In the Sixties, the Japanese factories, bent on the conquest of Europe, launched themselves into a crazy technological scramble for the winner's circle in the world racing championship.
The crankshaft has a special configuration: the three cylinder part has the crank pins at 120 degrees, the two cylinder part has the crank pins, in the usual Honda fashion, at 180 degrees. Both crankshaft are coupled together at the position of the camshaft drive.
The valve angle is 56 degrees, 24 degrees for the inlet and 32 degrees for the exhaust valves. Lubrication system is by wet sump, with two oil coolers in the sides of the fairing. There's also an oil temperature gauge. The oil temperature is checked, and the coolers can be covered more or less to maintain the oil temperature within certain limits
Power output is 38 bhp at 20,500 rpm. If you're surprised that this is not 21,500 rpm, just like the RC116 : the complicated crankshaft reduces the mechanical efficiency somewhat, which results in a slightly lower Pme of 15.5 kg/cm2 and a 1000 rpm lower engine speed for maximum power. The polyester petrol tank has an unpainted strip, to check the petrol level.
Champion of the Four-Stroke
Until the ban in 1967 on 50cc and 125cc engines with more than two cylinders and more than four cylinders in the larger displacements, Honda championed the four-stroke engine, which used extra cylinders to make up for the power it lost to two-stroke models. The error of this approach was apparent in the 125cc class where Honda, world champion with two and four-cylinder engines, competed against the increasingly potent two-stroke twins of Yamaha and Suzuki.
To counter this challenge, Honda engineer Irimajiri (designer of the marque's 50cc twins and its six-cylinder 250) created a five-cylinder four-stroke made entirely from magnesium, for 1966. This amazing feat of engineering had pistons just 35 mm in diameter and cranked out 23,000 rpm yet stalled below 17,000 rpm, forcing the rider to work the nine-speed transmission relentlessly to keep the engine running! The Honda 125 proved unbeatable in the hands of the talented Swiss cyclist Luigi Taveri.
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