Hitachi NR90GC - 3-1/2" Gas Powered Clipped Head Framing Nailer Technical Data And Service Manual page 47

Remove the Retaining Ring [37] with the retaining ring puller for C-type hole. Insert a finger into the outlet of
the Cylinder Ass'y [42] and push the tip of the Piston [39] straight up. Then the Piston [39] can be removed
from the Cylinder Ass'y [42]. If the Piston Ring [38] fits in the retaining ring groove inside the cylinder, insert a
small flat-blade screwdriver into the concave portion of the cylinder and fit the Piston Ring [38] in the piston
ring groove of the Piston [39]. Then the Piston Ring [38] can be removed by pushing the Piston [39] up
(Fig. 35).
Remove the two Piston Rings [38] from the Piston [39] being careful not to deform them.
Screw a bolt having 20 mm or more threaded portion (for example, Hex. Socket Hd. Bolt M5 x 20 [13]) in the
hole beside the outlet of the cylinder or insert a roll pin puller in the hole and tap it lightly with a hammer to
remove the Piston Bumper [40] (Fig. 36).
Cylinder Ass'y [42]
Piston Bumper [40]
Bolt having 20 mm or
more threaded portion
Fig. 36
--- 44 ---
Roll pin puller