Figure 3-34.Ring gap.
There is an additional groove cut into the piston just
above the top ring groove. The purpose of this groove is
to divert some of the intense heat that is absorbed by the
piston head away from the top ring. This groove is
called a HEAT DAM.
RING GAP (fig. 3-34) is the split in the piston ring.
This is necessary for installing the ring on the piston and
allowing for expansion from heating. The gap must be
such that there is enough space so the ends do not come
together, as the ring heats up. This would cause the
rings to break. There are a few variations of ring gap
joints (fig. 3-35). Two-cycle engines usually have pins
in their ring grooves to keep the gap from turning. This
is important because the ring would break if the ends
were allowed to snap into the inlet or exhaust ports.
Staggering the ring gap is also important as it prevents
blow-by. A significant amount of total blow-by at the
top ring will be from the ring gap. For this reason, the
top and second compression rings are assembled to the
piston with their gaps 60-degrees offset with the first
Rings must also be fitted for the proper side
clearance (fig. 3-36). This clearance varies in different
types and makes of engines; however, in a diesel engine,
the rings must be given greater clearance than in a
gasoline engine. If too much side clearance is given the
rings, excessive wear on the lands will result. If there is
too little side clearance, expansion may cause the lands
Figure 3-35.Ring gap variations.
Figure 3-36.Fitting piston ring and installing piston.