Hitachi NR90GC - 3-1/2" Gas Powered Clipped Head Framing Nailer Technical Data And Service Manual page 46

(1) Keep pressing the side of the housing.
(2) Push the tip of the cylinder.
Output section
Fig. 34
Cylinder Ass'y [42]
Piston [39]
Small flat-blade screwdriver
Piston Ring [38]
Piston [39]
Push in.
Push in.
Fig. 35
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