Frame, front suspension, brakes (with the same changes as described for the 125 cc twin RC143), all is new. Like with the 125 cc, the cylinders are inclined at 35 degrees and are cast in one piece with the top part of the horizontally split crankcases, which are cast in light alloy, as are the cylinder heads (cast in one piece for the four cylinders). The various covers are cast in electron, an alloy of aluminum and magnesium.
The double overhead camshafts are now driven from the center of the crankshaft by a gear train between the cylinders. To reduce the width of the engine, this gear train is partially situated behind the cylinders, and from there drives the magneto for the ignition. The valve angle is 76 degrees: 36 degrees for the inlet and 40 degrees for the exhaust valves. Bore and stroke are still 44 and 41 mm. Carburetion is by four Keihins with remote float chambers, with a rather steep downdraft angle for the inlet bell mouths; later they are replaced with concentrics with a short bell mouth in line with the inlet track.
There is a six speed gearbox and the engine has wet sump lubrication. Power output is given as over 38 bhp at 14,000 rpm. Maximum torque is 2.1 kgm at 12,000 rpm. Total weight of the bike is now 128 kg dry, top speed is given as over 220 km/h.
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