What is the valve clearance of an engine using hydraulic lifters when the lifters are completely flat?

The valve clearance of an engine using hydraulic lifters when the lifters are completely flat is important to understand because it can impact engine performance and operation. When hydraulic lifters are fully collapsed, it indicates that there is no hydraulic pressure inside them to maintain the proper clearance between the lifter and the valve. However, when this occurs, there is still typically a small amount of valve clearance required to ensure that the valve can fully close without interference.

This implies that even with the lifters being flat, there should still be some clearance present—meaning it must exceed zero. Having this clearance helps prevent potential problems such as valve-to-piston interference or excessive wear on valve seats.

In contrast, if the clearance were equal to zero, it could lead to issues such as valve float or damage due to the inability of the lifters to accommodate thermal expansion or operating tolerances. Additionally, the other options suggesting a negative value or doubling the specified amount also do not reflect the operational principles of hydraulic lifters.

Thus, the correct understanding is that there must always be a minimum clearance when lifters are flat, ensuring the engine operates smoothly without risking mechanical failure.