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between low-tension and ordinary piston rings?
tension rings are thinner and exert less pressure against cylinder walls
than conventional rings. This reduces friction, improving fuel economy
and cylinder sealing. Low tension rings are used in most engines today.
tension is described two ways. One is tangential tension, which is the
amount of force needed to squeeze the ends of the ring together. The
other is unit pressure, or the amount of pressure exerted by the face
of the ring against the cylinder wall.
the 70s, conventional piston rings had tangential tensions of up to
30 pounds. Compression ring tension specs for a Ford 302 V-8 used to
be 22 to 26 pounds. It is 14 to 16 pounds on later model versions of
the same engine. On some applications today, compression rings are rated
at 5 to 7 pounds.
amount of force the ring exerts against the cylinder wall (unit pressure)
depends on tangential tension as well as ring thickness and cylinder
bore diameter. Conventional oil rings exert pressures in the range of
180 to 240 psi. Low tension rings fall in the 90 to 160 psi range.
aftermarket low tension rings have a somewhat higher tension than the
OE rings they replace. If an OE ring specification calls for 6 to 12
pounds, an aftermarket ring may have as much as 12 to 16 pounds. Higher
tension is needed because rings are often installed in oversized cylinders.
Cylinder bores may also have more distortion than a new engine, so extra
loading improves sealing.
rings require rounder cylinder bores, which may require the use of torque
plates when honing certain engines. When heads are torqued, cylinder
bores can distort up to 0.0015" or more near the bolt holes, throwing
cylinders out of round.
obviously makes it more difficult for rings to seal properly. Simulating
bore distortion by bolting a torque plate to the block allows cylinders
to be honed so they will maintain their shape when the engine is assembled.
is essential that correct replacement rings be used. Conventional rings
designed for standard grooves must not be used in shallow groove pistons
designed for low- tension rings. Narrow, low-tension rings must not
be used in deep groove pistons.