Honda CB450 K5 Hi tech twin
1965 was a big year for Honda. The company lured the world’s top rider, Mike Hailwood, away from MV Agusta with a deal to contest the 250cc, 350cc and 500cc World Championship classes for the 1966 season, and boldly ventured into the ‘big bike’ market with its revolutionary CB450. Today, the 450 is regarded as a middleweight rather than a big bike, but back then it was aimed directly at the segment of the market so long the domain of the British big twins.
The parallel twins offered by BSA, Triumph, Matchless/AJS, Norton and Royal Enfield were themselves aimed not so much at the home market, but the USA, where they cut serious inroads into Harley-Davidson’s territory. Honda wanted a chunk of that, but initially at least the CB450 failed miserably in USA, due more to the styling that the specification. Ironically it was a big hit in the UK. The poms loved it so much they even nick-named it the Black Bomber, despite the fact that it was also available in red. There’s no doubt the new 450 was a spirited performer, and when it was released in February 1966, it enjoyed a sizeable price advantage over the British 650s, selling for just £350 in UK. Early road tests showed the 450 could sail past the magic ‘ton, which was not bad for an engine of just 444cc, but there were other issues that seriously polarised opinion. The Americans hated the look, particularly the humpy fuel tank. It was no lightweight either, tipping the scales at 187kg – 20kg more than a 6T Triumph. And whoever selected the ratios in the four-speed gearbox was clearly out of step with what was required. Around town, the 450 was a chore, with an ultra-low first gear (18.58:1) and a yawning gap to second (10.79:1). This was not so much a problem when changing up – the torquey engine bridged the gap nicely – but changing back from second to first in traffic was akin to throwing out an anchor. Ground clearance was a problem as well, both the footrests and centre stand easily made contact with the road when the machine was heeled over. Seeking to prove the CB450’s worth on the race track, Honda, via its UK dealer network, tried to enter the ‘Black Bomber’ in the prestigious Brands Hatch 500 Mile Race in 1966 – but the entry was vetoed when the F.I.M. (who controlled the International regulations for the race) banned it on the basis that it was “too advanced”! Clarifying the reasons for their curious decision, the organisation stated that the use of double overhead camshafts in a production machine required specialised servicing techniques that were beyond the average owner and not in the spirit of Production Racing.
It was seen by many as a back-handed compliment, and by others that the home fare was somewhat primitive. Honda did manage to steal some thunder by flying in Mike Hailwood from the previous day’s Dutch TT, and he was permitted to cut a few ‘demonstration’ laps on the CB450 prior to the start of the 500 Mile Race.
That engine, with its much-appreciated electric starter, featured a 180º four-bearing crankshaft, twin overhead camshafts and highly unusual torsion-bar valve springing, which enabled a 9,500 rpm red line – unheard off for a big twin at the time. Naturally, much was made of the torsion bar valve springing by the excited press, but in reality a conventional valve spring is actually just a torsion bar wound into a helix. Nor was the idea all that new. French Panhard Dyna cars of the early 1950s featured such a system in their 850cc opposed-twin engines. Honda also tried the system in their V12 Formula One engines, but discarded it in favour of helical springs. In the CB450, the camshaft lobe acts directly onto the rocker arm, while the valve is retained by a forked arm which itself is anchored to the torsion bar. The bar simply twists along its length to provide springing. One side effect of the system, not entirely desirable, is very high seat pressure, and the valve spring torsion bar also has a very finite life; shorter than helical springs. There is, however, a saving in weight over the normal helical spring and collet arrangement. In a departure from usual Honda practice, the camshafts were supported in bearings contained in
end plates on the cylinder head rather than running directly onto the casting or by the ball bearings used in the CB77. Crankcases were horizontally split, with the crankshaft supported in four roller bearings.
The impressive specification failed to translate into sales in what was intended to be the primary market, USA. Honda America’s National Service Manager Bob Hansen lobbied the factory to produce a racing version in order to demonstrate the model’s performance capabilities, and the result was a small batch of hand-built racers that were entered for the 1967 Daytona 200. They were fast all right, recording 137 mph on the bankings, but all four entries suffered problems of various sorts.
A gear up
Revolutionary as the CB450 was, more work was needed, and by November 1967 a completely new version, the K1, broke cover at the Earls Court Show in London. Most importantly, a fifth gear was added, virtually between the existing first and a slightly raised second (12.58:1), which transformed the riding experience as there was little need to go back to first other than for a complete stop. There were changes within the engine too, with larger inlet valves (up from 35mm to 37mm) and exhaust valves (from 31 to 32mm) and compression increased from 8.5:1 to 9.0:1. In search of a smoother power delivery the Keihin CV (constant vacuum) carbs dropped from 36mm to 32mm, but despite this, power rose to 45hp at 9,000 rpm.
LEFT The CB450 engine unit was compact, but tall and heavy. BELOW The CB450 camshafts bear directly onto hardened strips on the cam followers, with a separate arm retaining the valve which is controlled by a torsion bar shaft. BOTTOM The torsion bar controlling each valve twists inside a hollow tube.
New badges for the K5. Keihin CV carbs were difficult to tune on the original model but improved from the K1.