Honda's so­phis­ti­cated and weighty de­sign mile­stone

Classic Bike (UK) - - Contents -

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


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

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