WYVERN Model Gas Engine
Brad E. Smith
7574 So. 74 Street, Franklin, Wisconsin 53132
I never met a gas engine that I didn't like. Some I just
like more than others, and this was one of them. Our hobby of
building model engines really began in England, where model steam
engines that were built in the 1700's exist in museums. At
about the turn of the century, an English magazine called Model
Engineer began publication. This magazine is still published today,
twice each month, and bound volumes can be viewed in some of the
larger libraries in the U.S. Plans for many steam, gas (petrol, as
the English call it), and hot air engines have been published
through the years in this magazine. One of the major designers of
model gas engines was Edgar T. Westbury, whose works were always
published in the Model Engineer. He designed racing engines, model
boat engines, and this model stationary engine. He modeled the
engine after a typical Crossley stationary engine, built in the
late 1880's that was used to power machinery in English
factories. While not an exact replica of a particular engine, it is
representative of the engines of this period.
The model has the looks of a very early gas engine, with such
interesting details as a side shaft to operate the two valve
rockers, a counterweighted crankshaft, thin section flywheels, and
turned (marine style) connecting rod. The engine has a 1.250'
bore, by a 2.000' stroke, making this a long stroke engine. An
early style head is used, again copied from a Crossley, called a
'clerestory' head, from the internal shape of the
combustion chamber. The Crossley Brothers found that the shape of
the combustion chamber, and the position of the valves, had an
important bearing on efficiency. The clerestory head became popular
at that time.
The WYVERN casting kit and drawings are available from Power
Model Supply Company, Rt. 1, Box 177, N.W. Cor. Hwy. 67 & Long
Road, DeSoto, Missouri 63020. The castings, made by Woking
Precision Models in England (actually Scotland) are of very good
quality. They are of iron, aluminum and bronze (called 'gun
metal' in England). The drawings are clear and correct. A
construction article, written by West-bury, and the drawings were
published in The Model Engineer in 1963. This is available with the
castings from Power Model Supply. Also included are the valve
springs, the skew gears, two piston rings, and a 10 mm spark plug.
I didn't use the spark plug provided because I prefer to use
the smaller ?'-32 size plug, as it looks more in proportion on
An aluminum cylinder head is provided. Here is a small problem.
The intake valve is designed as a removable bronze housing that
contains the valve seat. No problem here. But the exhaust valve
seat is ground into the head. Aluminum will not hold up as a valve
seat very well. There are two solutions to the problem. One is to
make a removable bronze valve housing, with seat, for both the
intake and the exhaust valves. The other is to get a cast iron
head. This is the route that I took, although either way will work.
A cast iron head for this engine is sold by Tom Alexander, Box 125,
Iowa Falls, Iowa 50126. Tom also sells a different base for the
engine, which has feet protruding from the lower rim, so that the
engine can be anchored easily to a piece of wood or metal. The base
provided does not have such, and therefore would be difficult to
anchor to anything.
The crankshaft on my engine is a one-piece crank, with the two
counterweights attached to the crank throws with socket head cap
screws, counterbored into the weights. I believe in a one-piece
crank for a gas engine as the surest way to be assured that the
crank will stay together. The article calls for brazing a
crankshaft from pieces. The choice is up to you, but I have found
that it takes no longer to turn a crank from a solid bar, than to
prepare the pieces, braze them together, and finish machine. And a
solid crank is such a masterpiece, that the job has its own rewards
The engine uses a novel method for holding the flywheels to the
crank. It is done with a split tapered bushing and a tapered bore
in the flywheel. I doubted that this would hold very well, but this
method gives an extremely tight fit that won't work loose. Bore
the flywheel taper and the tapered bushing with the same lathe
setting. After the bushing has been turned, do not remove it from
the lathe, but coat it with 'Hi Spot' blue dye, and try the
flywheel on it. If all the blue doesn't wipe off, polish the
high areas of the tapered bushing until it does. Keep doing this
until there is a complete wipe of the blue dye off the bushing. You
will then have a very tight fit. Also, make a ream fit between the
bushing and the crankshaft. When you cut the bushing off, leave it
a little longer than the flywheel hub, so that it protrudes on the
large end. Remember that the large end goes against the shoulder on
It is difficult to get the intake valve housing to seat at the
bottom of the hole. I don't like this method of a metal to
metal seal between the head and the valve housing. Instead, I used
O-rings to seal here. The O-ring gives a tight seal and enables the
housing to be removed and replaced easily during engine set up.
