Hangar 9 P-51D Mustang Sport 40 ARF Assembly Manual page 43

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Step 6
Use a hobby knife with a #11 blade to remove the balsa for
the retract strut.

Step 7
At this time you will need to fuel-proof the area in the wing
for the retract. Use either a mixture of 30-minute epoxy
thinned with isopropyl alcohol or a fuel-proof gray model
paint to do so. Once everything has cured, use a trim seal
tool to neatly iron the covering back on the wing and into the
wheel well as shown.
Hangar 9 P-51D Mustang Sport 40 Assembly Manual

Step 8
Cut a 6-inch (152mm) piece of air line and attach it to the
retract air cylinder as shown.

Step 9
Install the retract mechanism in the wing using four 6-32 x
1/2-inch socket head screws and four #6 washers. Tighten
the screws using a 9/64-inch hex wrench or ball driver. Make
sure to use threadlock on the screws so they don't vibrate
loose in flight.

Step 10
The air line will now go through the small hole at the root
of the wing.

Step 11
Install an air line connector on the air line. Make sure this
connector will mate to the one at the fuselage as they are
difficult to remove once pressed on the air line.
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