The C3 refers to greater radial clearance to allow for high speed environments where excess heat is generated. Generally C3 bearings are used in engine and gearbox environments. You should always replace a C3 bearing with another C3 bearing, and on the same basis avoid fitting a C3 when not called for. C3 Bearings feel loose in normal ambient temperatures prior to fitting, and will show axial play until running at operating temperature. Manufactured to inch sizes to meet the requirements of many older British machines and many current American manufactured applications.