Offshoreonly.com - How is a leak down test performed?

Each cylinder is rotated to TDC (both valves closed).
Air pressure is applied through a leak down tester. The tester has an input pressure guage, a small orifice, then a cylinder pressure guage connected to the spark plug hole of the cylinder. The orifice allows a slow leak between the guages. If there's a lot of leakage in the cylinder (ie. by the rings) it will bleed out faster than can be supplied through the testers orifice. The difference between the input pressure, ie 50psi and the cylinder pressure, ie 40psi gives the percent leakage (20%). You can also listen to the crankcase, water jacket, carb, exhaust for the air hissing to determine the source of the leak.