Last year's star racing power unit, the Honda two-fifty four earned its laurels by both speed and reliability. As this drawing shows, the engine follows orthodox design apart from employing four valves per cylinder - unusual when the bore measurement is only 44mm. With a stroke of 41mm the engine is slightly over-square.

Drive to the twin overhead camshafts is by a train of spur gears located between the inner cylinders. The cams operate direct on inverted cups which fit over the coil valve springs and bear in the cam-box housing.

Cast-iron skulls form the combustion chambers in the light-alloy head and the chambers are of pent-roof shape closely matching that of the piston crowns as illustrated.

Unlike the 1960 carburettors which had guillotine slides, the instruments used in 1961 have cylindrical slides; the float bowls are underslung and surround the mixing-chamber bases to help immunise the fuel-air ratio from high lean angle variations.

The engine produces about 45bhp at its peak of 13,500rpm.