When performing a valve adjustment on a cylinder's valves, ____________.

When performing a valve adjustment on a cylinder's valves, it is essential that the valve being adjusted is in the closed position. This is because adjustments aim to set the appropriate clearance between the valve and its associated components, such as the rocker arm or pushrod. When the valve is closed, it ensures that the correct distance can be measured and set accurately, as there is no pressure on the valve from the camshaft, which could lead to improper clearance readings.

Adjusting the valves while they are open would produce inaccurate results, as the components would still be under tension, preventing a proper measurement of the clearance. This highlights the importance of performing adjustments under the right conditions, ensuring optimal engine performance and preventing issues related to valve clearance, such as excessive wear or poor engine operation.