Hangar 9 P-51 Mustang 1.50 ARF Assembly Manual page 43

Step 7
Carefully insert the stopper assembly into the fuel tank.
Note the position of the vent tube; it must be up at the top
portion of the fuel tank to function properly. Also, it may
be necessary to shorten the length of the fuel pickup
tubing to make sure the clunk does not rub against the
back of the fuel tank. You should be able to turn the tank
to any attitude and the clunk will fall to the lowest point.
(All directions except for having the stopper facing down.)
Note: If you are planning on installing the
Saito 200TI, the fuel tank will be oriented
differently. Make sure the tank is built so the
vent is pointing up with the fuel tank oriented
as shown in the picture.
Section 16: Fuel Tank Assembly
Step 8
Tighten the M3 x 20 screw carefully—do not over tighten.
This allows the rubber stopper to form a seal by being
slightly compressed, thus sealing the fuel tank opening.
Important: Be sure to differentiate between
the vent and fuel pickup tube. Once the tank is
mounted inside the fuselage, it will be difficult
to tell the tubes apart.
Note: The fuel tank will be installed in
a later section.
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