Hangar 9 Twin Otter ARF Assembly Manual page 22

Step 1
Position the firewall drilling template against the
firewall and flush with its edges. Tape it in place with
low-tack tape then use a felt-tipped pen to mark the
2-stroke engine mounting holes.
Step 2
Remove the template and use a 1/16-inch drill bit in a
pin vise to pilot drill the mounting holes.
Note: When drilling the upper hole use care not to drill
too far beyond the firewall and into the fiberglass of
the nacelle.
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Step 3
Use an 11/64-inch drill bit in a drill motor to enlarge
the mounting holes.
Step 4
Use a cutoff wheel in a rotary tool to shorten one of
the 6-32 Allen head machine screws to a length of 3/4
inches. Also modify one 6-32 blind nut by removing
one side of the flange as shown.
Step 5
Press the blind nuts into the mounting holes from
inside the nacelle. The modified blind nut is installed
in the left mounting hole against the side of the motor
box. Use the 6-32 x 3/4-inch screw with a #6 washer
and engine mount to draw the blind nuts into place
with a 7/64-inch hex wrench.
Step 6
Measure 13/16 inch (20mm) across from the lower
left-hand engine mount hole and drill a 1/8-inch hole
for the throttle pushrod.
Twin Otter ARF Assembly Manual