On an in-line 6 cylinder engine, when piston #5 is at TDC on its compression stroke, which valves can be adjusted?

When piston #5 is at TDC (Top Dead Center) on its compression stroke, it is the perfect moment to consider which valves can be adjusted. At this point in the engine's cycle, piston #5 is at the top of its stroke and has just finished compressing the air-fuel mixture, leading to the next combustion phase.

During this specific position in its cycle, it is important to note that the corresponding valves for piston #5 can be adjusted because neither the intake nor the exhaust valve for that cylinder will be in a position of motion. The exhaust valve for cylinder #5 would just be closing, and the intake valve is still closed, making this a prime time to make adjustments without the risk of interfering with valve operation.

In contrast, the valves of the adjacent cylinders, such as those for #4 and #6, would be in various stages of opening and closing due to their own cycles of operation. This makes adjusting them at this moment inappropriate or potentially damaging. Therefore, adjusting the valves for piston #5 is the correct action to take, as it ensures that the adjustments can be made correctly and safely without affecting neighboring cylinders.