A pair of Tom Senior engines

Posted by Chris Graham on 5th April 2023

Patrick Knight looks at an interesting pair of Tom Senior engines, which are excellent examples of fractional horsepower stationary engines.

Tom Senior engines

Tony Khun’s No.3 Simplex engine.

I am sure that we are all aware that the engines we see on today’s rally fields are gradually becoming physically small and smaller, year-on-year. There are several reasons for this but, as someone who – for various reasons – hasn’t exhibited an engine at a vintage event for a good many years, I’ll not express my thoughts on the subject here.  

Fractional horsepower engines, of any make, always seem to attract a great deal of attention when other engines hardly warrant a second look, so I was pleased to see two Tom Senior fractional horsepower engines, owned by Tony Khun, being exhibited at the Kingsley Autumn Crank-Up over the weekend 15th and 16th October. 

A 1922 advert for the Simplex engine.

The Tom Senior Engineering Company operated from three addresses over the years: the Victoria Works, Cleckheaton, Yorks, 1922; Westgate, Cleckheaton, Yorks. 1923; and Atlas Works, High Town, Liversedge, York, from sometime in the 1930s. The following specifications for Tom Senior ‘Simplex’ and ‘Superior’ engines have been gleaned from the pages of an undated sales catalogue and period advertisements. With this in mind, some of the measurements quoted may have changed over the years, as indeed did the prices, which I have deliberately not included to prevent complications. Both styles of engine were offered in gas or petrol forms and like so many fractional horsepower engines, Tom Senior engines were available as bare castings, machined castings or as a finished engine to suit the purchasers’ engineering skills and financial situations.

The Simplex
The smallest engine was the Simplex, which was offered in two sizes, and had a number of features that made it identifiable, including a removable cylinder head with the spark plug set at an angle of about 45 degrees. The No.1 engine had a 12mm plug, while on the other model an 18mm plug was fitted.

• The No.1 engine, rated at 1/16 horsepower had a bore and stroke measuring 1 inch by 1½ inches, and had flywheels measuring 5 inches in diameter with 1 inch faces. 

• The No.2 engine had a bore and stroke measuring 1¼ inches by 1½ inches and had 6 inch diameter flywheels.   

• The No.3 engine was rated at 1/8 horsepower and had a bore and stroke measuring 1½ inches by 2 inches. The flywheels were 7 inches in diameter with a 1 3/16 inch face. 

• The No.4 engine, rated at ¼ horsepower, was the largest in the range and featured a bore and stroke measuring 1 7/8 inches by 2 ¼ inches. The flywheel diameter is not mentioned. Simplex engines were normally fitted with battery and coil ignition although, when required a special sub-base could be supplied to allow magneto ignition to be employed.

Tom Senior engines

Here we have an example of the Superior engine with rotary side-shaft.

The Superior 
The Superior model was a well-made engine built to a high standard and it differed from the Simplex in several ways; the most noticeable being the rotary side-shaft to operate the vertically-opposed inlet and exhaust valves, which were located in special pockets to facilitate their easy removal for decarbonising etc.

• The No.1 engine rated at 1/8 horsepower had a bore and stroke measuring 1 ½ inches by 2 inches, and 7 inch diameter flywheels. 

• The No.2 engine, rated at ½ horsepower at 650rpm, had a bore and stroke measuring 2¼ inches by 3 inches, and the flywheels were 10 inches in diameter.

At some date, currently unknown, the Superior type A was introduced with a flyball governor but, at this time, I don’t have any details on this mode.

A more detailed image of the Superior engine.

This feature comes from the latest issue of Stationary Engine, and you can get. money-saving subscription to this magazine simply by clicking HERE


Pay just £3 an issue when you subscribe!

Subscribe to Tractor & Machinery magazine today and receive your first 6 issues for just £18!