The Engine Shop - 2011/06

June 2011 77
Eric Henderson | The Engine Shop [email protected]
A brief introduction
Left: The new O.S. FS-95V is a superb piece of aluminum casting
and sports a two-stroke-style muffler.
Below: All the parts that make this four-stroke engine, minus the
screws and bolts for more clarity.
Left below: The inlet and outlet valves are the same size and
are recessed in the head.
Also included in this column:
• Review of the O.S. FS-95V
four-stroke
(Editor’s note: Health issues have
necessitated a change of writers for this
column. The MA staff appreciates Joe
Wagner’s contributions to the magazine and
wishes him well. Eric Henderson has
stepped in to write “The Engine Shop” and
continue to keep readers up to speed on all
things engine related.)
FIRST, MY BEST wishes go out to Joe
Wagner and his family. I hope that Joe has a
speedy recovery and is soon back to his
former self.
Second, a quick personal introduction.
Engines and I go back to my preteens, when
I was flipping diesel engines to get my CL
airplanes to fly.
Then, after a brief foray into 500cc
British four-stroke motorcycle racing
engines with aluminum castings and
cylinder heads, I was back with my first
Merco .61 glow engine. That was soon
followed by the early YS 60 short-stroke,
pressure-regulated, two-stroke, tuned-pipe
screamer.
That was followed by a long procession
of O.S., Rossi, Moki, YS, Magnum, ASP,
Enya two- and 4-stroke power plants, to
name a few. They found their way into my
airplanes, cars, and boats, and they ranged
in sophistication from no-throttle CL
engines to the electronic-fuel-injected,
supercharged four-stroke variety. Add a
string of Desert Aircraft, 3W, and RCS
gas/ignition singles and twins, and you have
my model-engine résumé fairly wrapped up.
Next, the phone rang from Muncie, and
here I am in print again with MA, reviewing
and writing about my favorite area of the
hobby!
In this column I have the O.S. FS-95V
glow engine for review. The company
indicates that it is the first in a new line of
four-strokes, which prompted a much closer
look at all the “bits” to see if O.S. had
changed things. When an engine is the first
of a line, you often find new engineering
that will be carried forward in future
releases.
There is an obvious major external
change. Gone are the single rocker-box
cover and parallel pushrods. In their place
are two pushrods that spread out toward
individual black covers.
This configuration clearly indicates that
both valves are now also at an angle in the
head. The pushrod protectors are positioned
in a “V,” to line up with new cylinder head
design.
It’s reasonable to assume that the “V”
designation was derived from this feature
and could be a signature of future designs
and releases. It would normally be better to
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asymmetrical. The locations of the ports and
glow plug leave only a few places where the
bolts could go.
O.S. is clearly confident that this works.
My only comment is that one has to be
judicious when retightening the cylinder
head.
For cylinder head tightening on a model
engine, using an inch-ounce torque driver is
mandatory in my book. At first each bolt
should be tightened lightly and equally. This
translates to slight turn at a time on the four
bolts until you experience some resistance.
Then, and only then, hit them with the full
force of the torque wrench.
The angled valves can be opened with
pressure from my finger. It was reassuring
that they could not touch each other, even at
well above maximum opening settings.
One head-design item puzzled me. The
angle of the valve does not match the angle
of the pushrod to the rocker arm; the valves
are angled more. Therefore, neither pushrod,
when viewed from the front, is going to be
directly in line with its lifting action. The
valve-lifter end is square with the top of the
valve itself.
The purist in me
was uncomfortable
with the alignment,
but I could not see
why it would not
work. Call it an
aesthetics fetish if
you like, but time
will tell on this one!
While in the
“puzzled” zone, the
muffler and the
crankcase have
pressure and vent
nipples precast in
the mold. No
longer evident are
the brass screw-in
nipples that we
have grown to
know and love.
This is a brilliant
casting
achievement on
O.S.’s part. I
hastily add that the
78 MODEL AVIATION
but I’m writing this in winter, and until the
rain stops ...
You need only three tools to take apart
this engine and four to put it back together:
a 2mm and a 3mm Allen key for the bolts
and a Japanese Phillips-head screwdriver for
disassembly. The other tool I strongly
recommend for reassembly is an inch-ounce
torque driver, to correctly tighten the head
and crankcase bolts.
