A leak-down test pressurizes a cylinder at top dead center and reads what percentage of the air escapes, showing whether rings, valves or the head gasket still seal. With 100 PSI in, 90 PSI out is 10% leakage.
Warm the engine, bring the cylinder to TDC on the compression stroke, thread the tester into the plug hole and feed it 100 PSI. Then listen — where the air goes names the part. Breathers mean rings. The intake means an intake valve isn’t seating. Per Motor State, “air coming from the exhaust or header outlet is evidence that an exhaust valve is leaking.” Bubbles in the radiator mean head gasket. There’s no universal pass number; acceptable leakage tracks the ring package, and Motor State puts Dykes-style rings “in excess of 20%,” conventional rings at “roughly 8-12%,” gapless rings at “as little as 1-3%.” Log your own baseline fresh. And it isn’t a compression test — that one measures pressure made while cranking.
A leak-down number tells you whether the engine is worth freshening or worth pulling, before it decides for you in the middle of a feature.