The pin bore/piston pin sliding system must be in perfect condition in order to ensure reliable engine operation. If the surface roughness is too low, particularly when starting, this can cause galling of the pin bore. Therefore, depending on the pin bore diameter, a surface roughness of Ra = 0.63–1.0 μm is desired for the pin bore. Pistons with piston pins that move only in the piston (shrink-fit connecting rods) generally have slightly greater surface roughness values, in order to increase oil retention, particularly under less-than-ideal running conditions.
Other detailed measures are often necessary in order to ensure lubrication under all operating conditions. These include oil pockets (slots) or circumferential oil channels for improved lubrication in the pin bore.
The clearance of the piston pin in the pin boss is important for smooth running and low wear of the bearing surfaces. The fitting clearance increases with increasing operating temperature, which is caused by the greater heating of the piston relative to the pin, and possibly the different thermal expansion of the piston and pin materials. For pistons of Al alloy, this increase can be approximated as: increase in clearance = 0.001 x pin diameter [mm] The increase in clearance for a 30 mm diameter piston pin is therefore approximately 30 μm.
Previously, very tight clearances were typical, so that the piston pin could be inserted only in a preheated piston. Today, the clearance is considerably greater, and the piston pin is inserted into the pin bore at room temperature. This prevents deformation of the skirt due to shrinkage stresses and potential galling of the piston pin in the piston when starting at low temperatures. When designing the minimum clearance, differentiation must be made between a floating pin bearing or a piston pin with a shrink fit in the pin end bore. The floating piston pin is the standard design and is the variant that can be specifically most highly loaded in the pin boss.
