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.

Piston rings are usually made of fine grained alloy cast iron. This material possesses excellent heat and wear resisting qualities inherent in its graphitic structure. The elasticity of this material is also sufficient to impart radial expansion and compression which is necessary for assembly and removal of the ring, and particularly to enable it to exercise flexible pressure on the cylinder valves.

1. Compression rings
2. Oil control rings

Piston (Image credit: btechgurus.blogspot.com)

Piston is one of the most important part in a reciprocating engine in which it helps to convert the chemical energy obtained by combustion of fuel into useful mechanical power. The purpose of the engine is to provide a means of conveying the expansion of the gases to the crankshaft via the connecting rod, without loss of gas from above or oil from below. Piston is essentially a cylindrical plug that moves up and down in the cylinder. It is equipped with piston rings to provide a good seal between the cylinder wall and piston. The efficiency and economy of the engine primarily depends on the working of piston. It must operate in the cylinder with minimum friction and should be able to withstand a high explosive force developed in the cylinder and also the very high temperature ranging from 2000 C to over 2800 C during operation. The piston should be as strong as possible, however its weight should be minimized as far as possible in order to reduce the inertia due to its reciprocating mass. The top of the piston is called head. Ring grooves are cut on the circumference of the upper portion of the piston. The parts below the ring grooves is called skirt. The portion of the piston that separates the grooves are called the lands. Some pistons have a groove in the top land called a heat dam which reduces heat transfer to the rings. The piston bosses are the reinforced sections of the piston designed to hold the piston pin or wrist pin.

There is problem of cylinder wear in I.C. engine and this is solved by use of cylinder liners. Cylinder liners are in the form of barrels made of special alloy iron containing silicon, manganese, nickel and chromium. They are cast centrifugally. It is now customary to fit cylinder liners on engines of cars and commercial vehicles. These liners are of the oil hardening type and offer considerably longer life for the engine. 

The cylinder liners are of two types:

1. Dry liners - Dry liner is made in the shape of barrel having a flange at the top which keeps it into position in the cylinder block. The entire outer surface of the dry liner bears against the cylinder block casting and hence has to be machined very accurately from the outside also. Thus it is not in direct contact with the cooling water and hence is known as dry liner. Its thickness ranges from 1.5mm to 3mm. It is used mostly for reconditioning warm cylinders.

2. Wet liners - A Wet liner forms a complete cylinder barrel. It is provided with a flange at the top which fits into the groove in the cylinder block. At the bottom either the block or the liner is provided with grooves, generally three in numbers, in which the packing rings made of rubber are inserted. The liner is in direct contact with the cooling water and hence is known as wet liner. The outer surface of the liner does not require accurate machining. Wet liners are thicker than dry liners, ranging from 1.5mm to 6mm.

Crankcase is attached to the bottom face of the cylinder block in I.C.engine. It acts as the base of the engine. It supports the crankshaft an camshaft in suitable bearings and provides the arms for supporting the engine on the frame. The oil pan and the lower part of the cylinder block together are called the crankcase.