BUILDING AN ECONOMY

HIT & MISS ENGINE

Part four

by John Merrett

Many stationary engines of this type had the same style of simple silencer  consisting of two dished halves creating an expansion chamber, one with the inlet connection and bolted together to form a narrow annular exhaust passage. I expected the two halves to be supplied as basic castings but looking at the drawing it became obvious that these halves were to be machined from solid bar!

I contacted the supplier who duly despatched a hefty chunk of 3” dia. MS. Bar about 3” long. The machining of these two thin walled dished and flanged components involved a lot of careful planning and a great deal of precise lathe work. The inside and outside radii had to be produced carefully and the depth of the inside radius  checked very precisely to obtain a thin wall but not so thin that the inner radius broke into the outer.

The amount of material left after the two halves were finished was probably about 5% so you can imagine the quantity of swarf produced. However many scouring pads it would have made I have no idea but I can reveal that I nearly lost sight of the lathe in it all, and had to dig it out at least three times.

The final operation was to machine up the little spacer washers, drill the bolt holes through the to halves and then bolt together.

In preparation for the rectangular petrol tank, the wooden frame seen above, was bonded into the base casting using Araldite.
The main components of the petrol carburettor are machined from brass and gunmetal bar material. The body is in the form of a venturi with a vertical spray bar and needle to control the petrol supply. Surrounding this bar is a rotating sleeve, cross bored the diameter of the venturi throat with an external lever to act as a throttle. The inlet is capped with a drilled disc to form a choke control.
Getting quite heavy now with the flywheels keyed in place, not so easy to lift on and off the bench!

Note the silencer screwed to the exhaust outlet on the cylinder using a commercial 1/4” BSP. male to female pipe elbow.

The arrangement of the starting handle boss and the dog slipped over the crankshaft end (driven by the flywheel key) was an arrangement dreamt up by me and is a sort of inside out version of the starting handle system that I recall on my father’s Ford 8 back in the late 1940s.
The twin rising cams of the dog were produced by a combination of end mill steps which were blended into cams by using a miniature, high speed grinding wheel on a flexible drive. The ends of the cam lock into the pins of the dog and when the engine fires it ejects the handle off the end of the crankshaft.
Apparently the cowards way of starting the engine is to employ an electric drill with a rubber wheel in the chuck and apply it to the flywheel rim.

Because the valves are exposed on this engine, the technique for starting is to hold the rocker down with a finger, spin up the flywheels and release the valve. With carburettor set up correctly the engine should fire up.

We are, however, getting ahead of ourselves as we haven’t got a petrol tank yet.

This picture is included as it shows the starting handle, the dummy magneto and a different view of the carburettor.
You may recall in picture 35 the wood guides bonded into the inside of the cast aluminium base. These guides are to locate the petrol tank correctly to line up the filler and feed pipe to the already drilled holes in the ends of the base.

Having a sheet of copper handy of about 20 SWG. this was used to construct the tank by folding up four sides and two ends with appropriate tabs for soldering. The unsoldered tank was tried in position and the inlet and pipe feed hole centres marked in situ. The feed pipe was formed and vee’d at the suction end to ensure a clear passage on the tank bottom at the suction end and silver soldered into the tank end.

At the other end a brass bush was silver soldered in to take the filler which had to be screwed in when the tank was fitted into the base. The tank was pickled in citric acid to thoroughly clean the copper and could then be soft soldered with Tinman’s solder and Baker’s fluid flux, without any fear of melting the connections.


The petrol tank was eased into the wooden guides and the filler screwed into the bush in the end of the tank. The tank was then fixed to the wooden guides by applying silicone sealant which bonded to the tank and the wood.


Go to part one

Go to part two

Go to part three

Go to part five