Piston skirt is the part below piston ring land. Itsfunction is to keep vertical position of piston during reciprocating action,which is leading part of piston. The skirt part slide with high speed incylinder under side thrust action, so it is badly worn. When designing skirtpart, it should have enough bearing area to reduce beta ratio and form oilfilm; assure right cylindrical shape of skirt part and keep minimal gap withcylinder, position pin properly to prevent piston inclining in cylinder.

To make piston skirt part with good leading action and goodsealing property without increasing friction loss, piston skirt and insidecylinder should keep minimal space which changes with temperature. Transect ofskirt is usually designed to be oval, long axis is perpendicular to piston pincenterline. Piston skirt enlarges in piston pin centerline direction while minishes in perpendicular direction when working.

When piston operating, the temperature is unevenlydistributed along altitude direction, more near to the top, the temperature ishigher, and temperature gradient of land ring is bigger than skirt part. So inorder to assure small space and well-distributed, surface of piston should bedesigned in principle to be shape of taper or step type which have small upperpart and large down part.