The OS FS-20 is a legendary, vintage 4-stroke glow engine manufactured by Ogawa Seiki (O.S. Engines) between 1985 and 1989. At the time of its release, it held the distinction of being the world’s smallest mass-produced 4-stroke engine designed for radio-controlled airplanes. Prized by collectors and vintage RC enthusiasts, it is highly regarded for its smooth, realistic scale sound and high low-end torque.
Displacement: 3.56 cc (0.217 cubic inches)
Bore: 18.0 mm
Stroke: 14.0 mm
Practical RPM Range: 2,500 – 12,000 RPM
Power Output: 0.31 – 0.37 hp at 12,000 RPM
Weight: 265 grams (9.35 oz)
Torque Over Speed: Unlike screaming 2-stroke engines, this 4-cycle motor values torque. It swings larger propellers (typically ranging from 9×5 to 10×5) to pull scale models gracefully.
Glow Plug Requirement: It specifically requires a long-reach hot plug, ideally the O.S. Type “F” Glow Plug, to maintain stable combustion in small-displacement 4-strokes.
Simplified Carburetor: The stock intake features an air-bleed carburetor. This requires meticulous needle adjustments to achieve a reliable low idle.
Precise Engineering: Early production iterations did not utilize backplate or intake manifold paper gaskets. Instead, they relied strictly on metal-to-metal tolerances.
Camshaft Design: Disassembling the timing gear reveals a unique vintage feature: the camshaft rides on a loose single steel ball instead of a modern caged ball bearing assembly
