You’ve heard the term ‘Black Bomber’ before, but many people use it on the wrong machines. The ugly, odd looking, small bike you see here has one of my absolute favorite engines. It uses NO valve springs! It has Steel cast-in Hemispherical Combustion Chambers with short and very efficient intake and exhaust ports. In 1965 when the 1st Black Bomber was released – 43 Horsepower from 444cc! was big news and that fact was used in the corporate advertising of the time.
Quite the hot-rod this bike was! Rumors still say there are really 2 stories behind it’s nick-name –
One says the bikes (available only in Black) were Japan’s weapons to do battle with the British 650’s of Triumph and BSA which it would outrun and outlast them – thus “Black Bomber” (war machine on which to do battle)
Of course the rumor that is not as flattering is that salesmen gave the odd-looking bikes that name because they weren’t selling and had therefore ‘bombed’. Since all were Black – thus “Black Bomber”. I’ve never liked this story, so I made-up the first one!
This example was a project dream come true because it took me over 1 full year to restore due to parts availability. There are hundreds of “K0” model-only parts and 99% of those parts were taken by aliens to other planets long ago!
When Honda’s engineers decided to build the CB450, they could have just made a bigger machine out of existing engine designs for the 450. They already had great running 250cc and 305cc Twins. They had mastered the use of high compression, single overhead cams, rockers and rocker shafts, dual springs, and combustion chamber designs on these smaller machines….Why not just make things bigger like everyone else?
No – Honda engineers did an almost “complete rewrite” of the Cylinder Head Design. Never before, Never Since. The CB/CL450/CB500T DOHC would mark the beginning and end of this unique design. What’s so different?
As mentioned, No Valve Springs are used! This 4-Stroke engine doesn’t use coil valve springs. Instead, it uses torsion bars and followers that close the valves.
Both Intake and Exhaust Ports are as close to “perfect” as any mass-produced machine had ever achieved. The elimination of coil springs allowed for an almost straight and uncluttered path for fuel and air to enter the highly prized “Hemispherical” chamber. Coil springs and the valve angles got in the way of perfection, so the engineers designed the new “Torsion Bar System” in their quest for Perfection.
Dual Overhead Camshafts – Unheard of in 1965 for a production machine! Again, the engineers determined that only a “pair” of camshafts could give them the control they wanted in the perfect Cylinder Head Design – One camshaft for Intake and one for Exhaust. Both would be driven by a single cam chain of 128 links.
Hemispherical Combustion Chambers were formed in Steel and were cast inside the Aluminum Head. – Look close at the photos and you’ll see the Steel/Aluminum chambers.
Special, Domed, Performance Pistons with special shapes were used to minimize the squish band (that area on the outside of the cylinder/pistons where heat can soften ring lands and cause damage.
These innovations have captured my interest and amazement for years now. I have to ask “Why not today?” I can only guess that the parts and machining required are very complicated. Add to that, the assembly steps are far more difficult and it starts making sense….. Cost, weight, labor, and size probably made these obsolete. It doesn’t mean they are any less fascinating…… I still hold them as one of my all-time favorite engines. Take some time and look at these photos. You’re looking as a design that hasn’t been used in over 45 years. I love these little engines!
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