ENGINE STRIP: GOLD WING

Honda's sophisticated and weighty design milestone

The component content of Honda’s GL1000, displayed for your delectation

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In 1972, Soichiro Honda had given his research department, led by Soichiro Irimajiri, a free hand to create the ultimate big, longdistance motorcycle. The result was the M1 prototype (or AOK) with a 1470cc water-cooled flat-six. A longitudinal crankshaft and horizontally-opposed pistons were chosen to keep weight low for manageability, smooth power delivery and to suit the M1’s borrowed BMW R75/5 gearbox and shaft drive. The experiment undoubtedly influenced the layout of the subsequent GL1000, designed for production under project leader Hisaho Nozue. Rather than being wholly radical, the power unit used proven features, brought together for the first time in a two-wheeler.

The engine revealed here is from 1976. Unusually for a Japanese unit, it has a vertically-split crankcase. The crankshaft is on the gasket-less central joint’s axis, in the upper part of the case, with the five-speed transmission underneath it. The cylinder bores are integral with the crankcase casting and contain shrunk-in iron liners. The crankshaft has three split-shell main bearings, supported by half-housings in substantial walls in the right-side crankcase, completed by bolted-up bearing caps. In addition, a ballrace in the crankcase rear wall supports the rear end of the shaft, which rotates clockwise as seen from the front of the unit.

Two pulleys for the camshaft belts are keyed in tandem to the front of the crankshaft ahead of the front crankcase wall. Discs adjacent to the pulleys guide the toothed belts, which drive larger camshaft pulleys with 2:1 speed reduction. Each belt is tensioned on its smooth side by an adjuster pulley, bearing on the lower run of the front belt and the upper run of the rear one. Thin metal plates behind the belts insulate them from engine heat and a fourpiece cover encloses the drives in an oil-free compartment.

Parallel to the crankshaft, the four-lobe camshafts are sited centrally on the two cylinder-head castings. They bear partly in the head metal and partly in bolted-down holder castings that also carry the rocker spindles above and below each shaft. Rockers on the upper spindle open the inlet valves, supplied from above, while the lower pair arranged between them open the exhaust valves into their downward-facing ports. Light coil springs on the spindles laterally locate the rockers, which run directly on their spindles. The valves are conventional, with double coil springs topped by retainers held to the valve stems by split collets. Shrunk-in guides are fitted with oil seals and the combustion chambers contain hard rings for the valves to seat on, while the spark plugs thread into the head from above, between