Generally, washers are defined as thin disc-shaped plates, typically with a hole through the center, that are used in conjunction with a threaded fastener such as a screw or a bolt. The usual functions of washers include distributing load, creating separation, preventing corrosion and wear, locking fasteners, and reduce vibration.

A flat washer, also known as a plain washer, is a type of washer that is a flat annulus or ring which is used to spread the load of a screwed fastener and prevent damage to the surface being fixed.

Flat washers are generally used to increase the bearing surface of a nut or fastener’s head. It spreads the clamping force over a larger area. Washers may be useful when dealing with soft materials and oversized or irregularly shaped holes. It is usually assembled around a bolt or screw, between the bearing surface of the fastener and the part to which it is attached. Flat washers are used to improve stress distribution, and to span large clearance hole.

A B.I. flat washer means it’s black iron instead of galvanized iron. The latter has zinc to prevent corrosion. The disadvantage of this, however, is it can’t be used in gas lines as the zinc starts to react with gas, causing flaking, overtime.