When asked if he wants a piston
ring set with moly, chrome, or cast iron faced top compression
rings the mechanic often returns the question to the
jobber salesman by saying, "What should I use?"
There is some confusion in the
trade as to what type ring set should be used. Hastings
would like to offer some suggestions to help the consumer
make the proper decision for his particular application.
The single most important factor
to be considered in selecting the proper top compression
ring face coating material is the service requirements
the engine will be operated under. Will the engine be
subjected to unusual speed or load operation, stop and
go - short trip driving, or operating in a high dust
or dirt environment? if, for example, the vehicle is
a passenger car operated by family members for what
could be termed the average driver, it really doesn't
matter which type is selected from a standpoint of the
life of the engine and piston rings. On the other hand
if one of the above mentioned conditions is going to
exist on a regular basis then no doubt one type of ring
face coating will be more appropriate than the others.
The three popular types of top
compression ring face coatings, chrome, moly, and cast
iron, each has advantages of its own with respect to
operating conditions. Moly has a very high resistance
to scuff. Chrome has good resistance to scuff but does
not exhibit moly's oil retention capabilities. Plain
cast iron is a durable wear surface in normal operating
conditions and is less costly than the moly or chrome
For typical light duty service
where the vehicle Is not subjected to long periods of
high speed or load operation and is run primarily on
paved streets, plain cast iron is a good choice because
piston ring cast iron Is very durable when not subjected
to unusual dirt or heat conditions.
When faced with continuous high
speed or severe load conditions, the engine will be
subjected to long periods of high temperatures. Moly
Is then a good choice because of its scuff resistance.
Moly, which is an acronym for molybdenum, inherently
is quite porous in its applied state which results in
excellent retention of oil in the face of the ring.
Moly also has the highest melting point of the three
popular face coatings which results in its capability
to live better under more severe operating conditions,
or more specifically to resist scuffing and scoring.
In a dusty environment such as
gravel pits, sand or rock mines, or operating on a dirt
or unpaved roads, chrome is the best choice. As mentioned
earlier, moly because of Its porosity holds oil on the
O.D. face of the ring which helps inhibit scuffing.
Pores on the material also can serve as a trap for foreign
materials, however. Because of the incoming air/fuel
mixture probably will contain some abrasive contaminant
In a dusty environment, chrome with its smoother O.D.
surface is a logical choice. Chrome's extreme density
and hardness resists the Impingement of dirt Into the
face of the ring which accelerates cylinder wear, and
actually contributes to the exhaust gases carrying some
of the airborne contaminant out through the exhaust
system. Chrome has more resistance to scuffing and scoring
than cast iron but somewhat less than moly.
All in-all when the engine experiences
normal driving conditions and is properly maintained
with respect to oil and air filter changes, any of the
three coatings functions equally well. It is the installer's
expert judgement in analyzing the primary use of the
engine that should lead him in a direction of which
ring set will be the best possible choice for that particular
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