GREAT PLANES Piper J-3 Cub Instruction Manual page 7

❏ 2. Extend the marks on the firewall to locate the position
for the engine mount on the firewall.
❏ 3. Cut the Engine Mount Template from the back cover
of the manual and tape it to the firewall as shown. At the
locations on the template, drill four 7/32" [5.6mm] holes in
the firewall for the engine mount blind nuts.
❏ 4. Install four 8-32 Blind Nuts to the inside of the firewall.
Pull the blind nuts into the back side of the firewall using
8-32 Socket Head Cap Screws with a flat washer under
the head of the screws. Fit the two halves of the Engine
Mount together. Use four #8 Flat Washers and four 8-32 x
1" Socket Head Cap Screws to secure the engine mount to
the firewall. Do not tighten the screws at this time, as the
mount must be adjusted for the engine. Depending on your
engine selection, it may be necessary to trim the top center
stringer to allow for positioning the engine mount.
❏ 5. Test fit your engine into the mount. Adjust the width of
the rails to fit the engine snugly. Tighten the mount screws
to allow marking the engine mounting holes without moving
the rails.
❏ 6. Position the engine on the engine mount rails so the
propeller thrust washer is 5-1/4" [133mm] ahead of the
firewall. Use a Great Planes Dead Center Hole Locator
(GPMR8130) (not included) or a sharpened piece of wire to
scribe the four engine mount holes onto the rails. Use a
center punch at the marks to prevent the drill bit from
wandering, then drill #29 pilot holes through the rails. Be
sure to hold the drill perpendicular to the rails. If you have
access to a drill press, this is a good tool for this purpose.
Use an 8-32 tap to tap the holes for the 8-32 screws. Use
four 8-32 x 1" socket head cap screws to secure the engine
to the mount.
❏ 7. Drill a 3/16" [4.7mm] hole in the firewall for the throttle
pushrod. The hole location will depend on whether you are
installing a 2-stroke or 4-stroke engine.
❏ 8. Roughen the outside surface of the 11-3/4" [298mm]
Throttle Pushrod Tube with coarse grit sandpaper. Insert
the pushrod tube through the hole in the firewall. Push it in
until it is flush with the front of the firewall. Use medium CA
to glue the tube to the firewall, but leave it free inside the
fuselage until the servos are installed.
Note: The engine has been removed from the above picture
for clarity.
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