Hitachi NR90GC - 3-1/2" Gas Powered Clipped Head Framing Nailer Technical Data And Service Manual page 48

(2) Reassembly
Reassembly procedures should be followed in the reverse order. Note the following points.
Drop the Piston Bumper [40] in the Cylinder Ass'y [42] facing its stepped side inward and push it in until it
passes through the stopper portion by means of the handle of a hammer. You feel a click when the Piston
Bumper [40] passes through the stopper portion (Fig. 37).
Apply designated oil to the inside of the Cylinder Ass'y [42] and the O-ring groove on the outside of the
Cylinder Ass'y [42] according to 10-1.
Mount the two Piston Rings [38] to the Piston [39] being careful not to deform them.
Be careful of the following when mounting the Piston [39] to the Cylinder Ass'y [42].
(1) Position each split of the two Piston Rings [38] at an angle of 90 degrees with respect to the exhaust
port aligning these two splits in 180-degree opposite positions (Fig. 38).
(2) If the Piston Ring [38] fits in the retaining ring groove inside the cylinder, insert a small flat-blade
screwdriver into the concave portion (7 places) at the end of the cylinder and lightly push the Piston [39] in
fitting the Piston Ring [38] in the groove of the Piston [39]. Do not push the Piston [39] in forcedly.
Otherwise, the Piston Ring [38] may be damaged (Fig. 39).
(3) Check that the tip of the driver blade is shown in the outlet. The Piston [39] cannot be mounted if the tip of
the driver blade contacts the Piston Bumper [40] or the bottom of the cylinder.
(4) Face the chamfered side of the tip of the driver blade in the direction opposite to the exhaust port of the
Cylinder Ass'y [42] (Fig. 39).
Handle of a hammer
Cylinder Ass'y [42]
Stopper portion
Piston Bumper [40]
Fig. 37
--- 45 ---