Model Review

Allman engine

The Allman engine.

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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.