John Smyth 4-1/2 hp Restoration – Part 5 of 5

It's finished! Peter Rooke completes the restoration of his John Smyth engine.

| October/November 2016

This is the last in a five-part series on Peter Rooke's restoration of a 1914 John Smyth engine. Read Part 1, Part 2, Part 3 and Part 4 for earlier stages of the restoration.

Fuel pipe

A long run of pipe was needed to from the fuel tank at one end of the cart to the mixer at the other. Steel pipe with a nominal size of 0.25 inch was used. The pipe was secured to the mixer by what looked like a 0.5-inch pipe thread, but the thread pitch was 18 threads to the inch (TPI) rather than 14 TPI. Nothing to match this was found so this clamping nut had to be made, but it’s not easy to turn a taper thread without the correct attachment for the lathe.

To make the nut, a piece of hexagonal steel was mounted in the lathe chuck then a hole bored to a diameter of 0.718 inch and depth of 0.5 inch. The lathe was set up to screw cut 18 TPI and a parallel thread cut to fit a minor diameter of 0.84 inch. Pipe taper threads normally contract by 0.75 inch over 12 inches, translating to 0.0035 inch per revolution for an 18 TPI thread. One of the flats of the hexagon was marked with a daub of red ink as a reference point. A very slow feed was used to give complete control and enable counting the number of turns. It was then a case of increasing the cut of the thread by 0.0035 inch then cutting on this setting for eight threads deep, increase the cut again for seven threads and so on, reducing the number of threads cut until the start of the nut was reached.

A spigot to fit the internal dimensions of the inlet was made, with a shoulder and fiber washer that would be pressed against it when tightened. The new nut was put on the spigot and then a pipe elbow was brazed to the spigot. Elbows were used to align the mixer inlet with the pipe from the fuel tank, and a tap was added.

To make assembly and disassembly easier, a pipe union was fit between the pipe from the fuel tank and the tap assembly. When tightened to the cylinder head inlet pipe, the mixer was twisted too far around to align with the fuel pipe. Gasket paper was cut to fit over the connecting thread of the mixer, proving sufficient to correct the alignment. With the tank centered on its mounts the outlet from the fuel tank was slightly out of alignment, so the pipe was bent just a little to pass down the side of the engine to the mixer.


The governor collar was cleaned before refitting, and it was then noticed that it had a brazed repair and the gap between the collar shoulders varied. The original roller fitted to the end of the governor lever had a step worn in it from this defect. To correct this, the shoulders were filed true and a new roller was made from brass, fractionally undersized so that it was an easy fit. The edge of the catch plate was trued up and made square on the bench grinder.


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