September 13, 2019.


John R. Bentley 2019.
Bruce Stewart & Co. Imperial Engine
The Imperial Marine Gasoline Motor

A scratch-built miniature replica of Bruce Stewart & Co. Ltd.'s  "IMPERIAL  MODEL A"  engine




Model constructed in 1995

More than a century ago the Bruce Stuart Company began manufacturing a range of gasoline engines. At first, land-based versions were produced but the potential market for small marine engines was ripe for this island based company and they stepped up to the challenge by offering a line of seawater cooled engines. These two-stroke engines were used in the small boats of the inshore fishery on the Atlantic Coast.

The Model A was not their first but it was produced early on and was likely introduced well before the beginning of World War I. Versions producing the same horsepower continued to be produced by the company with occasional updates (and bearing new model letters) until the late 1950's.  The engine was advertised as having a 4 1/2-inch bore and a 4-inch stroke*. It was rated at 5 horsepower at 600 revolutions per minute, weighed 230 pounds and had a 16-inch flywheel. In addition to some smaller and larger engines, there were also 2A, 3A and 4A versions of the "A" denoting the number of cylinders. These produced 10 HP, 15 HP and 20 HP respectively. Early Imperials like this one used a standard "jump-spark" ignition but the company later changed to the more waterproof and faster igniting "make-and-break" system.

* considering the low rpm surely that figure represents only the part of the stroke within the displacement portion of the cylinder volume - i.e. above the top of the exhaust port.





The little motor presented in these photos is not much bigger than a baseball and was entirely fabricated from cast iron, steel and brass without the use of any castings. It is a scale model of the single cylinder Imperial 5 HP Model A.

This model sports a working water pump for cylinder and manifold cooling and a handmade spark plug. It is a 2-stroke, valveless 3-port design like the prototype. The bore is .757 inches with a .800 inch stroke (.5625 above the port). Displacement is 0.29 cubic inch (4.8 cc) and the compression ratio is 7.5:1. The cast iron flywheel has a 3.34-inch diameter. The piston is a long skirt type with a baffle on top designed to facilitate the simultaneous outward flow of exhaust and inrush of fresh mixture with a minimum of interference.

For purposes of display, I used a Dumas propeller (3" dia. x 2" pitch) which I had previously purchased nearly ten years earlier as an unfinished casting from James Bliss Marine in Boston in the early 1980's.  Apparently as of this writing (Sep. 2019) this beautiful prop is still available online from Dumas as #3108.



A close cropped shot showing the spark plug, cylinder priming cock and the timing interrupter
(beneath the timing lever and its associated sector plate are the vertical water pump and two check valves)











Constructing the water jacket and cylinder assembly was interesting (and quite fun actually!)




The model's piston and cylinder are iron. It is a venerable combination for sliding surfaces still used in many car engines today for the rings & cylinders. The surrounding water jacket was turned from steel. The cylinder was pushed in from the bottom and the two pieces with all necessary protuberances (and the bottom flange) were then silver brazed together.

As I was writing this I remembered that the cylinder's inner portion was distorted during construction and I had to make a new one. I wondered if perhaps the old cylinder might possibly have survived the intervening twenty-four years. Sure enough - I found that blackened old thing at the bottom of my junk box!

I put it in the lathe to clean it up in order to take a few pictures which follow:

The inner portion of the cylinder assembly
(junk - this one was damaged during construction)



Two of the three cylinder ports: exhaust (upper) and intake (lower)



Threaded bushes for the priming cup and spark plug



These (as well as the upper part of the cylinder) are encompased by the cooling jacket and surrounded by seawater.


Bypass port admits the fuel/air mixture from the base (crankcase)





Below: the bypass channel is visible on the outside of the cylinder
(the green vertical channel to the right of the propeller blade tip)



Well suited to the small inshore fishing boats of those days the 3-port, 2-stroke design made for a very simple engine.

There was no reed valve and no rotary valve as the piston and ports served this purpose. It could be started in either direction. The basic engine had only three moving parts - a piston, a rod and a shaft. This design was inexpensive to manufacture and had little to go wrong (at least on the inside). Of course a standard plunger pump for cooling and ignition timing gears were required to complete this simple power plant.




A full size manifold from an early Imperial dwarfs the model



( Yes, I know the carburetor was a bit twisted around on the manifold when I took that photograph ! )

It is interesting to note that the cast iron "blob-shaped" manifold acts to handle both the intake of mixture from the carburetor and the exhaust output as well as being water cooled itself. I chose to make mine appear in the gray color of cast iron as the restored full size engine which I was copying had the manifold painted with high temperature silver paint. At least some of the original manifolds may have been painted the same green as the rest of the engine as the picture below suggests.

Later models of the Model A Imperials used different manifolds some were castings and some I have seen were machined from stock. The latter may have been replacements for OEM units which had rusted out.  Unlike water from lakes or rivers, using seawater for cooling was not kind to engines in our area due to the salt content.



I scaled down the full size Imperial's Schebler Model D 260 carburetor



The Schebler employed a curved elbow air pipe inside to take advantage of the low pressure area which occurs in part of the bend. This creates enough vacuum to draw liquid fuel through a needle valve instead of using a straight restricted venturi as in modern carburetors. I copied the Schebler on the outside but omitted an internal float as it is unnecessary and impractical in such a small size. Gravity feed from a fuel tank located on a similar level is adequate. This is normal practice in model boats and airplanes which use internal combustion engines of this size.

Ignition and water cooling components
including the timing lever, interrupter, spark plug, priming cup and the water pump with check valves


A rotating spark timing contact inside the top circular black & bronze housing is turned at shaft speed by a set of miter gears driven from the crankshaft. The top and side external high voltage terminals are not yet installed. The upper side of the green painted timing gear is visible at the bottom of its vertical shaft in this picture.

"Jump spark ignition" used on this engine was very much like that on a modern engine. The main difference was that the ignition coil didn't just produce one spark on per explosion. The battery-operated coil in the jump spark system had an integrated buzzer like that on a Model T Ford coil. When connected to a battery the buzzer caused the coil to produce a continuous series of high voltage sparks. The coil's primary circuit was connected in series to the battery through the timing interrupter by means of a side terminal on the interrupter.

Inside the interrupter a rotating contact driven from the crankshaft touched the fixed terminal once each revolution. This rotating contact was connected to a top terminal on the interrupter. The intermittant buzzes of high voltage developed in the coil's secondary winding passed through an external wire to the spark plug. Forward and reverse timing were controlled by a handle which turned the black insulator (along with its fixed contact) through a considerable arc to advance or retard the spark.

In the photo below you can see the teeth of the mating timing gear on the crankshaft beneath the gray manifold. The teeth were marked out using a dividing plate on the lathe then hand cut with a file ...and some patience!


The Imperial floating in mid air!



I included tiny drain cocks for the carburetor bowl and the crankcase. These units are very small but they do function.



During construction in 1995 I made rough pencil sketches which show the major portions making up this single assembly. This H-shaped lower half of the crankcase is also visible in many of the photos. For inclusion on this page I added color to the sketches and also numbered the 18 individual steel pieces. These pieces were silver brazed together to create the lower crankcase.

Original pencil sketches of the subassemblies and individual parts of the lower crankcase




(This is not CAD.  Before we all had computers on our desks I used to own a pencil.)





Two inspection covers give access to the water jacket and the crankcase







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John R. Bentley 2019.