from the Solvang Vintage Motorcycle Musuem:
"This 450cc bike was the first double overhead cam engine in a Honda street bike. This engine will rev up to 10,000 rpm, an engine speed that would turn British bikes into a pile of broken bits. The Black Bomber was Honda’s message to the British “Here we come” (the 4 cylinder CB 750 was the message “too late – here we are”). The Black Bomber was considered too old in its styling and was changed to a more modern look in 1968. Collectors, of course, want the “ugly” Black Bomber.
Unlike automobiles in the USA market, Honda motorcycle models were not designated strictly by calendar year. Honda distinguished models by affixing a K-number suffix to the model, typically beginning with K0 at the model's introduction, sometimes with K1. Individual K models were offered for one or more years, depending upon sales rates and remaining inventory. For example, the initial CB450: the 4-speed Black Bomber,is designated CB450K0. It remained in Honda sales literature through 1968, alongside the next model, the CB450K1.
It was unique for a road bike in having twin overhead cams with torsion bar valve springs. One of the most impressive parts of the bike to this day is the engine which, when viewed in profile, has a very classical twin cam shape -- like that of a Manx Norton.
The CB450 has a stormer of an engine when it is carefully set up. The main weakness of the bike is that it is very heavy (at about 430lbs) and ithe gear ratios are quite spaced over the 4 speed box with a low top gear. Even so it is generally reckoned to be the best 450/500cc Honda twin until the introduction of the CB500 water-cooled twin of the nineties."