How is the effective piston area of a cone calculated?

[454Casull]

It probably isn't pir^2... Anybody know?

[roddyama]

Usually the diameter is measured as the actual cone diameter plus half of the surround on both sides. Then it's pi X (dia/2)^2

[Ron E]

Yes, it is:
Sd = pi * radius ^2

or, since radius = Dia/2

Sd = pi*(Dia/2)^2 = (pi*(Dia)^2)/4

As Rodd says, you should include half of the surround. The easiest way to measure diameter is to measure across the diameter on opposite sides of the surround. See picture.