Piston forms the lower part of the combustion chamber.It seals the cylinder and transmits the gas pressure to the crankshaft through connecting rod.The Piston absorbs heat of combustion and this heat must be conducted away if the metal temperature is to be kept in safe limit.

Property of material of piston

1.Material require similar properties to those for cylinder liners and covers.

2.It must have a long fatigue life to survive the fluctuating mechanical and thermal stress.

3.The metal must resist high temperature creep, corrosion and erosion

4.Readily conduct heat for cooling but have limited thermal expansion so that working clearances are maintained with the liner and piston rings.

5.The crown must withstand the high gas load and transmit the force from this to the piston rod.

Function :-

1.It provides means where by gas loads are transmitted to the connecting rod and crankshaft system.

2.It acts as a crosshead (4 -stroke ) to resist cylinder wallside load in the connecting rod

3.It is a barrier for the gas and oil sealing element (piston rings )

Constructional feature

Piston are cast in chrome -molybdnum alloy steel and machined on all surfaces.Ring grooves are cut into the wall,and to reduce wear and fretting of groove surfaces they are chromium plated and ground.

The piston diameter tapered inward on top of piston rings to allow for thermal expansion.

Composite pistons are made in two parts and used highly rated slow and medium speed engines.

1.Crown :- Top part of piston

2.Skirt :-Bottom part of piston


Shape may be concave or convex

Concave crown is suitable for efficient combustion

Convex crown is suitable for high mechanical stress due to thick material.

Crown carries compression ring in the grooves.


It takes side thrust due to connecting rod angularity (trunk type engine ).
It also guides piston in the liner and prevents rocking.

It accommodate oil scraper rings in the grooves.

Made up of wear resistant and low co – efficient of expansion material e.g. cast iron

Fig:-1.Two- stroke piston

2.four stroke piston