What is the real function of the ring?
OK, RINGS?? What are the
functions of the piston rings? The most common answer is " To create a
seal for compression." OK, the sealing off of the gases for the
compression portion of the stroke is a major function of the piston
ring, BUT this is not all the ring has to accomplish. Let's look at at
what else is going on with the rings.
We'll start from Bottom Dead Center (BDC). At BDC, the crankcase is
highly pressurized, the tuned pipe is sucking its hardest ,and the
cylinder is filling with the fresh charge via the transfer tunnels. The
ring has, hopefully, collected a bit of oil from the cylinder wall and
transferring heat to the cylinder wall. The piston now begins its race
towards Top Dead Center (TDC), the ring's radial tension is keeping it
as tight against the cylinder wall as it can. The ring is collecting oil
from the cylinder wall and getting ready for the compression portion of
the stroke. Once the exhaust port has been closed, (controlled by the
piston crown edge, or in the case of pistons fitted with a Dykes ring,
the ring itself controls port timing), the ring is retaining oil to help
with the compression seal, as well as lubrication. The gas pressures
from the compressing gases keep the ring tightly sealed against the
- As the piston approaches TDC, cylinder pressures are building
rapidly, the piston crown is getting very hot, and the ring is doing its
best not to allow any of the pressures to blow past it. Then combustion
occurs (hopefully at about 11-14 degrees ATDC). The ring now takes on
its role as a heat transfer. The crown temps are very high and rising
and if these temps do not get reduced the aluminum piston will surely
melt. The ring will try and absorb as much of this heat as it can and
transfer it to the cylinder walls where it can be dissipated . Ridding
the piston of these high temperatures is a very important function of
the piston ring.
The style of ring, number of rings, and its location on the piston is
very important in assuring that this heat dissipation successfully takes
Now, the piston is back at BDC and it all starts over again.
The piston is subject
to more abuse then any other part of the 2 stroke engine. It is
constantly being pushed on, sucked on, fired on, squeezed on.... well,
you get my point.
It is the weakest link in the engine.
Over the last decade or so, advances have been made in piston technology.
It has been found that adding Silicon to the Aluminum will reduce
piston expansion caused by the extreme heat that is present within an
engine. This reduction in thermal expansion reduced piston seizures.
Silicone also adds strength to the aluminum and reduces wear.
Pistons are often though of as being a perfectly
round cylinder. In actuality, pistons are tapered from the top to
the bottom. Why?... Well, different parts of the piston are subject to
different levels of heat and since heat tends to expand metals, then it
stands to reason, that the areas which are subjected to more heat will
expand more then the areas that are subjected to less heat. The top of
the piston is obviously subjected to the most amount of heat, as well as
pressure, and therefore; will expand more.
Pistons are not only tapered, but oval ground at the
skirt. Once again, this is to accommodate the different temperatures
that are present on different parts of the piston.
So, when measuring a piston for wear, one needs to measure on the skirt faces at the widest point
which is always below the wrist pin. Measuring anywhere, but the widest point will give an inaccurate measurement.