Leak-down tester

Compression tests should normally be done with all spark plugs removed to maximize cranking speed.

Cranking compression is a dynamic test of the actual low-speed pumping action, where peak cylinder pressure is measured and stored.

Leak-down primarily tests pistons and rings, seated valve sealing, and the head gasket.

This can be as simple as a breaker bar on a crankshaft bolt in an automatic transmission vehicle, or leaving a manual transmission vehicle in a high gear with the parking brake locked.

In the United States, FAA specifications[1] state that engines up to 1,000 cu in (16 L) engine displacement require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle.

While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate.

A locomotive engine which gives a leak-down of 10% on a leak-down tester is virtually perfectly sealed while the same tester giving a 10% reading on a model airplane engine indicates a catastrophic leak.

Also, leakage paths in cylinders can be turbulent at fairly low flow rates.

At low and modest leakage percentages, there is little or no difference between single and dual gauges.

Any pressure above 15 psi (100 kPa) will function just as well for measurement purposes although the sound of leaks will not be quite as loud.

The exact test pressure tolerated before rotation is highly dependent on connecting rod angle, bore, compression of other cylinders, and friction.

There is less tendency to rotate when the piston is at top dead center, especially with small bore engines.

Maximum tendency to rotate occurs at about half stroke, when the rod is at right angles to the crankshaft's throw.