Hangar 9 Twin Otter ARF Assembly Manual page 23

Step 7
Use a ruler to measure the vertical centerline of the
firewall and mark it with a felt-tipped pen. Measure
down 1 9/32 inch from the top of the firewall and mark
a horizontal line.
Step 8
Use a drill motor and 5/16-inch drill bit to drill a hole
5/32 inch each side of the centerline.
Twin Otter ARF Assembly Manual
Step 9
Use a round file to remove the material between the
holes, creating a slot measuring 5/16 x 5/8 inches.
Step 10
Place a #6 steel washer onto each of the 6-32 Allen
head machine screws. Insert the screws and washers
in the engine mounts, noting that the modified screw
locates in the lower of the two holes under the beveled
edge of the right-hand mount.
Step 11
Apply a drop of threadlock to each of the screws then
use a 7/64-inch hex wrench to secure the mounts to
the firewall.
Step 12
Perform steps 1 through 7 beginning on page 20 to
install the cowling on the nacelle. Set the front face of
the cowl 3 7/16 inches from the firewall when taping it
in place to drill the mounting holes in the nacelle.
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