De­vel­op­ment of the sin­gle

Classic Dirtbike - - Hedlund -

A sep­a­rate fea­ture on Nils Hed­lund proved he was still work­ing away with the four-stroke en­gine con­cept and had pro­duced a com­pact unit con­struc­tion ver­sion of his dohc mo­tor for use in mo­tocross and speed­way. Us­ing a 99mm stroke, Hed­lund of­fered two bore sizes of 80mm and 88mm to give 498cc and 602cc re­spec­tively, with the larger mo­tor be­ing in­tended for the 750cc class in­tro­duced around 1968.

For MX en­gines a three-speed clus­ter was all that’s needed thanks to the torque pro­duced by the lower com­pres­sion and mild cam tim­ing, as op­posed to the more rad­i­cal spec of the speed­way con­fig­u­ra­tion which would have only a coun­ter­shaft in place. With plenty of ex­pe­ri­ence fix­ing en­gines in pad­docks, Hed­lund was well aware of the need for sim­plic­ity in the de­sign of his en­gine and it bris­tled with fea­tures to sim­plify the work of main­te­nance. For in­stance, the whole en­gine is bolted to­gether us­ing the same size Allen bolts – one key fits all – and all tim­ing de­tails were marked on the fly­wheel and vis­i­ble through an ac­cess plug in the crank­case.

Op­er­at­ing the cams was a chain from the crank to a sprocket on the in­let cam, be­hind which was a gear cog that con­nected to the ex­haust cam and both cams op­er­ated a rocker sys­tem to open the valves. As on the ear­lier en­gine, a Stefa ig­ni­tion unit with an ex­ter­nal HT coil was op­er­ated by the ex­haust cam. To en­sure the freest ac­tion ev­ery­thing in the cylin­der head ran on nee­dle roller bear­ings, in fact such bear­ings were used through­out the en­gine apart from one ball bear­ing on the drive side crank­case. Hed­lund of­fered two types of oil­ing sys­tem – a sim­ple os­cil­lat­ing pump for the to­tal loss speed­way unit and a Tri­umph plunger pump for the MX ver­sion. Oil was car­ried in the gear­box cast­ing and for the MX ver­sion a small oil fil­ter un­der­neath the unit makes sure it is clean be­fore it heads for the en­gine. Con­nect­ing the crank­shaft to the gear­box is a sim­plex chain which ran through a Nor­ton/ ESO type clutch and the fi­nal drive sprocket was on a splines on the main­shaft and held in place by a cir­clip. To save weight, Elek­tron was used for ma­jor cast­ings with the crank­case be­ing ex­cep­tion­ally stiff thanks to rib­bing and the at­trac­tive gold an­odized en­gine weighed in at 88lb. All in all a very smart en­gine unit.

ma­chines but look­ing at con­tem­po­rary weights it was still lighter than Match­less’ G80. How­ever, de­spite his weight mis­giv­ings, Bash­ford rock­eted up Hawk­stone Hill and ac­com­plished the drop back down with ease, im­pressed by the no-fuss power de­liv­ery of its DOHC en­gine, a valve ar­range­ment more com­mon with vi­o­lent power de­liv­ery on road race ma­chines. At all times this unit be­haved im­pec­ca­bly and there wasn’t a trace of mega­phoni­tis at any point in the range yet snap­ping open the throt­tle un­leashed the horse­power in shov­el­fuls.

Back in the pad­dock, aching and sweat­drenched, Mike ad­mit­ted to feel­ing punch­drunk and throt­tle-happy from his ex­pe­ri­ence but com­posed him­self to talk to Tib­blin and Hed­lund about the bike.

It was one of three mod­els built though a fur­ther ten frames were made and our fea­ture bike could well have been from that later batch. The en­gine, a dou­ble over­head cam unit had a bore and stroke of 80mm x 90mm which, ac­cord­ing to Mo­tor­cy­cling, is 499cc ca­pac­ity but our cal­cu­la­tion sug­gests this gives 452cc, though a 99mm stroke with 80mm bore will give 498cc... a mistype per­haps. Though the crankcases owe their ba­sic de­sign to the Al­bin mo­tor, the rest is pure Hed­lund. A chain run­ning in a tun­nel cast in the cases, bar­rel and head con­nected the crank­shaft to the in­let cam, which in turn is con­nected to the ex­haust cam by a gear train. In­stead of the cams run­ning di­rectly on the valve stems they used a rocker sys­tem sim­i­lar to that of BMW rac­ers. Pow­er­ing the spark plug was a Stefa fly­wheel mag­neto run­ning off the ex­haust cam and feed­ing fuel in on the 1964 ma­chine was a Bing car­bu­ret­tor.

To keep weight as low as pos­si­ble, the half-gal­lon ca­pac­ity oil tank is mounted on the en­gine plates above the BSA scram­bles gear­box and the petrol tank is of light­weight glass fi­bre. Han­dle­bar con­trols were sin­gled out for spe­cial praise, be­ing Magura and su­perbly made. Also praise­wor­thy were the forks with Ce­ri­ani yokes and Nor­ton legs. Less im­pres­sive was the lo­ca­tion of the ex­haust pipe and gear lever though Mike al­lowed this suited Tib­blin and his di­men­sions but not Bash­ford, still as he said the bike wasn’t built for him.

Rear hub is likely CZ un­less any­one can say dif­fer­ent, CZ hubs are pop­u­lar with specials. Note also the drillings for pull ad­justers rather than snail cams for the wheel. In­side here is a small Stefa ig­ni­tion unit which pro­vides ac­cu­rate sparks. Front...

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