(This housing has to be removed to insert and replace the exhaust
valve.) The drawing shows the sizes needed for the O-ring seal.
An O-ring was also used as a head seal. Cut the groove in the
head, so that the O-ring seals against the end of the cylinder
liner. Again, this makes a perfect seal that can be removed and
replaced easily without scraping Permatex off the surfaces, or
making new gaskets. Standard Buna N (Nitrile) O-rings seal up to
250° F. This is within the temperature range of model gasoline
engines, and I have had no problems with any O-rings. A Viton
O-ring is available, which will withstand temperatures up to 500°
F. You can use these rings if it makes you feel better. They are
available from industrial seal companies, or from Power Model
Supply. Be sure to always lubricate the O-ring before installing
Another reason for using O-rings on the cylinder head is that it
permits a metal to metal seal of the head to the cylinder. This is
important on this engine because one side shaft bracket is mounted
to the head, and the other is mounted to the frame. A metal to
metal fit on the head-cylinder joint means that the two brackets
will stay in line when the head is removed and replaced, no matter
if the torque is different on the head nuts.
The carburetor is a little gem. It does however need a choke. In
the construction article, Mr. Westbury states that it doesn't,
but it appears that the article was written before the first engine
was built. I turned the lower part of the carb, so that a thin
brass choke band could be fitted. The carb works perfectly. The
venturi is formed by the conical valve. The screw on top limits the
valve's lift, thereby varying the venturi effect. Again, I used
an O-ring between the carb flange and the head. The O-ring groove
is machined into the cylinder head. Two needle valves are provided,
one for gasoline, or petrol as the English call it, and the other
For the main bearings, I used Oilite sintered bronze flanged
bushings. These, I split in half with a 0.010' thick Dremel
slitting saw, held in the milling machine chuck.
The cams are held on the shaft with a set screw during set up
and initial test running. When the exact position is found, they
are drilled for 1/16' Spirol pins.
The valve springs that were provided with the kit were much
stiffer than necessary to operate the valves. These I replaced with
springs from the local hardware store. When the engine was running,
I held my fingers on the rocker arms to reduce the spring tension.
I found the engine increased speed when I did this. This indicated
that the springs were still too heavy. I kept reducing the tension
on the springs by cutting the coils shorter, until I reached the
optimum spring pressure. Any more and the engine would begin to
die. The engine runs cooler after doing this, and wear on the gears
will be less, as the shaft turns much easier.
On this engine, I tested compression ratios from 8.5:1, down to
4:1 to find the ideal ratio. A high ratio puts more stress and wear
on the engine's moving parts. The ideal ratio was found to be
about 5.12:1. It is easy to calculate the compression ratio before
you make the piston.
The formula is: compression ratio = (piston displacement +
clearance volume) divided by clearance volume.
I adapted the drawings for a Stuart Turner steam engine governor
to this engine, downsized to fit the model. The early governors
were close relatives of the steam engine governors, so it fit right
in. It is driven by an O-ring, and controls the engine speed
The lever on the side shaft bearing contains the ignition
points, which can be advanced or retarded while the engine is
running. This way the ideal setting can be found.
The engine was brush painted with synthetic automotive enamel,
over three sanded coats of primer. The builders plate (cast by me)
was screwed to the cylinder. The engine was mounted to a steel
plate, edged with aluminum. The top of the plate was tiled with one
inch square ceramic tiles, around the engine base, to represent an
engine room floor. The unpainted metal surfaces were gun blued with
a product called Brownells Oxpho-Blue, available from Power Model
Supply. After using other metal blueing compounds, I found this to
be the best.
I designed the muffler along the lines of a muffler that I had
designed several years ago for an 1800 HP diesel engine. This one
uses steel wool, held in place with window screen as an element. It
is very effective in keeping the engine exhaust quiet without
causing back pressure.
For best operation, put a check valve in the fuel tank.
Don't rely on the conical valve in the carb. I used a Briggs
& Stratton fuel check valve (part PIPE-FUEL #293700). For
ignition, I run the engine on two 'D' cell batteries, and a
model airplane style coil. If you are wondering about the lack of
glass oilers, the very early engines used brass oil cups.
The WYVERN 'petrol' engine was really fun to build. It
was different than building 'just another horizontal hit and
miss engine'. It looks impressive, both standing and