What is the correct procedure for adjusting valve tappet clearance?

The correct procedure for adjusting valve tappet clearance is to adjust at Top Dead Center (TDC) at the end of the compression stroke. This position ensures that both the intake and exhaust valves are fully closed for the cylinder being adjusted, allowing for an accurate measurement of the clearance between the valve and the tappet.

Adjusting at TDC at the end of the compression stroke means that the rocker arms are not under pressure from the camshaft, thus providing the best conditions for setting the clearance accurately. This ensures consistent performance and prevents the risks associated with improper valve operation, such as misfires or excessive wear on components.

In this context, the other methods mentioned are not suitable. For example, adjusting at the bottom of the compression stroke would mean the valves could be slightly open, leading to incorrect clearance measurements. Adjusting while the engine is running could result in unsafe conditions, as the moving parts create significant risk, and it is challenging to get an accurate reading under such circumstances. Additionally, adjusting only when the engine is cold is not critical in this case—warm engines can expand and operate differently, but the focus should be on the correct mechanical position, which is why TDC at the end of compression is essential.