[2] Although the model was designated 550, the actual capacity was 572.5 cc (34.94 cu in).

[3] Honda developed a completely new, unusual design of engine to compete in the middleweight-sector with twin overhead camshafts acting on rockers, having screw-adjusters for clearance which actuated the sixteen valves (four per cylinder).

The engine featured a standard oil-cooler and a distinctive, unusual exhaust system, a first for Honda, with cross-over pipes directly in front of the engine linking cylinders one to four and a separate pair of pipes connecting cylinders two and three.

The CV carburettors were of a new type using mixture-enriching internal fuel passages for cold-starts, with careful engineering of the inlet tracts to achieve smooth gasflow.

[2] The machine was equipped with transistorised ignition and electrics were 12 volt.