The carburetor comes off first, with a
screwdriver. Find the right fit to the
Phillips-type head, because these screws
chew up too readily if the head is not a good
fit. (I have surmised, for a while, that O.S.
does not want you to overtighten these
screws, because they might distort the
carburetor body or snap the bolts if you
tighten them with an Allen wrench.)
It is worth noting/saluting that the
carburetor can be rotated 180°, to allow the
throttle arm and main needle assembly to be
located on either side of the FS-95V. In
earlier engines you could find yourself in a
situation in which the throttle pushrod
wanted to go through the center of the tank.
Problem solved!
The inlet manifold simply pushes into an
O-ring seal in the carburetor. Four short
3mm screws hold the backplate in place.
There is no gasket or O-ring, as would be
found on most two-strokes. There is no need
for a perfect seal, because the lower
crankcase area is not under much pressure
during operation. O.S. has done a super job
of machining and mating these surfaces.
The propeller end of the crankshaft does
not use the more traditional Woodruff key.
Instead, a flat area is ground on one side of
the shaft to accept a drive washer with a
matching center cutout. Two propeller
retaining nuts are supplied; they are cut so
that they jam against each other, to prevent
the propeller from flying off inadvertently.
The valve covers, aka rocker-covers, are
held in place with two 1.5mm bolts. The
inlet header pipe and valve covers are
machined to such a fine finish that no
gaskets are used.
The cylinder head is held on with only
four bolts that reach well down into the
crankcase. Not only is this a four-bolt head,
but the distribution of the bolts is
camshaft casing is also extremely thin and
of excellent caliber.
What concerned me was that the fuel
tubing might not want to stay on a nipple that
has no increase or change in width near the
end. I understand that it could not be cast that
way, but silicone fuel tubing loves to come
off when it get oily or old.
The crankcase vent line is noncritical
other than the inconvenient mess if it fails.
The muffler exhaust/pressure feed to the tank
is much more important for successful
operation. (During testing the pressure and
crankcase vent lines stayed firmly in place. It
helps if you clean the metal with alcohol
first.)
The engine uses a 16002 main ball
bearing and the same front thrust bearings as
many of the current O.S. engines do. There
are also two small bearings that support the
camshaft. Experience dictates that it would be
good practice to use the crankcase vent as an
after-run oiling port, to prevent these bearings
from rusting in more humid climes.
During normal running, the oil that
The two-needle carburetor can be positioned with the
throttle linkage on either side of the FS-95V.
O.S. uses a one-piece cam unit with a
healthy cam lift and a good, long
dwell of the open valve to get a
strong-running engine.
The FS-95V features a lightweight
ringed piston with full walls
that prevent piston rock at high
rpm.
The plastic bag contains a battery, a switch, an Spektrum AR600
receiver, and a TM1100 telemetry module.
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blows by the piston will provide adequate
lubrication to the main bearings. The rear of
the crankshaft is hollow, and there is a
connecting hole in the shaft directly in front
of the helical cam drive gear.
When the piston moves on its down
stroke, it creates unwanted pressure in the
lower crankcase. The pressure is
momentarily relieved through the crankcase
vent nipple. A byproduct of this action is to
move oil through the system. This oil also
lubricates the cam gearing and the valve
lifting system.
The piston then moves up again, which
results in a pulsing of pressure. Without some
form of venting, the engine would run
roughly.
Judging from a visual check, the cam
profile has an aggressive look. The initial lift
is quick, and the opening dwell of the valve
has a “sporty” duration.
The FS-95V is clearly designed to be light
and powerful. The connecting rod looked
unique in that it had a phosphor bronze
bearing in the big end but nothing in the little
end. The wrist pin was not retained by
conventional circlips. Instead, it is allowed to
float in the piston and is restricted only by
two Teflon wrist pin end pieces.
The ringed piston is scalloped on the
inside but retains full outside walling. This is
great for piston wear and will prevent piston
rock that often plagues four-strokes. The ring
is free to rotate in the piston groove, and the
cylinder liner does not have to be aligned in
any particular way for reassembly.
Reassembly is fairly straightforward, but
take care to put everything back the way it
came apart. Marking the pushrods as left and
right will help you keep the original valve
clearances.
The camshaft has a timing dot that is
critical to line up correctly. The FS-95V’s
timing dot should be at the bottom of the
camshaft, while the piston is at top dead
center (TDC).
