Hitachi NR90GC - 3-1/2" Gas Powered Clipped Head Framing Nailer Technical Data And Service Manual page 49

Exhausting
direction
Exhausting
direction
Fig. 38
Push lightly.
Piston [39]
Chamfered side of
the driver blade
Fig. 39
Split of the Piston Ring [38]
Split of the Piston Ring [38]
Small flat-blade screwdriver
Push lightly.
Piston Ring [38]
Piston [39]
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Cylinder Ass'y [42]