Desirable Characteristics of the Piston

• Silent in operation.
• Seizure between piston surface and inside of cylinder should not occur.
• Sufficiently resistant to corrosion against combustion.
• Short in length so that engine size (length of cylinder) is decreased.
• Light in weight.
• High thermal conductivity to help quick heat transfer to reduce temperature of top of the piston (for higher compression ratio and to avoid detonation which may otherwise be caused due to pre-ignition).

The piston is subjected to side thrust when the connecting rod is inclined with respect to the cylinder axis. The skirt is provided with bosses in the inside to support piston pin. The skirt must be of sufficient length to resist tilting of the piston under load. It is kept quite close fitted in the cylinder but even then it is separated from the cylinder walls by means of a thin film of lubricating oil. This oil film helps in smooth running of engine.

The method of eliminating slapping tendency of pistons is to offset them from the cylinder axis (center line) towards the major thrust side. The piston pin-hole is not symmetrically placed as usual, but is slightly offset.

As the piston approaches TDC during compression stroke, the offset causes it to tilt slightly so that its top skirt surface on the minor thrust side is placed against cylinder wall.

Simultaneously the bottom surface of the piston skirt on the major thrust side is also placed against the cylinder wall. When the TDC is passed the inclination of the connecting rod forces the piston against the major thrust surface. However, since the lower surface of the skirt is already in contact with the cylinder wall because of the offset the piston skirt very smoothly wipes into full wall contact, thus avoiding piston slap.

If the piston offset is located towards the minor thrust side, a higher mechanical advantage is obtained. Moreover, lesser piston to cylinder friction is caused. However, it will not decrease the piston slap. Therefore in racing car engines the pistons are offset towards minor thrust side since noise and life are not as important as performance in such engines.