upside down piston, push the ring to about 1 inch from
elasticity after considerable use. Piston rings are often
the bottom of the cylinder. Remove the piston and use a
quite complicated in design, are heat treated in various
feeler gauge to check the clearance. If the clearance is
ways, and are plated with other metals. There are two
too large, oversized rings are required. If the clearance
distinct classifications of piston rings: compression
is too narrow, remove the ring and file the ends until the
rings and oil control rings (fig. 3-30).
clearance is correct. To file the ring, first place a file in a
The principal function of a compression ring is to
bench vise; then hold the ring with both hands and
seal in the fuel-air mixture as it is compressed and also
compress the gap while filing. This should ensure the
seal in the combustion pressures as the mixture burns
squareness of the clearance. File only a small amount at
during the compression and power strokes. All piston
a time, checking your clearance often. If you remove
rings are split to permit assembly to the piston and to
too much at one time, the ring is unusable. Clearances
allow for expansion. When the ring is in place, the ends
are specified in the manufacturer's manual and should
of the split joint do not form a perfect seal; therefore, it
be carefully adhered to.
is necessary to use more than one ring and to stagger the
Rings must be fitted also for the proper side
joints around the piston. If cylinders are worn,
clearance (fig. 3-31). This clearance will vary in
expanders are sometimes used to ensure a perfect seal.
different types and makes of engines; however, in the
(See figs. 3-27 and 3-30.)
diesel engine, these rings must be given a greater
The bottom ring, usually located just above the
clearance than in the gasoline engine. If too much side
piston pin, is an oil-regulating ring. This ring scrapes
clearance is given the rings, excessive wear on the lands
the excess oil from the cylinder walls and returns some
will result. If there is too little side clearance, expansion
of it, through slots, to the piston ring grooves. The ring
may cause the lands to break.
groove under an oil ring is provided with openings
through which the oil flows back into the crankcase. In
some engines, additional oil rings are used in the piston
skirt below the piston pin.
Connecting rods must be light and yet strong
enough to transmit the thrust of the pistons to the
Prior to installing a new ring, the ring end clearance
must be checked. To check the clearance, first push the
ring about 2 inches into the cylinder. Then, using an
Figure 3-31.--Fitting piston ring and installing piston.
Figure 3-30.--Piston rings.