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Orenstein & Koppel SB (1938): single-cylinder diesel tractor with thermosiphon cooling, 10.3 kW output and fewer than ten units built from the Further Models series
Orenstein & Koppel was a German manufacturer best known for light and industrial railways and construction machinery, alongside which it also built small farm tractors. The SB was built in 1938 and is among the rarest models of the brand: according to available records, fewer than ten units were made.
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The single-cylinder diesel engine has a displacement of 1,470 cm³ with a 105 mm bore and a 170 mm stroke, a clearly long-stroke ratio typical of slow-running single-cylinder diesels of the period. At 1,400 rpm it delivers 10.3 kW, equivalent to 14 horsepower, driving the rear wheels through a three-speed gearbox. Cooling is by thermosiphon, meaning circulation of heated coolant without a water pump.
Long-stroke single-cylinder diesels with thermosiphon cooling were a deliberately simple, low-maintenance design for small farm tractors, offered by several manufacturers in the 1930s alongside the better-known Lanz Bulldog. Omitting a circulation pump reduced the number of wearing parts but required a sufficiently large coolant volume.
Since the SB was built in single-digit numbers, buying a used example is practically out of the question; surviving units are mostly found in collections or museums. Anyone caring for such a vehicle should check the thermosiphon cooling for deposits, since without a circulation pump these impair cooling performance especially strongly.
Sources: SpecCodex-Register: Kennwerte des Modells SB
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