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The differential pressure test is designed to check the compression of reciprocating engines by measuring the leakage through the cylinders caused by worn or damaged components. The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across that orifice will result. The restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:

The orifice size is selected on the following:

Engines up to 1,000 cubic inch displacement: 0.040 inch orifice diameter, 0.250 inch long, 60-degree approach angle.

Engines in excess of 1,000 cubic inch displacement: 0.060 inch orifice diameter, 0.250 inch long, 60-degree approach angle.