When refitting the cylinder head, you
must ensure that the copper head gasket does
not get misaligned and damaged. A drop of
oil will keep the gasket “stuck” to the head
during this process. Line up all parts and
lightly screw in the bolts.
Setting the piston to TDC on the
compression stroke takes the valve spring
pressure out of play while you fit the head.
With the asymmetrical bolt pattern, an inchounce
torque driver/wrench will reduce the
risk of distorting the cylinder head.
To test the engine, the last item I fitted
was the supplied-in-the-box type F glow
plug. I used Cool Power 20% nitro aircraft
blend fuel with 18% lubricant content and
Master Airscrew 12 x 6, 13 x 6, and 14 x 6
propellers. The FS-95V fits the same
mounting holes as the FS-91S II, 61FX,
65LA, 75AX, 95AX, and FS-110 engines,
which made it easy to fit to the test stand.
Making a departure from the more
traditional engine reviews, I configured the
test mount with a throttle servo, Spektrum
AR600 2.4 GHz receiver, and new
Spektrum TM1100 telemetry module. It
June 2011 79
took only a few minutes to bolt the rpm
pickup onto/into the backplate and add a
temperature sensor wire, which I simply
rigged as a noose to hold it against the
cylinder head. I wrapped the AR600 and
TM1100 telemetry bits in a plastic bag
to protect them from fuel and exhaust
contamination.
Telemetry allows you to read the
temperature and engine rpm from a safe
distance. The rpm reading on the
transmitter display screen updates
rapidly and accurately.
I carefully monitored the head
temperature changes, because this was a
new power plant being run hard on a
relatively cool day. I watched the head
temperature track the power requested
by the throttle stick. You could also see
the rpm go up as the engine was broken
in more and more.
It was a 38° day, with 15 mph winds
cooling everything. It was not only safer
doing the testing this way, but also
much more comfortable on a cold day. I
sat “remotely” in my vehicle most of the
time with the heater on.
The FS-95V fired up with the first
spin of the electric starter. The main
needle was initially set at three turns
out. There was never a need to touch the
low-end needle factory setting. I used
the 12 x 6 propeller first, to avoid
overloading moving parts during the
initial break-in process.
During this time, I recorded the rpm
relative to approximate throttle
positions. Most model engines have
neither a linear nor a directly
proportional response to the throttle
stick. The FS-95V had a reasonably
good rpm-to-stick position ratio.
With an idle start point of 2,210 rpm,
I saw 5,700 at the one-quarter position,
7,100 at the half position, 9,200 at the
three-quarters position, and 10,500 at full
throttle. A bit steep initially, but nothing
that a bit of throttle curve could not fix.
The response was consistently
smooth and rapid. There was a
perceived increase in exhaust note and a
power surge upon the application of full
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throttle.
O.S. has redesigned its Jet Stream
muffler with a new shape, to obtain a
better four-stroke sound. There was a
distinct “bark” when running at full bore,
but overall it was a pleasant sound that I
could talk over!
After using a full 24-ounce tank of
fuel, I could begin the rpm measurements.
My findings were as follows.
• 12 x 6 propeller: idle rpm, 2,600; fullthrottle
rpm, 11,700; head temperature,
97°
• 13 x 6 propeller: idle rpm, 2,510; fullthrottle
rpm, 11,300; head temperature,
102°
• 14 x 6 propeller: idle rpm, 2,210; fullthrottle
rpm, 10,700; head temperature,
109°
Please note that O.S. recommends only
13 x 7-9 to 15 x 6-8 propellers. The smaller
propellers should be used for break-in
only.
The test rpm figures were obviously
above the O.S.-specified practical rpm of
11,000. I recommend dropping to 15%
nitro or use a bigger propeller if the engine
revs too high for the valve train.
This engine design is 10 grams lighter
than the O.S. FS-91, which is all goodness
when it comes to that all-important powerto-
weight ratio. Getting 10,700 rpm with a
14 x 6 propeller on a nonpumped,
nonsupercharged power plant is wonderful.
This strong-performing four-stroke runs
smoothly and sounds great! Check the
website for pricing.
The O.S. FS-95V specifications are:
• Bore: 1.14 inches (29mm)
• Stroke: .93 inch (23.6mm)
• Volume: .95 cu. in. (15.59cc)
• Weight: 20.88 ounces (592 grams) MA
Sources:
O.S. Engines
(217) 398-8970
www.osengines.com
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