THE GAUGE - Most engine builders have a drawer in their rollaway dedicated to storing all the "cheap compression gauges" that they have used the poor judgment to buy during their career. However in the very front of that drawer is the Snap-On gauge that gets used regularly. This gauge is preferred not just for it's good accuracy and durability, but rather for one simple design feature. The adaptor hoses, of the Snap-On gauge, that screw into the spark plug threads has a Schrader air fitting at the spark plug tip location. That is, the pressure is sealed off at the face of the dome in the cylinder head, which gives the truest representation of the exact combustion chamber volume. Most other automotive gauges have this air seal fitting mounted in the gauge body, at the end of a 16" hose. This means that the air volume inside that hose (usually about 3-4 cc) is added to the combustion chamber volume during a measurement. The end result of his added volume is a reading that is 20 - 35 psi lower than the true reading. For similar reasons, the tapered rubber "hold - on" type gauges are virtually useless. Besides indicating the added 2 cc of the threaded spark plug hole itself, these gauges are notorious for leaking as well.
