Valves used for isolating or directing fuel flow must be of the positive stop variety, i.e., a ball valve that turns through no more than 90 degrees. The reason for this guideline, which is an ABYC standard, is to facilitate a quick, at-a-glance indication of a valve’s open or closed position. Traditional rotary “washing machine” gate valves, while not uncommon, do not meet this standard, and the handles are often mild steel, making them rust-prone. The same holds true for tank sight-glass valves; these too should be of the ball rather than gate variety. Finally, any valve that relies on an external stem spring should not be used. The spring applies tension to a tapered plug, thereby maintaining a liquid-tight seal. The spring itself, however, is nearly always made from mild steel; when it rusts and crumbles, a fuel leak is sure to follow.