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Cylinder Pressure

By “Skip" Millard

Originally published in the Austin-Healey Magazine

Looking for a easy method to check your engine for bad valves; excessive ring blow-by; defective head gasket; cracked water jacket? A simple, all in one tester can be made by using an old sparkplug. Discard the porcelain and electrode leaving only the threaded collar section. To this weld or braze a male quick disconnect fitting from an air hose.

An air compressor is necessary so now take a trip to your neighborhood gas station for the actual testing. Remove all spark plugs. Place your thumb over the spark plug hole of the cylinder to be tested and tap the starter solenoid button to rotate the crankshaft until compression forces your thumb off the plug hole. The piston should now be at TDC with valves closed, or, firing position. Now screw your home made tester into the plug hole and attach the air hose and listen.

If this occurs at:

Trouble is:


Bad Intake Valve

Tail Pipe

Bad Exhaust Valve


Excessive Ring Blow-by

Adjoining Cylinder

Blown Head Gasket or Warped Head

Hiss or bubbles in radiator indicates a leak into water jacket from head gasket or cylinder sleeve. Check each cylinder in order, first finding TDC for each cylinder being tested. Piston will not move when air pressure is applied if piston is within 5 degrees of TDC.

Good Luck

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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