When operating an aircraft with a constant-speed propeller, which procedure places the least stress on the cylinder components?

Adjusting the manifold pressure before the RPM when decreasing power is the correct procedure for minimizing stress on the cylinder components of an engine equipped with a constant-speed propeller.

When an aircraft reduces power, if the RPM is decreased before lowering the manifold pressure, there can be a sudden reduction in engine load which leads to rapid changes in cylinder pressure, temperature, and load conditions. This can cause increased thermal and mechanical stress on engine components. By reducing manifold pressure first, this allows the engine to slowly decrease its pressure and temperature, thus promoting a smoother and more gradual reduction in power that reduces the risk of thermal shock and excessive strain on the cylinders.

Choosing the order in which to reduce manifold pressure and RPM has significant implications for engine longevity and performance. Practices that promote smoother transitions tend to prolong engine life and maintain its reliability.