Model Review

By Staff
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The Allman engine.
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View of radiator connections, and valve cage assembly.
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View of cam gear, hit and miss gear. The governor is in the pulley.

32 South Street, Apt 1 Clinton, New York 13323

The first time I saw the Allman model was at the North American
Model Exposition in Ann Arbor, Michigan in 1991. The Allman is the
third in a series of limited run scale engine models made by DeBolt
Machining. The model is of an inverted vertical, single flywheel
side crank engine, with an unusual valve gear and hit-and-miss
governor. It certainly isn’t just ‘another push rod
engine’. This engine also sports a cast iron radiator, and a
gas tank to be mounted below the floor level. The governor is
located inside the belt pulley on the offside of the engine. The
connection from the cam to the exhaust valve is an unusual system
of rocker arms that wraps around the engine from one side to the
other. Some other features include oil lubrication, a ported
scavenging exhaust, and a neat looking muffler. It runs on regular
unleaded gasoline and Coleman lantern fuel. I prefer the latter,
because it doesn’t smell as bad (although gasoline has a more
familiar smell). There is a small thumb screw on the governor
mechanism for speed control.

Since there are no living Allman engines that I know of, and the
only reference to one that I have seen is a period drawing
reproduced in the American Gas Engines, it is hard to compare the
model to the original. I am told DeBolt used patent drawings in
their research. The original presumably had a jacketed cylinder.
The model has no cooling passages in the cylinder at all. The
piping from the (functional) radiator to the engine ends at the
cylinder casting in blind, tapped holes. It seems that this
arrangement can provide only a token amount of cooling, but I have
run my engine for hours with NO water, and the engine doesn’t
seem to mind. This was done, no doubt, to keep the price of the
model down. The original used a gaseous fuel (probably natural gas
or producer gas), and hot tube ignition.

The model has a rather conventional fuel mixer, and uses high
tension ignition with a 10mm spark plug. This was done to save the
owner the hazards and extreme aggravation of hot tube ignition.

I purchased this engine from DeBolt as a mechanics kit. They
sent me the engine in two batches of parts. The smaller parts came
first, all sealed in plastic, and grouped by assembly order. All
parts were CNC machined, including the dome head bolts. The
castings, most of which came later in a second batch of parts, were
a bit rough, with a few flaws here and there, but nothing that
detracts from the operation or appearance of the engine. The model
is made of iron and bronze castings and steel and brass stock. A
painted, brass tag with a DeBolt serial number was also included.
The model is a substantial hunk of iron. It is not as heavy as some
I have seen, but it will still build your muscles.

The assembly instructions that came with the model are very
brief, and some potentially confusing assembly drawings are relied
on to complete the model. The only problems I had were assembling
certain parts before others, having to remove them to put other
parts on. This is probably due to my chronic reluctance to follow
directions. Each part has a drawing and numbered for
identification. Drawings for a base are included, as well as timing
and wiring diagrams. No photos of the model are included, nor is
there any indication of what the overall model will look like when
finished. Overall assembly went well, and was completed in the
course of a couple of evenings. Tolerances in parts were good, if
rather tight in a few places. Some filing was needed to fit some of
the parts properly.

Starting the model was a project in itself. After setting the
timing as per the instructions, I spent considerable time tweaking,
adjusting, cranking, and cussing. I found that my engine likes the
timing back a tooth from the setting shown in the instructions. The
engine is very sensitive to the needle valve setting (? to
1/3 turn open), and it seems to like some
choking (? to 72 open). The governor needs to be very free, and
well oiled (WD40 is too thin: I used 30W). I had a problem with a
leaky exhaust valve, which made the engine impossible to start.
After disassembling the cylinder for the third time to lap the
valve, I decided that the return spring for the cam rocker arm was
keeping the valve open slightly. A tighter exhaust valve spring
fixed the problem. Another modification was to remove the wire that
keeps the needle valve from turning, and place a coil spring around
the stem. This keeps the needle valve in place better, and saves
the skewered fingers I would get from the wire.

The engine doesn’t have any too much power due to its small
bore, and I have actually run it with the governor removed
completely, something I would never do with the Perkins model. This
is good, in that the engine can’t destroy itself in the event
of a runaway. At higher speeds, however, the engine can turn over a
6 volt magneto well enough to light a small light without lugging.
Because of the inverted cylinder, the oil feed to the cylinder
needs to be watched or the engine runs smoky, and the plug tends to
foul. My first runs were without a base, and with no radiator
attachments. The cylinder became warm, but not hot; cooling is not
a problem. After all adjustments are made and the engine is broken
in, it is fairly easy to start, but picky about fuel/air settings.
The spark needs to be quite hot, or the engine will not sustain
itself. This was the main cause of my problems in getting the
engine running right at first. I was using a six volt battery on my
coil, and a 12 volt motorcycle battery works much better. I have
run the engine quite a bit since I first got it together. I find
that it is quite easy to set up and run. It is a good engine to
start for a few minutes ‘just to watch it run.’ I mounted
it on an oak base, which is somewhat different than DeBolt’s
suggestion. I made it more to the form of the classic engine skid.
I put the spark coil in the base with the tank, and made external
connections to a motorcycle battery for quick connection.

The model has a different appeal than other ‘farm’
engines. This is the type of engine one would find illustrated in
pre-1990 engineering texts or trade journals, for use in small
factories or shops. It is an interesting model, with all the
mechanisms out in the open. It is different enough to draw
attention, but conventional enough to be easily understood. (The
Otto-Langen model is often mistaken for a hot-air engine). It is
well executed, and a pleasing model overall.

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