Hitachi NR90GC - 3-1/2" Gas Powered Clipped Head Framing Nailer Technical Data And Service Manual page 50

When mounting the Retaining Ring [37] to the Cylinder Ass'y [42], fit the Retaining Ring [37] in the groove
securely facing the split to the no-concave portion of the cylinder. Face the sagged side of the Retaining Ring
[37] to the Piston [39] (Figs. 40 and 41).
Exhausting
direction
Retaining Ring [37]
Piston [39]
Retaining Ring [37]
Face the split of the Retaining Ring [37] to the
no-concave portion of the Cylinder Ass'y [42].
Fig. 40
Cylinder Ass'y [42]
Face the sagged side of the Retaining
Ring [37] to the Piston [39].
Fig. 41
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Cylinder Ass'y [42]
Concave portion (7 places)