What does a feeler gauge specifically measure in an engine?

A feeler gauge is a precise tool that measures the gap or clearance between two parts in an engine, most commonly used to check valve clearances. Valve clearance is the space between the valve stem and the rocker arm or cam follower, which is crucial for ensuring proper engine operation. Adequate clearance allows for thermal expansion of the components during operation and ensures that the valves fully close, preventing loss of compression and maintaining optimal engine performance.

Using a feeler gauge, a technician can insert the gauge into the gap and gauge the thickness to determine if it falls within the manufacturer’s specified measurements. If the clearance is too tight or too loose, it can lead to performance issues or engine damage.

The other options pertain to measurements that require different tools and techniques, such as compression testers for cylinder compression, viscometers for oil viscosity, and fuel pressure gauges for fuel system pressure. Each of those tools is designed for specific parameters that are not measured by a feeler gauge. Thus, the correct answer highlights the specific application of a feeler gauge in measuring valve clearance.