Figure 3-26. - Cam-ground piston action.

There are two types of piston skirts in most engines - FULL TRUNK and PARTIAL SKIRTED (SLIPPER) (fig. 3-28). The full trunk type of skirt has a full cylindrical shape with hearing surfaces parallel to those of the cylinder. This gives it more strength and better control of the oil film. The partial skirt or slipper skirt has considerable relief on the sides of the skirt. Removal of the skirt in these areas serves the following purposes:

Lightens the piston, which, in turn, increases the speed range of the engine.

Reduces the contact area with the cylinder wall, which reduces friction.

Allows the piston to be brought down closer to the crankshaft without interference with its counterweights.

Figure 3-27. - Cam-ground piston.