FAA Powerplant Written Practice Test

What is the purpose of the stem keys installed on the valve stems?

1. To adjust valve timing

2. To secure the valve spring retaining washer

3. To limit valve lift

4. To improve valve sealing

The correct answer is: To secure the valve spring retaining washer

The purpose of the stem keys installed on the valve stems is to secure the valve spring retaining washer. These keys play a critical role in the assembly of an engine's valve system by ensuring that the retaining washer, which holds the valve springs in place, does not move out of position during operation. This secure positioning is vital because it maintains the proper spring pressure on the valves, which is essential for their correct operation and to prevent any potential failure that can occur if the springs lose their tension or alignment. The other options, while related to valve function, do not accurately describe the specific role of the stem keys. Valve timing adjustments are made through different means such as camshaft position, and limiting valve lift is typically managed by the design of the cam profile. Improving valve sealing involves the shape and fit of the valve against the seat rather than the means of retaining the springs. Thus, the correct understanding of the stem keys highlights their fundamental purpose in securing components crucial for the reliable operation of the engine’s valvetrain.