Tank cooled

The earliest "flat" heads were a slightly domed and uncooled casting. The cooling water pipe did no more than allow the water to pass from the crankcase to the cooling tank. The water pipe is at the front of the engine. There being no scope for it within the flat head the priming cup was fitted in the crankcase top flange. It seems the design suffered from cracking and was soon modified. Few survive as most were replaced by Jellymould (JM) heads. The early pistons used a blade rather than ramp deflector. This style of piston remained in use until the end of the Jellymould. Neither flat nor  JM heads should be fitted over a ramp deflector piston as it would raise the compression ratio too high.

The next head seems to be an attempt to maintain the flat head at a more even temperature by grafting a cooling ring onto it. This resembles a ring-doughnut and very few were made.

The Jellymould  head is near fully-cooled and was a commercial success. It is however very susceptible to frosting and many have suffered as this one has.

The most common half-cooled head appeared in several almost identical forms which overlapped each other in production: The Victory head appeared in late 1918 and remained in use until perhaps 1920 or 1921. In about 1922 the "Victory" legend was removed and replaced by an "S" (standard?).   Examples of the 1922 model exist with a Victory and it seems batches were made with praffin and "Victory" heads.

The 1923 model introduced the option of dedicated Paraffin or Petrol  heads and from that time the heads are so marked. The latter have an upward angled spark plug hole and could be used with a paraffin ring if desired. The petrol only option seems not to have been much used before 1931 but was a common retrofit. A factory issued petrol-only model has a different carburettor without the drain.

Finally for the 1931 model the head bears the legend "New Model".

Other variants exist on for example the "Admiralty lighting sets" and the Town-gas model. Pictures of these would be most welcome.

Hopper Cooled

The so called "Appletop" was introduced with the 1923 model and continued through to the end of production. In 1932 the hopper was changed to petrol and an optional compression ring could be purchased to lower the compression ratio. The only differences for this model are longer head studs with brass nuts and a drain tap on the end of the water inlet pipe. This style was not intended for constant full output use. For some reason it seems to have sold especially well in Scotland.

The cistern hopper (colloquially known as the "Bog-top") was introduced in 1935  for use in Benford concrete mixers. Production in the 5hp size was not large. Some of the features of this model appear in rebuilt or late-production engines destined for other uses, presumably as stocks of spares were consumed.