What position should the piston be in when checking valve tappet clearance?

When checking valve tappet clearance, it is essential for the piston to be at the top of the power stroke. This position ensures that both valves are fully closed, allowing for an accurate measurement of the clearance between the tappet and the valve stem.

During the power stroke, the piston is at the point where it has completed the intake and compression strokes and is about to start the power phase. At this moment, the valves are seated, meaning there is no pressure on the tappets from the valve springs, which is critical to get a precise measurement. If the piston were at a different position in the stroke cycle, such as the bottom of the stroke or halfway up, one or both of the valves may still be partially open, leading to an inaccurate measurement of the tappet clearance. Thus, positioning the piston correctly is fundamental for proper valve adjustment and engine performance.