What aspect of a valve spring is typically being checked using an illustration?

The aspect typically being checked with an illustration is the installed height of the valve spring. Installed height refers to the distance between the bottom of the spring seat and the top of the spring when it is installed in the engine. This measurement is crucial because it ensures that the spring operates correctly, providing the necessary tension and allowing for proper valve closure and operation.

The correct installed height is essential for maintaining the engine's performance, especially at high speeds when the springs need to effectively control the movement of the valves. If the installed height is incorrect, it can lead to issues such as valve float or improper sealing, which can degrade engine performance and reliability.

In the context of valve springs, options like spring tension, spring diameter, and coil count, while relevant to valve spring performance, do not pertain specifically to what is typically illustrated when checking the spring's installation in an engine. Illustrations primarily focus on the position or measurement of the spring once installed rather than these other factors.