Development of the single
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...