Why do mechanics pull a propeller through prior to starting some reciprocating engines?

Pulling a propeller through prior to starting some reciprocating engines serves as a preventive measure to check for hydraulic lock. Hydraulic lock occurs when a fluid, typically fuel or oil, accumulates in the combustion chamber, preventing the piston from moving. If there is a significant buildup of liquid in the cylinder, turning the propeller by hand can help identify this condition before starting the engine, which is crucial to avoid potential damage.

When mechanics pull the propeller through, they can feel resistance if there is liquid present in the cylinder. This method provides a quick and effective way to ensure that the cylinders are clear before attempting to start the engine, thereby enhancing safety during the engine start-up process.