Retaining rings are circular fasteners that fit over shafts or inside cylindrical products such as pipe. There are two main types: internal and external. Internal retaining rings have a diameter specified for the bore and groove sizes they will fit into. External retaining rings, also known as retaining rings for shafts, have a diameter specified for the shaft and groove sizes it fits around. Both industrial retaining ring types, also known as a retaining clip, have a thickness dimension. Measurements will be expressed in inch or millimeters. Retaining ring material varies based on application. Stainless steel retaining rings are made from one of the most common ring materials.