Two types of pistons have been used in 20 Series pumps and motors.
Prior to any piston replacement it is necessary to check the construction
of all pistons in the cylinder to determine if they are the welded steel or
plastic filled type. Replacement of plastic filled pistons with welded
pistons may be accomplished by following the procedure outlined below.
Plastic filled pistons may not be used to replace welded piston
When replacing plastic filled pistons with welded pistons during overhaul
or repair of a unit, it is mandatory that the welded pistons be
incorporated symmetrically in multiples of three (either three, six, or all
nine). See illustrations.
P = Plastic Filled
W = Welded
Do not replace welded piston assemblies with plastic filled piston
assemblies; however, any plastic filled piston assemblies that you have
in inventory can be used to replace existing plastic filled pistons that you
To install the cylinder block assembly, it is necessary to check the
alignment of certain parts. There is no special relationship of pistons to
bores, springs, etc.. However, the alignment of the ball guide and
cylinder block splines is critical. The undersized tooth in the spline of
the cylinder block must line up with the missing tooth in the ball guide