The torque required to tighten a fastener is reduced when friction is reduced. If a fastener is dry (unlubricated) and plain (unplated), thread friction is high. If a fastener is wax coated or oiled or has a zinc phosphate coating or cadmium plating, friction forces are reduced. Each of these coatings and combinations of coatings has a different effect. Using zinc-plated hardened flat washers under the bolt (capscrew) head and nut reduces friction. Dirt or other foreign material on the threads or clamping surfaces of the fastener or component can increase friction to the point that the torque specification is met before any clamping force is produced.
Even though varying conditions affect the amount of friction, a different torque value cannot be given for each. To ensure that they are always torqued accurately, most OEMs recommend that all fasteners be lubricated with oil (unless specifically instructed to install them dry), and then torqued to the values for lubricated- and plated-thread fasteners. When locking compound or antiseize compound is recommended for a fastener, the compound acts as a lubricant, and oil is not required.