Piston pin or wrist pin or gudgeon pin connects the piston and small end of the connecting rod. Piston pin is generally hollow and made from case hardened steel heat treated to produce a hard, wear resisting surface. Piston pin may be selectively fitted and, if supplied with the piston, are not interchangeable. The external bearing surface is finished to a very high degree of accuracy to ensure correct fit in the piston and connecting rod. Piston pin should be inspected for wear, cracking or pitting. Circlips should always be renewed and where soft end pads are fitted, check that they are not loose or cracked. Circlips are types of rings which prevent pin to come in contact with cylinder wall.