Machine screws are small-diameter fasteners that fit most high-volume machining applications and can be used on almost any type of surface. They are straight-bodied fasteners with flat ends and have full threads to either fit in a pre-tapped hole or match with a machine screw nut for firmer tightening. Standard machine screws join metal objects together through existing tapped holes and are typically used in high-volume automotive or machinery assembly and repairs because of their uniformity. Tamper-resistant screws require special tools for installation and removal. Sealing machine screws have an O-ring under their heads for sealing out air, chemical vapors, and moisture.