O.S. GF- 40 Close-Up At a time when several of the model engine manufacturers are scaling back production due to the slow economy, O. S. keeps turning out new engine designs almost faster than we can bring them to our readers' attention. This month, we will take a look at their latest “gas burner,” the O. S. GF- 40. This will be the fifth engine in the O. S. gasoline-engine lineup. Others include three fixed-wing aircraft engines and the GT15HZ, . 92 cu. in. helicopter engine. Where the previous engines were of 2-stroke design, the new GF- 40 is a 4-stroke. Specifications include a bore of 1.575 inches ( 40.0mm) and a stroke of 1.252 inches ( 31.8mm), making the engine of “oversquare” design with a displacement of 2.439ci ( 39.96cc). The engine develops a whopping 3.75hp at 8,600 rpm and has a bare weight of 41. 27 ounces. The muffler adds 4 ounces and the ignition module another 3. 35 ounces. Prop sizes range from 18x8 through 18x12 for sport flying and through 20x8 or 20x10 for scale with an 18x8 propeller recommended for break-in. The engine utilizes a single ring, aluminum piston running in a chrome-plated, aluminum cylinder, i.e., no separate steel or brass sleeve, and it is equipped with a specially designed Walbro pump carburetor. An unusual feature is the means of crankcase ventilation, which O. S. terms PCV (Positive Crankcase Ventilation). A term coined by the automobile industry for our smog engine crankcase ventilation systems, i.e., the PCV valve. The O.S. system now uses a one-way valve in the crankcase that opens on the upstroke of the piston drawing fresh air into the crankcase. On the down stroke, the valve closes and the air is forced through the hollow-drilled crankshaft and tangent hole into the camshaft area. The flow then goes up through the pushrod tubes into the valve chamber and on into the combustion chamber via holes surrounding the intake valve—a rather “tricky” way
of increasing lubrication to the valve train area and preventing excess
crankcase oil from blowing out the crankcase vent. Needless to say,
keeping the pushrod tube O-rings and valve chamber gasket in good
shape is critical.
attaching it to a pressure tap on the Pitts
muffler solve this problem? All my model
engine experience consists of .45- to .60-
size displacement engines. I have always
used muffler pressure on the vent line. I
am looking forward to your reply. I have
enjoyed reading your column over many
years and will continue to do so. Your work
is greatly appreciated.
Martin Prestigiacomo
Turning the other way allows fuel to go
from the tank to the carburetor. In the case
of the Sullivan Fuel Filler Valve (pictured),
this is a push-pull operation.
Your engine has a pump carburetor and
the vent line must be left open. Otherwise,
a vacuum would be created in the tank and
the engine could not draw fuel. There is no
need for muffler pressure.
O.S. engine dilemmaS
U My older O.S. Surpass with pump runs
but starves for fuel at max rpms after a few
seconds (occasionally it runs without stalling). The mixture seems correct, but I suspect the pump is having issues. Do I dare
disassemble the pump to try a fix? Also, I
have an O.S. FSR . 61 circa 1983, which has
been flawless until lately. It now surges at
half-throttle and won’t die or slow down
even with the throttle completely closed.
Any help would be appreciated.
Raymond Johanson,
Fort Ripley, MN
Perfect for giant-size airplanes, the new Fuel Filler Valve from Sullivan Products works great and is easy to install. The red portion pushes and pulls in and out for fueling and defueling.
Martin, it is very important
that the baffles be installed
for proper cooling, i.e., they
direct the incoming air over
and through the cylinder fins.
Without them, the air just
passes through the cowl.
Remove the baffles
from the replacement
cowl, or make new
ones, and install in
the original cowl.
All you have to do
to solve your tank-filling problem is to
install one of the commercially available refueling valves in the carburetor fuel
line. Turning one way shuts off the carburetor, allowing fuel to fill the fuel tank.
Raymond, your first problem
could possibly be the pump,
which you definitely do
not want to disassemble.
Once the seals are broken,
replacement parts are not
available. However, your
problem is more than
likely due to fuel foaming. Be sure your fuel
tank is soft mounted, i.e.,
wrapped in foam rubber,
and the propeller and
