3W state in their instructions that the engine must not be
mounted directly to metal stand-offs, as unequal lengths, or
unequal tightening of the bolts could cause crankcase dis-
tortion. Therefore we made a mounting plate from 2 layers
of 3mm carbon-composite plywood, glued together with
24hr laminating epoxy, and bolted the engine to this with
M5 bolts, washers and lock-nuts. Then the carbon-ply
mounting plate was secured to the firewall using 4 stand-
offs of exactly 30mm length, with the M6 bolts and washers
included in the hardware.
As it is a single-cylinder engine we chose to use anti-vibra-
tion mounts in the carbon-plywood mounting plate, made
from the usual ‘well-nuts’ (or short lengths of hard silicone
or rubber tube) to isolate the mount from the firewall, and it
certainly does help to prevent transmission of vibrations to
the airframe.
Four 9.5mm diameter holes were drilled in the carbon plate
for the M6 bolts, and the well-nuts were inserted from the
front face, with the excess length trimmed flush with the
back surface. The M6 bolts should be a fairly tight fit inside
the well-nuts or tubes used. If you use short lengths of hard
rubber or silicone tube, instead of the well-nuts, then you
will need to add a rubber washer on the front face, instead
of the flange on the well-nuts. Rubber washers can be eas-
ily cut from motorcycle tire inner tube with a sharpened
brass tube. An M6 steel washer is used on the front face,
under the head of the M6 bolt.
On the back face, between the carbon/ply plate and the alu-
minum standoffs, another 16mm (5/8”) Ø rubber washer
was added. Use Loctite on the M6 bolts that pass thru’ the
isolation mounts in the carbon plate to secure them firmly
into the stand-offs - and be careful not to tighten the bolts
up so much that you completely compress the rubber and
eliminate the anti-vibration system! The photo (above)
shows the order of assembly of the parts.
You need to cut a hole in the firewall for the rear-mounted
3W carburettor, and the 3W instructions include a full-scale
drawing of the engine mount positions and carburettor
which is very useful when making the mounting plate and
carb cut-out.
We highly recommend that you fuel-proof all bare wood
inside the motordome/tank base area with a thin coat of
laminating resin, as these carbs can spray a thin mist of gas
(petrol) from the venturi.
The header/manifold was custom-made to suit this installation, but it is quite long, which meant
that we needed to shorten the entry into the Greve tuned pipe by 45mm to correct the tuned
length.
Composite-ARF Extra 300SX
(2.6m span)
25
(above) 3W 80cc Xi motor
installed, with custom header and
Greve tuned pipe. Note position
of Ignition unit, and cable -ties
which secure the 2 receiver Nicads
inside the motordome.
(above) Shows the order of assem-
bly of the anti-vibration mounts in
the carbon-ply mounting plate.
(below) Standoffs are bolted thru’
the firewall with the supplied M6 x
20mm allen bolts & washers. M6 T-
nuts are also included in the hard-
ware, which can be used with
some engine mount types.