adjust the pressure regulator on the tester so that the regulator pressure gauge reads around 20 psi. With a co-worker securely holding the propeller blade, slowly open the air valve. You should use care when opening the valve since the propeller will rotate quickly if the piston is not at TDC. Once you have the valve open, the person holding the propeller wants to ensure that the piston is in its TDC position. This is done by slowly rotating the propeller in the direction of normal rotation. Reaching TDC is indicated by a flat spot or sudden decrease in force required to turn the propeller. If the propeller is pulling in the direction of normal rotation, then you have passed TDC. If this happens, you want to back up about one-half revolution and rotate it through again in order to eliminate the effect of backlash in the valve operating mechanism and to keep the piston rings seated on the lower ring lands.