The Honda CB400SF motorcycles are equipped with a 399 cm3 four-cylinder four-stroke engine of the NC23E modification. Engine power is 53 hp at 11000 min-1, torque 37 Nm at 10000 min-1. The engine is designed to use gasoline with an octane rating of at least 95. The cylinders are in-line, with forced water cooling. The engine is interlocked in the crankcase with the gearbox, clutch and alternator.
The engine uses a DOHC type gas distribution system, with four valves per cylinder and two camshafts located in the block head and driven by a chain from the gear of the middle journal of the crankshaft. The camshafts rotate on plain bearings.
The crankshaft of the engine is composite, non-separable, the piston stroke is 42 mm, the cylinder diameter is 55 mm, the compression ratio is 11.3. The cylinders are cast from aluminum alloy with cast-iron liners pressed into them. The cylinder heads are also made of aluminum alloy by casting with subsequent machining.
The pistons are injection molded and have two compression rings and one oil scraper ring.
The engine uses a combined lubrication system. Part of the details (cylinder walls, camshaft lobes, piston pins) lubricated by splash or oil mist. Friction pairs requiring a constant presence of a large amount of oil (crankshaft and camshaft journals), are lubricated with oil coming from the oil pump under pressure. The trochoid type oil pump is driven by a separate chain drive from the gearbox input shaft. Since the oil carries with it the wear products of the engine parts, before it gets back from the oil pan to the friction pairs, it is pumped through a paper full-flow oil filter located in the front of the crankcase.
The engine power system includes an air filter with a paper filter element and four Keihin VP22 constant vacuum carburetors, one for each cylinder.
The clutch basket is also located in the same crankcase with the engine («multi-disk» oil bath clutch) with six friction discs. The gearbox is two-shaft, six-speed, with the inclusion of the first gear down from «neutral», the remaining gears are sequentially switched on by turning the pedal up.