E-Mail: aavolyn@bellatlantic.net

How many times have we heard the question, "What side clearance should
our piston or packing rings have? This is not a question to be taken lightly. For any piston
ring, rider ring, or mechanical packing ring to function effectively, it must be able to move
freely in its groove. To correctly provide for this clearance, two factors must be taken into
consideration, as follows:

a. The expansion rate of the material.
b. The clearance for the ring to float when fully expanded at operation
temperatures.
With the introduction of thermoset plastics (bakelite, micarta) and the thermoplastics
(TeflonŽ, Nylon, Ryton) expansion rates jumped dramatically. Now we have to be concerned
with anywhere from .010 to .020 per inch coefficients of expansion. Also, the amount of
temperature rise in some materials even affects this coefficient.

Some materials have different expansion rates in one direction versus another. Because of
the way the thermoset plastics are formed, there is a greater expansion rate perpendicular to
the laminated layers than parallel to them. TeflonŽ materials are affected by the direction in
which the material is molded. For packing ring installation, you must allow an additional .004
to .005 side clearance after the rings are fully expanded due to temperature. This allows the
ring to float freely.

MATERIAL SIDE CLEARANCES

BRONZE .0015/Inch
TEFLON .020-.022/Inch
CARBON .001-.002 Total
MOLIPAK .015/Inch (Side Clearance)
.005-.007/Inch (End Gap)

616 ORANGE ST.

P.O. BOX 1097

MILLVILLE, NJ 08332

Telephone: 856-327-8040

Fax: 856-327-9595

E-Mail: aavolyn@bellatlantic.net