单项选择题 During （）, the starting air valves should be closed to prevent the engine from turning.

单项选择题 Due to （） action of pistons some relative motion between parts in contact in the coolant supply and return system must occur.

单项选择题 Doors are generally formed in the （） of the both in-line and Vee engines to give maintenance access to the crankshaft main and connecting rod large end bearing.