Valve clearance changes on opposed-type engines using hydraulic lifters are accomplished by:

In opposed-type engines that utilize hydraulic lifters, valve clearance is automatically maintained by the design of the hydraulic lifter system, which adjusts for variations in the clearance as the engine operates. However, if there is a need to specifically change valve clearance or address issues that have arisen, the most practical approach is to replace the push rods.

Push rods play a critical role in connecting the camshaft to the hydraulic lifters, transmitting motion that opens and closes the valves. If push rods are bent or worn, they can affect the overall clearance between the valves and seats, leading to insufficient clearance or excessive play. By replacing the push rods, not only can valve clearance be adjusted effectively, but it helps maintain proper engine timing and performance.

While the other options involve methods that may affect valve operation, none provide the direct means to adjust clearance in the same way that replacing push rods does. For instance, adjusting valve seats would potentially involve machining or shimming, which can be more complex and may not address wear in the push rods, and changing the camshaft would generally alter the timing and lift characteristics rather than the clearance itself. Replacing lifter assemblies could be necessary for other reasons, but in terms of adjusting valve clearance specifically, the focus on push rod