During a cylinder leakage test, the piston must be at top dead center on the exhaust stroke in the cylinder being tested.

During a cylinder leakage test, the piston must be at top dead center on the exhaust stroke in the cylinder being tested.

• A. True
• B. False
• C. It doesn't matter
• D. It must be at bottom dead center

Answer: (A)

For a cylinder leakage test you need the cylinder sealed so pressurized air can’t escape through the valves. The position at top dead center on the exhaust stroke is where both the intake and exhaust valves are closed, isolating the cylinder. With the valves shut, any loss of pressure must come from the rings or valve seats, not from an open valve. If you tested with a valve partially open (for example at another stroke), air would escape through that valve and give an unreliable reading. So the piston being at TDC on the exhaust stroke provides the proper, sealed condition for an accurate leakage test.

For a cylinder leakage test you need the cylinder sealed so pressurized air can’t escape through the valves. The position at top dead center on the exhaust stroke is where both the intake and exhaust valves are closed, isolating the cylinder. With the valves shut, any loss of pressure must come from the rings or valve seats, not from an open valve. If you tested with a valve partially open (for example at another stroke), air would escape through that valve and give an unreliable reading. So the piston being at TDC on the exhaust stroke provides the proper, sealed condition for an accurate leakage test.