Abstract: A source of gas pressure from an internal combustion engine is made available by tapping off the exhaust products within the combustion chamber, preferably just prior to exhausting the gas out of the engine. A pressure tap fitting communicates with the combustion chamber through the cylinder wall. The opening into the combustion chamber is closed off during most of the stroke of the engine by the skirt of the piston. As the piston, during the power stroke of the engine, is driven down the cylinder wall, the crown of the piston eventually clears the opening in the pressure tap. The exhaust gas, under very high pressure, enters the tap just prior to dumping the exhaust gas through the exhaust ports of the engine. Thus, a useful source of pressure is derived from the engine without affecting the operating efficiency of the engine.