Pistons are usually small in diameter than the bore of the cylinder. The space between the cylinder and the cylinder wall is called the piston clearance. This clearance is essential for the following reasons :
- It provides a space for a film of lubricant between the piston and cylinder wall to reduce friction.
- It prevents piston seizure. Due to the very high operating temperature, the piston and cylinder block expand. The cylinder is cooled better than the piston, hence sufficient clearance should be provided for the piston to expand, failing which the piston seizure will result.
- If there is no clearance between the piston and the cylinder, it will be impossible for the piston to reciprocate in the cylinder.
The piston clearance amount depends upon the cylinder bore size and the metal used in the piston. But it is generally 0.025 mm to 0.100 mm. This clearance is filled with oil so that the piston and rings move on oil films. Proper clearance should be maintained between the piston and the cylinder wall. If the clearance is too small, there will be a loss of power from excessive friction, severe wear, and possible seizure of the piston in the cylinder.
If the clearance is too much, a piston slap will result. A piston slap means sudden tilting of the cylinder as the piston is down on the proper stroke. The poison shifts from one side of the cylinder to the other with sufficient force to produce a distinct noise. As the piston warms up the clearance is reduced and the noise usually disappears. In order that fixed clearance may be used without risk of seizure, special alloys have been introduced and many designs of pistons are in use. These special designs involve cam grinding to non-circular forms, semi-flexible skirts incorporating oblique slits, controlled distribution, and the like expedients.