FAA Powerplant Written Practice Test

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

1. Adjusting the rocker arms

2. Replacing the camshaft

3. Replacing the pushrod

4. Adjusting the valve seats

The correct answer is: Replacing the pushrod

In opposed-type engines that utilize hydraulic lifters, valve clearance adjustments are achieved by replacing the pushrods. Hydraulic lifters automatically maintain the desired valve clearance during engine operation, allowing for self-adjustment as the engine runs. However, if the clearance is found to be out of specification, this indicates that the pushrods may be the issue, either because they are worn or because the incorrect length has been installed. Unlike adjustable rocker arms, which can be tuned to set the proper clearance, hydraulic lifters rely on an internal mechanism that compensates for wear and thermal expansion, thus making the pushrods critical to achieving the correct valve movement. Replacing them ensures that the length is appropriate for the specific engine configuration, helping maintain optimal performance and function of the engine. Alternatively, adjusting the rocker arms does not apply to hydraulic lifters, as their function is different than conventional ones that require precise manual tuning. Likewise, the camshaft and valve seats play more direct roles in timing and sealing rather than in maintaining valve clearance, which is specific to the hydraulic system and its components.