When piston rings are new, a period of running is
necessary to wear the piston rings a small amount, so
they conform perfectly to the cylinder walls. The
cylinder walls are surfaced with a tool called a hone,
which leaves fine scratches in the cylinder walls (fig.
3-37). The piston rings are made with grooves in their
faces, which rub against the roughened cylinder walls,
serving to accelerate ring wear during the initial stages.
As the surfaces wear smooth, the rings wear in.
Extreme pressure may be applied to high spots on
the piston rings during the wear-in period. This can
cause the piston rings to overheat at these points and
cause damage to the cylinder walls in the form of rough
streaks. This condition is called scuffing. New piston
rings are coated with a porous material, such as
graphite, phosphate, or molybdenum. These materials
absorb oil and serve to minimize scuffing. As the rings
wear in, the coatings wear off.
Some piston rings are chrome-plated. Chrome-
plated rings provide better overall wearing qualities.
They also are finished to a greater degree of accuracy,
which lets the piston rings wear in faster.
Figure 3-37.Piston ring wear-in.
Connecting rods connect the pistons to the
crankshaft. They must be strong enough to transmit the
thrust of the pistons to the crankshaft and to withstand
the internal forces of the directional changes of the
pistons. The connecting rods (fig. 3-38) are in the form
of an I-beam. This design gives the highest overall
strength and lowest weight. They are made of forged
Figure 3-38.Connecting rod construction.