Only 64 were made, the last in 1938. In 1934, Stuart Turner introduced the S type steam engine. This was not a model but a fully enclosed single-cylinder commercial power unit of 1 to 1.5 HP designed to drive generators or fans or pumps. It stood 22 inches tall with a 10-inch flywheel and weighed about 120 lb. Preston Services have stated that there were both slide valve and piston valve versions, only 247 were made between 1934 and 1962. In 1938, Stuart Turner developed a single-cylinder 2-stroke diesel with crankcase compression, the model H. At the time this was the smallest commercial diesel engine in the world. These engines were sold mainly to the RNLI for charging batteries. The company produced a number of products for the military during World War II (1939–45), including combined boiler, steam engine and generator plants, designed to be used by resistance fighters or troops operating covertly behind enemy lines to power radio transmitters. The first of these modest-sized steam generator plants was called the Firefly, and was based around the Stuart Sirius engine, later ones named Mk814 had a bespoke single-cylinder piston valve engine. Stuart Turner also produced many internal combustion generators with uses such as charging aircraft batteries and providing dummy runway lights on fake airfields. Stuart Turner engines were built into the back of AEC fuel bowser trucks to drive a Zwicky Ltd pump to transfer fuel to aircraft. These P5XC engines were referred to in-house as the Zwicky engines. Stuart Turner continued to make engines (especially marine engines), pumps, generators, and model kits throughout the post-war years, though several of the pre-war model internal combustion engines did not reappear. The pre-war single-cylinder diesel engine was joined by a new twin-cylinder 2-stroke diesel in 1953, the H2/H2M/H2MR. Whereas the single had used crankcase compression to scavenge the cylinder, the twin-cylinder had two air pumps. Rated at 9 HP, the capacity was 780 cc, and weight 240 kg. The Single Cylinder Model H was updated to the H1, and many of its detail design features were changed, and ultimately it looked very much like a H2 minus a cylinder. Only 15 H1's were made. The single-cylinder diesel was phased out in 1960 and the twin in 1968, with only about 400 of each model produced.