When a liner and piston
rings have run in, the asperities, or high spots have been worn
off the rubbing surfaces, and ideally, what is left is a
microscopic corrugated surface which will hold the lubricating
oil. Combustion, especially of residual fuels, will produce
carbon which tends to build up around the topland of the piston.
As the carbon builds up, it will eventually touch the liner
surface, removing the lub oil film, and, because the carbon is
abrasive, it will polish the liner surface, removing the
microscopic corrugations, and giving it a highly glazed surface
to which a lub oil film cannot adhere. Carbon build up can be so
severe that it jams the top ring in its groove. The dual action of
further carbon abrasion, and unlubricated contact between rings
and liner leading to microsiezure will accelerate the wear rate
of the liner leading to blow by, loss of power, possible seizure
and danger of crankcase explosion.
The anti polishing ring has a slightly
smaller diameter bore than the liner in which it is
located, and is slightly larger than the top of
the piston. When the piston is on top dead centre the
bottom of the anti-polishing ring is just above the top
surface of the top piston ring.
The anti polishing ring is a clearance fit in a step
in the top of the liner, and can be replaced when it
wears. On some engines it incorporates a cooling space,
and so the anti-polishing ring (or fireband) is
fitted with O rings for sealing.
As the piston
reciprocates in the cylinder, any carbon build up forming on the
top of the piston will be removed by the anti polishing ring.
This ensures a good lubricant film on the liner surface,
reducing wear and giving a more consistent lub. oil consumption.