1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Alloy Dommie Head CC's

Discussion in 'Other Norton Motorcycles' started by Connie73, May 16, 2018.

  1. Connie73


    Jan 27, 2015
    Hi All,

    Trying to work out how much my compression ratio will increase when i change heads over to fix a spindle issue. New head has had 6thou taken off the deck. I have a calculator to plug the numbers in but need to know the cc amount for the head??
    Anyone ever measured this for 500cc dommie.
    Dommie Nator likes this.
  2. texasSlick

    texasSlick VIP MEMBER

    Jan 2, 2013
    Here is a formula that I worked out for calculating the compression ratio change when adding a base shim. This should should work for you, without the need to know CC volume. In your case, the "shim" thickness is minus 0.006 inches. Plug that in to the formula with the stroke in inches.

    Here is a formula which can be used to either calculate the thickness required to achieve a desired CR, or to determine the CR that will result from a given thickness:

    h/s = (CR1 - CR2)/((CR1 - 1)(CR2 - 1))

    h = thickness of shim
    s = stroke of engine
    CR1 = known compression ratio before shimming
    CR2 = compression ratio after shimming

    Note: the formula is dimensionless - this means thickness of shim and stroke can be in inches, mm, or furlongs....just be consistant! Also consider that one gasket at cylinder base is the norm and using a shim will require two gaskets....the compressed thickness of the second gasket should be considered part of the overall shim thickness.


    PS: I measured the CC volume in my Atlas head. I leveled the head both ways, then added water with a syringe calibrated in ml until the meniscus formed on the deck lip. If you have a spigot, be sure to subtract the spigot volume. I think this method is accurate to +/ - 2-3 ml.
    Last edited: May 16, 2018