keeps the piston straight in the cylinder as the engine warms  up from initial startup. As the piston heats up during warm  up, the part of the piston in line with the pin has more mass  and expands more making the piston completely round.  At low temperatures, the piston is oval shaped and, when  it warms to operating temperature, it becomes round. This  process reduces the tendency of the piston to cock or slap in  the cylinder during warm up. When the engine reaches its  normal operating temperature, the piston assumes the correct  dimensions in the cylinder. As many as six grooves may be machined around the  piston to accommodate the compression rings and oil rings.  [Figure 1-15]  The compression rings are installed in the  three uppermost grooves; the oil control rings are installed  immediately above the piston pin. The piston is usually  drilled at the oil control ring grooves to allow surplus oil  scraped from the cylinder walls by the oil control rings to  pass back into the crankcase. An oil scraper ring is installed  at the base of the piston wall or skirt to prevent excessive oil  consumption. The portions of the piston walls that lie between  ring grooves are called the ring lands. In addition to acting  as a guide for the piston head, the piston skirt incorporates  the piston-pin bosses. The piston-pin bosses are of heavy  construction to enable the heavy load on the piston head to  be transferred to the piston pin. The piston pin joins the piston to the connecting rod.  It is machined in the form of a tube from a nickel steel  alloy forging, casehardened and ground. The piston pin is  sometimes called a wristpin because of the similarity between  Figure 1-15. Machined rings around a piston.  Piston Pin Figure 1-14. Piston assembly and types of pistons.  1-13 Aluminum plug Piston pin Compression rings Oil control ring Recessed head Piston Piston pin boss Flat head Recessed head Concave head Dome head Slipper type Trunk type