ARB RD40 - Manual (Page 22)

3 Installing the Air Locker
20
From the outside of the housing, assemble the small O-ring over
the top of the short length of seal housing tube protruding through
the bulkhead fitting.
While holding the seal housing tube into the bulkhead fitting, insert
the small drilled end of the center compression nut over the
extended tube as shown in the assembly diagram (Fig.14.), and
screw it into the bulkhead body, and lightly tighten.
NOTE : Be sure to insert the correct end of the center
compression nut into the bulkhead body. The thread
has been partially relieved on the bulkhead side of the
center compression nut to visually identify its
orientation. (Fig.14.)
Figure 14.
NOTE : Excessive tightening of the center compression nut is
not necessary to form a good seal around the tube
and may damage the O-ring, the seal housing tube, or
the threads of the compression nut.
NOTE : Make sure the seal housing tube is all of the way into
the center compression nut while you are tightening
it.