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Thread: Difference between a 13mm and 14mm Brake master cylinder?

  1. #1

    Difference between a 13mm and 14mm Brake master cylinder?

    My single disk 1985 R80 came with a 14mm master cylinder. I ordered a spare brake assembly including lever housing and now see that its a 13mm. I think I can use it as its a whole assembly. I'm wondering if the difference is the 14mm is for dual disks as its pushing more fluid. Anyone know the reason for the two sizes?


  2. #2
    Liaison 20774's Avatar
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    Don't know the reason, but certainly a 14mm diameter (being bigger) pushes more fluid for a given stroke. IIRC, that means you would have less travel at the hand lever to get the same push at the caliper. RealOEM shows the two diameters for the '85 R80 with no other explanation:

    http://www.realoem.com/bmw/showparts...47&hg=32&fg=72
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    Registered User ANDYVH's Avatar
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    True that for a given stroke, a larger bore piston will move more fluid than a smaller bore. But hydraulic brakes are not so much about flow, as brakes "flow" very little fluid. It's the pressure in the system that does the work. All the "flow" has to do id siplace the caliper piston out just enough to take up the running clearnace between the brake pads and the rotor. When the master cylinder piston is moved it only has to move a little fluid to push the caliper pistons out and apply force at the brake pads. Ouput pressure at the master cylinder is greater for the 14mm bore versus the 13mm bore, assuming the same lever effort is applied. Let's assume the brake lever is the same for both and the force applied by the hand on the lever is the same. The 14mm bore master cylinder will produce more applied pressure for the same applied effort, or, equal pressure output with less applied effort.

    Less lever effort also means you should have better feel for braking effort applied. Its all about the master to slave cylinder ratio that determines how much effort is required at the hand lever for the desired braking result. So perhaps BMW did a mid-season change to the master cylinder to improve feel at the lever.
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    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by ANDYVH View Post
    .Ouput pressure at the master cylinder is greater for the 14mm bore versus the 13mm bore, assuming the same lever effort is applied. Let's assume the brake lever is the same for both and the force applied by the hand on the lever is the same. The 14mm bore master cylinder will produce more applied pressure for the same applied effort, or, equal pressure output with less applied effort. .................
    Actually it is opposite, 13 mm will give more pressure, and more stopping force at the same grip pressure, but more travel too, and in this case the difference is about 14%.

    I will not argue either way, but the longer travel may give better feel do to the travel difference, I don't know if the nerves and brain in our bodies is more sensitive to pressure differential or movement.
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    Curmudgeon nrpetersen's Avatar
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    It may even be some esoteric Govt standards somewhere on the allowable brake handle forces for a certain deceleration.
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    Rpbump USN RET CPO Rpbump's Avatar
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    Just 1mm. Sorry LOL
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  7. #7
    Bluenoser
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    The larger diameter piston is for more hydraulic capacity as with going from a single disk to a dual disk. So the 14 mm would be the one for a dual disk set up and the 13mm for a single disk set up.

    If your running the 13 mm on a dual disk set up you won't get as much maximum braking effect. It will work but you will notice a difference when applying maximum braking.

    The larger diameter piston pushes more fluid, which the dual disk set up would call for. I'm actually surprised that the difference is only 1 mm as it's been my experience that the difference in master cylinders on other brands is more than 1 mm depending on the front brake set up ie: 1 disk/2 disk.

    If you can return it and get the right master cylinder for your application you would be better off.
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