A radial engine that has been shut down for some time should be turned through by hand for what purpose?

Turning a radial engine by hand after it has been shut down for some time is an important step to check for hydraulic lock. Hydraulic lock can occur if any of the cylinders have accumulated fuel or coolant (in the case of liquid-cooled engines), which would prevent the engine from turning freely. By rotating the engine by hand, the pilot or mechanic can ensure that all pistons are able to move without obstruction. This is crucial because if an attempt is made to start the engine without first checking for this condition, it could lead to severe damage or engine failure when the starter engages.

While inspecting fuel lines or spark plugs is an important part of engine maintenance, these procedures are generally performed separately and would not specifically address the risk associated with hydraulic lock. Lubricating the engine is also necessary maintenance, but it typically requires a different process than just turning the engine by hand. Focusing on checking for hydraulic lock ensures safe engine operation before attempting to start it.