my MGB gearbox conversion but I cannot get the clutch to work properly. It does not disengage fully making gear selection very difficult. I am using the Marina master cylinder and the MGB slave and get about 1/4" movement at the slave for the full pedal travel. The MGB slave has a much larger bore than the Marina one so this is reasonable but is it enough? Does the MGB master have a larger bore? I have bled the system several times using different methods so I don't think that's the problem. Any thoughts anyone?

  • #2
    Re: Clutch problem

    A friend of mine who was until recently a Rover mechanic and trained on the Marina/MG range advised me when bleeding the clutch to force the slave pistion all the way into its bore before bleeding. It's quite easy to do with a lump of wood and an assistant. This reduces the chance of air getting trapped in the slave cylinder. Sorry if this sounds like teaching to suck eggs but you must make sure the outlet from the slave cylinder is at the top of the cylinder. I used the procedure above and ended up with a well bled clutch where before I was having real difficulty in getting much in the way of clutch travel.

    Hope it helps.

    Cheers Alan

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    • #3
      Re: Clutch problem

      Thanks Alan. Don't worry about the sucking eggs thing - I am grateful for any help. The method you describe was the second method I used after the 'normal' method failed. Method 3 invlolved priming the slave and pipe off the car before connecting up and bleeding in the normal way. Method 4 was to inject fluid into the slave cylinder bleed nipple filling the system from the bottom up. Each method resulted in 1/4" total movement at the slave. Also, the relative movement of master and slave are consistent with the size of the bore's which suggests the system is working as well as it can. I think either I need a larger bore master cylinder (anyone know what size bore an MGB master cylinder is?) or there is something wrong with the clutch itself. I am reluctant to seperate the engine and gearbox again for obvious reasons but I am running out of idea's.
      Sorry for 'going on' - I'm calm now....

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      • #4
        Re: Clutch problem

        Message originally posted by: John Wood
        Hello Rob.

        the problem you describe with your cluth was a common one in the 70,s when i was working in a austin morris franchise. the problem being that the clutch master cylinder causes a vacuun in the clutch pipe when the pedal is released during the bleeding process. This vacuum is so strong that it often causes a permanent air bubble in the pipe. which just gets pushed up and down the pipe , getting no nearer to the slave cylinder,with every stroke.
        We found this very frustrating back in the 70,s and it,s got no better.
        The cure is to pressure bleed the clutch from the top of the master cylinder, using a very simple apperatus.
        take the top off your gearbox filler squeezie bottle.The one with the spout on it.You should find that it screws straight on to the master cylinder. connect a length of sceenwash tube to top. connect a screenwash one way valve on to tube . connect foot pump or cycle pump to tube.
        Fill up master cylinder. Pressurise system and open bleed screw. do not press clutch pedal during bleed process'
        Good luck J.

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        • #5
          Re: Clutch problem

          Hi Rob, Got to pondering over your problem,so set down and made some calculations based on your info.I'll keep away from the maths so's not to bore you.
          firstly with a marina master cylinder o.065 bore and a slave of 1", for every 1" of master piston movement the slave moves 0.38" However with a slave of 1.25"[mgb]you only get 0.248"for the same input.With an MGB master[0.750"] and a MGB slave you get 0.36[virtually same as standard marina].Basic hydraulics says that if you increase bore size/area you'll get less movement but more force.
          I agee with the other comments posted ,but beleive you simply won't obtain your objective with the units you have fitted,you need a master cylinder bore of 0.750 or greater if you keep the mgb slave.
          Best Wishes
          Alan[ex RAF technician]
          Try AP racing or someone like J.E.M [01455 230626] they may be able to come up with same bolt fixing configuration in 0.750" bore size

          Comment


          • #6
            Re: Clutch problem

            Thankyou for your comments.
            John, this is brilliant. Yes, the gearbox oil squeezy bottle lid fits perfectly (there are several sizes but I found one that fits). I attached a tube, tyre valve and foot pump and it worked a treat - bled the system in no time and no need for an assistant.
            Now the bad news... the clutch still doesn't fully disengage!!!!
            Alan, I agree with your calculations - what I couldn't find out was the size of the MGB master and/or the required movement at the slave. I will try a 3/4" master.
            What have other MGB gearbox users done?

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            • #7
              Re: Clutch problem

              For my Type 9 conversion, which was originally designed for the MGB, I got hold of an MGB master cylinder. It has the same hole centres as the Marina, albeit the base plate is a little smaller. So I made up a small ally backing plate to fit against the pedal box.
              Marlin Roadster, LWB...1860 B Series + Ford Type 9
              Renault Espace 54mm front calipers, vented discs, cycle wings and adjustable tie-bars.

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              • #8
                Re: Clutch problem

                Message originally posted by: Alan Hogg[ex RAF,ex AP LOCKHEED]
                Rob,The mgb master cylinder is 0.750. bolt centres are same,so should work[BMC,Leyland Rover Mg did something right!],incidently a guy by the name of Alex Romni in New York USA has a mgb powered roadster that he is trying to convert to left hand drive[see forum entries back in January]

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                • #9
                  Re: Clutch problem

                  Fitted an mgb master cylinder over the weekend which fixed the problem.
                  Thanks everyone.

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