The Needle Stop Screw /chuck nut is an important component part of the airbrush that locks the needle securely onto the needle assembly, which is protected by the airbrush metal tail piece. The needle chuck allows smooth movement of the needle as it passes back and forth through the airbrush nozzle tip, so that the liquid makeup flows properly. This is a precision manufactured part specifically designed for the "Mist"ique Airbrushes and needles, and made of a strong, durable metal alloy. Remember, this screw holds the needle in place, so when you pull the trigger back the needle is simultaneously drawn back. These little screws have a way of getting lost when taking an airbrush apart, so it's best to keep an extra on hand. Your airbrush will not function properly without it!