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Master Cylinder, what does it mean?

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    Master Cylinder, what does it mean?

    Can someone 'splain this to me...

    I removed the MC from my car during my upgrade to an X3 booster/MC. The X3 one is labeled "25 22" front-rear which sounds good.

    I'm looking at what I pulled which I thought was a 25mm one, but it's actually labeled "31 20". Hmmmm..... No P/N on it.

    I disassembled it and measured the bore as 20.6mm.

    So, what does the 31 mean? Do master cylinders even have different bores? Is bias done through some other internal trickery?

    BTW, my 320i booster and the master cylinder weigh 9.5 lbs, the X3 one seems featherly light in comparison and is a tad over 6lbs.
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    Last edited by hoveringuy; 11-15-2020, 10:19 AM.


    urious about this as well. I thought about swapping over the seals from another MC to refurb my aluminum Girling.
    01.5 S4 Avant - Sold
    88 325iS - In pieces
    91 e34 - Totaled by some dumbass
    90 325i - Daily
    06 e91 - Sold

    feedback thread


      Assumed they use the same casting for many range of pistons. Which sucks trying to figure whats what later on...

      I was up above it, Now I'm down in it ~ Entropy - A Build thread.


        I wish I knew about the master question, but rear bias is also done with the prop valve below the master on the firewall
        Originally posted by priapism
        My girl don't know shit, but she bakes a mean cupcake.
        Originally posted by shameson
        Usually it's best not to know how much money you have into your e30


          Why have two different master cylinder bores AND a rear bias valve? I dont understand why BMW did/does that.