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1940 Ford Master Cylinder Question.....

Discussion in 'The Hokey Ass Message Board' started by bobbyb, Jul 19, 2010.

  1. bobbyb
    Joined: Jun 28, 2009
    Posts: 151

    from ohio

    What is the bore of a stock '40 Ford master cylinder? Am looking to use a dual cavity cylinder. Looking at previous posts most use a 67/68 Mustang, but some posts suggest a larger than stock bore. I could not find any post that listed the stock '40 bore. I am using the standard '40 brakes front and rear on my project car. If a larger bore is large?
    Thanks in advance.
    Joined: Feb 24, 2009
    Posts: 111


    If I remember correctly stock was 1 in. bore. You can measure it at the snap ring with a set of calipers. You will find that the bore size before the snap ring may be .025 to .030 bigger than the actual bore size, it's for assembly purposes.
  3. thunderbirdesq
    Joined: Feb 15, 2006
    Posts: 6,809


    I think they're 1.125"...
  4. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,589

    V8 Bob


    The info I have shows the '39-'48 master has a 1 1/16th (1.0625) bore. You would not want to increase the bore size on a replacement master, as that would just increase pedal effort for the same pressure out. The popular '69-'70 Mustang has a 1" bore and works great on the '40-'48 Fords, as this allows a reduced pedal effort without running out of pedal travel IN MOST CASES! But you HAVE to check to be sure the master can be bottomed out BEFORE the pedal hits the floor, or you will not benefit the dual master safety advantage in the case of a hydraulic failure, and have a difficult time attaining a good bleed. The common late model master cylinders have different measured full strokes, ranging from just over an inch to 1.5 or more inches. and this is where you can run into trouble. The full stroke X the pedal ratio = the required pedal travel.

    I would suggest buying a power disc/drum master, as these work just like drum/drum masters, but allow for a disc upgrade latter without having to change the master, and they usually have a deeper push rod bore that works better with manual linkage. Plumb the larger disc chamber to the fronts, and use 10 lb residuals front and rear. (Check for internal residuals in the master you buy/use, but many after '70 did not have them, both drum/drum and disc/drum.)
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