What is Piston Slap?


Staff member
Iowegia - USA
A quote from Heywood's book on Internal combustion Engines regarding piston friction: "The side thrust (force) [equation referred to]is transmitted to the liner [cylinder wall] via the rings and piston skirt. It changes direction as the piston passes through top- and bottom-center positions. Since the friction force changes sign at these locations and the gas pressure during expansion is greater than during compression, the side thrust during expansion is greater. The psiton skirt carries part of this side thrust [force] so it contributes to piston assembly friction. The large contact area between the skirt and liner, relative to the ring contact area, results in higher loading [pressures] and promotes hydrodynmaic lubrication. Piston skirt areas have been reduced substantially in recent years to reduce piston mass(which reduces side thrust) and contact area. An additional reduction in side thrust, leading to reduced skirt friction, has been achieved with the use of an offset wrist-pin. By offsetting the pin axis by 1-2 mm without changing the vertical location, the crank angle at which the piston traverses the bore and "slaps" the other side of the cylinder is advanced so it occurs before combustion has increased the cylinder gas pressure significantly." I would say that piston slap is reduced by increasing the thickness of the oil film (increasing viscosity) of the oil. Piston slap is my view is a mechanical impulse force that is reduced by the cushioning effect of the the oil film.