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What generally causes a piston to not
operate freely is either a dent in the valve casing (usually
caused by a sharp object striking the outside of the casing) or
a hard blow to some part of the instrument that may cause either
distortion around the ports or a brace to push into the casing
wall. If no damage has been done to the casing, then the piston
itself is probably bent or damaged in some way.
There are various methods employed in
the industry for correcting these problems and as a general rule
the best of these methods adhere to one very important
principle: the less metal removed from either the casing or the
piston the better.
If the casing is dented attempts should
be made to safely raise or knock the dent out. Straightening of
slide pipes may help to true up a distorted port. Bent or
damaged pistons must be straightened, repaired or replaced. Only
after these avenues have been thoroughly pursued should any
cutting of the metal be done. If the piston is close to
performing properly, lapping compound should be used to remove
the remaining amount of metal at the trouble area.
If more aggressive cutting needs to be
done scrapers or reamers are generally used. The cautious
technician, however, will cut or scrape away only enough metal
to get the piston working close and then remove the last bit of
metal with lapping compound. The actual lapping itself is an
easy process and will provide truly professional results.
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