What follows is a description of a small spark ignition freelance petrol engine.
Bore 9/32" Stroke 5/16" = .32 cc.
Having already built a 3/8 x 3/8 bore and stroke pushrod engine I wanted to build a smaller but overhead cam engine. I decided to go long stroke because with such a small bore I felt it was needed to get enough compression. I used materials throughout which suited me and the particular application, i.e. small cap head bolts for the valves, but probably would not suit the purist - but then it is my engine.
I made no drawings as I had clear in my mind the general design. I started with the bottom end with the stroke and a big end diameter leading to an overall crank size which then allowed me to create a crankcase around it. Lubrication is by wet sump with a dipper on the conrod. Once the crankshaft / case was completed I turned my attention to the crank follower and its housing together with the rear gear case. I worked out roughly the distance I would need between the crankcase and the camshaft to fit in the lower part of the cylinder head and room for the piston / barrel to fit in between.
With a rough idea now of the camshaft / crankshaft centre distance I could design the gear train. A sensible width of gear case together with the centre distance told me I would need four large gears together with the half size gear on the follower. With a suitable d.p. I found the number of teeth and a form tool was ground on the end of a piece of tool steel for both large and small gears. This was mounted sideways in the lathe tool holder on centre height and a presized blank carefully racked out, in the time honoured fashion. With the centre reamed three gears were parted off, a suitable taper to fit the camshaft was then machined in the end of the last blank and this too parted off. I now very carefully measured the exact gear centre distance with the minimum possible running clearance. This enabled me to exactly position the gear spigots in the case. The camshaft gear had a large hole behind it to spigot onto the end of the camshaft bush in the head. This now gave me the centre around which the head would lie and the gear case and crank follower assembly could be completed.
The cylinder head was the only point at which I made a large scale drawing to make sure the valves and plug would fit into such a small bore, and that the valve angle would give a satisfactory combustion chamber shape. This also allowed the spark plug angle and position to be accurately determined. Because clearances are so tight I could not drill and tap the spark plug hole through into the head in the conventional way. I therefore thought I would use a ring fire plug though not using the body in the normal way, as the earth electrode, but the cylinder head itself. Using a centre electrode of 1mm diameter I drilled a hole into the combustion chamber on the plug axis with a diameter that would give the correct spark gap between the centre electrode and the cylinder head. I then followed down with a 4 b.a. tapping drill just far enough to leave a land for the spark gap hole about .02" long. The hole was then tapped 4 b.a. to the very bottom. The plug body was made with a thread long enough so that the body sealed to the head on the drill point taper at the bottom of the hole, not on the shoulder above the thread. The hole in the head was now filed to a shape so that the spark could only occur in one place, not all around the electrode. The valves and seats followed normal practice with the valves machined from small cap head screws. The rocker arms were mounted on eccentric bushes to allow adjustment. The contact breaker was mounted on the rear of the gear case and the cam driven off the end of the crank follower. The assembly can be turned to adjust timing.
The carbureter was my own idea. I felt that the jet should be variable and in the area of maximum depression behind the barrel to give a better chance of controlling such tiny amounts of fuel required by a petrol engine, unlike a glow or diesel engine. The size of the jet, the angle on the needle and the amount of rotation can all be adjusted to obtain satisfactory results.
The engine has been run but those of you with sharp eyes will have spotted that I have broken the glass plug insulator which will have to be repaired (when I can find a spare moment) before I can continue with any further trials.