FAA Powerplant Written Practice Test

How are valve clearance changes accomplished on opposed-type engines using hydraulic lifters?

1. By adjusting the valve seat

2. By replacing the pushrod

3. By tightening the rocker arm screws

4. By modifying the valve lengths

The correct answer is: By replacing the pushrod

In opposed-type engines that utilize hydraulic lifters, valve clearance changes are primarily accomplished by replacing the pushrod. Hydraulic lifters are designed to automatically adjust to variations in valve clearance due to their design, which compensates for thermal expansion and other factors. However, if there is wear or if the pushrod is not of the appropriate length, replacing the pushrod becomes necessary to maintain the correct clearance between the valve and rocker arm. Ensuring the proper length of the pushrod is vital; it directly affects how much the lifter can extend, thereby influencing valve actuation. If a pushrod is too short, it may not allow the lifter to reach the required hydraulic setting, consequently leading to a greater clearance and potential engine performance issues. Conversely, a pushrod that is too long can create excessive pressure within the lifter, possibly leading to valve train problems. Thus, while adjustments to the rocker arm screws and other methods might seem viable, they do not directly apply to hydraulic lifters in the same way as pushrod replacement does. The unique nature of hydraulic lifters makes pushrod length the critical factor in ensuring proper valve operation and clearance in this engine type.