PHQ250JHMAVL Jackleg MK 1 Parts and Repair Manual PDF

Front Cylinder Assembly Inspection
Page 23
63. The clearance between the outer diameter of a new piston stem and the inner diameter of the
bronze front cylinder washer liner (C1517) is nominally 0.047mm (0.0015”). The clearance
between a worn piston stem and a worn bronze cylinder liner bore is 0.160mm (0.0063”) or
larger. If the cylinder liner does not visibly appear to be worn it still should be tested with the
piston (B2334) installed in the normal operating position in the cylinder bore with the stem
through the cylinder liner.
64. To measure the compression fit of the piston and cylinder
liner place the drill cylinder body flat on the bench and
advance the piston fully through the liner. With one hand
gripping the spline end of the piston and the other the front
part of the cylinder body push the piston back into the drill as
far as it will go then pull it rapidly forward until it stops on the
cushion of air above the liner.
Hold a thumb against the
splines of the piston and
slowly pull the piston out
of the cylinder. One should
be able to pull the piston
face approximately three
quarters of an inch further
through the cylinder.
65. An alternate method to measure the compression fit of the piston and bronze cylinder washer
liner is to hold the cylinder in a vertical position and push the piston up from underneath to let it
free fall down into the cylinder. The piston should bounce and then move slowly to the bottom of
the bore against the cushion of air.
66. A good air cushion prevents the piston from bottoming too hard on the cylinder liner during
the down stroke and provides a bounce to start the piston on its backward stroke during operation.
If the piston head has proper clearance with the cylinder bore and there is no bounce of the piston
on a cushion of air during the test then the bronze front cylinder washer liner (C1517) must be
changed to improve compression.
NOTE: It is very important that the distinct “pop pop” sound of a good air cushion is heard
during this test, not the jarring “clank” sound of metal on metal impacting of the two parts.
Version 200711