The shape of the piston head is designed to suit the type of combustion chamber of the engine and to improve turbulence of the air as it is being compressed. The head of the piston can also be designed with recesses or depressions to provide clearance for the heads of the valves.
Petrol engines use a slipper-type piston (Figure 6.10). Cutting away the skirt of the piston not only reduces the mass of the piston, but also provides clearance for the crankshaft balance weights when the piston is at BDC. This enables shorter connecting rods to be used, permitting a more compact engine design. On some engines, the balance weights are contoured to avoid interference between the weights and the pistons.
The combustion pressures force the piston downwards, and the connecting rod offers resistance, but it does this at an angle. The result is a side thrust of the piston against the cylinder wall, as shown.
The piston also has a side thrust during the compression stroke, but this is on the opposite side of the cylinder. Also, this is a lesser thrust because the downward force from compression is much less than the downward force of combustion.
The thrusts are sometimes referred to as the major and minor thrusts. Because the thrust during the power stroke (major thrust) is most important, this side of the engine is often referred to as the thrust side of the engine. It is necessary to know about the thrust side of an engine because the pistons in most engines have to be installed in a particular way. Pistons are often provided with a mark to show how they should be fitted in relation to the front of the engine.
Pistons are often designed with the bosses for the piston pin slightly offset. With this design, the bosses are moved a little away (offset) from the centreline of the piston. This is towards the major thrust side (Figure 6.12).
The offset, of about 1.50 mm, alters the angle of the connecting rod slightly and so transfers some of the force away from the thrust side of the piston. There is less tilt applied to the piston during the power stroke and so there is less chance of piston slap.