Honda RC149 5 cylinder 125cc Grand Prix bike.
Rather, we’re talking about the middle 1960s, when that key question turned into a technological battle between two-stroke and four-stroke 125cc race bikes. No special allocations or dispensations allowed—just bring your best eighth-liter bike to the track and we’ll settle things at the checkered flag.
In 1961, 1962 and 1964, Honda four-strokes won three 125cc world titles. But by the middle 1960s two-strokes had evolved into an undeniable force. Debates surrounding engine design had real merit: In a two-stroke engine, every crankshaft rotation yields a power stroke, while four-strokes produce power only upon every other crankshaft rotation—one rotation is “wasted” pushing out gasses. There is no shortcut here; to produce equal power in a four-stroke, engine speeds must increase substantially.
The obvious answer to this dilemma was to create an engine with more cylinders. But they would have to be tiny bores combined with an extremely short stroke to allow ultra-high engine speeds—all of which invited disaster given the materials and technology of the day. But Honda did it.
In 1965, Honda unveiled an incredible machine. It demonstrated just how far Honda engineers could reach beyond convention to produce a winning design. Boasting five cylinders with a bore measuring a minuscule 34.0mm and a stroke of just 27.5mm, this machine had one of the smallest cylinder bores Honda had ever attempted. And it revved to an unheard-of 21,000-rpm redline. Into this 1.33 inch diameter cylinder Honda stuffed four tiny valves and a centrally located spark plug. The next year, Honda engineers made minor changes to the bore/stroke figures, raising the redline to 21,500 rpm and creating the RC149 shown here.
Team technicians noted that these engines were among the most challenging of all Honda race bikes to wrench on. Not because they weren’t built well; they were incredible. The problem was they were so small your fingers felt like huge sausages. And if you tried to lap a valve the usual way, you’d twist the valve head right off.
With 34 horsepower sent through an eight-speed gearbox, the RC149 was good for more than 130 mph. In 1966, Honda won at Hockenheim in West Germany, Sachsenring in East Germany, Brno in Czechoslovakia, Dundrod in Ulster and Monza in Italy, beating the two-stroke-mounted competition for the world championship. That may have defined the four-stroke versus two-stroke argument. But the debate raged on for one simple reason: That was the last year Honda’s exotic works bikes contested the 125 Grand Prix class.
Wikipedia The 124.4 cc (7.59 cu in) air cooled inline 5 had a bore and stroke of 35.5 mm × 25 mm (1.40 in × 0.98 in) produced 38 bhp (28 kW) at 20,500 rpm.
The cylinders were split into two banks, one of 3 cylinders with a 120° crankshaft and the other a bank of two with a 180° crankshaft. The two crankshafts were connected together with gears. Where the two crankshafts met, between cylinders 3 and 4, there was a gear train to drive the camshafts. The crankcases were cast from magnesium and 4 valve heads were fitted.
The engine has wet sump lubrication with twin oil coolers fitted in the fairing and used electronic ignition. 5 carburettors were fitted and tuning was difficult. Each carburettor had 5 jets to adjust and were mounted on rubber stubs. Different length stubs were also part of the tuning process.
Transmission was via a multi-plate dry clutch, 8 speed gearbox and chain drive.
Credits to the photographer and staff writer, names of which were not published.
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