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Douglas Models D.T.5 500cc and D.T.6 600cc O.H.V.

Announcements about the availability of the new model were made during mid-1928, with a footnote to the effect that a 350cc version would eventually be available. But it never materialised and instead there was a 600cc version. The standard 500cc model sold for £85 and produced about 27bhp, but for an extra £10, a works-tuned engine could be fitted which would boost the power output to 32bhp. These engines were prepared under the personal supervision of Freddie Dixon.

One of the earliest orders came from AJ Hunting, the managing director of International Speedways. Confident that the new sport was about to enjoy a tremendous popularity, as it had done in his native Australia, he ordered a batch of 50 machines to help reinforce his position as one of the top promoters.

Someone once described the Douglas as looking fast even when it was standing still, and it is doubtful whether such a purposeful looking machine had ever been made before. Though fitted with large diameter wheels (28 x 2 1/8), it was still lower than most machines by virtue of the swan neck which dropped its top tube below the steering head. The frame itself was of duplex tube, full cradle type strengthened in the vicinity of the Gearbox to accept the high torque loadings.

It was fitted with a centre spring front fork of Douglas design having about 1? movement and controlled by an external Andre damper. The handlebar carried the essential controls, a lever throttle, air and magneto advance/retard lever and the all important ignition cutout button. If the throttle was opened too quickly, there was a tendency for the engine to hesitate, so the use of the cutout in conjunction with a fixed throttle opening often provided a better means of control when riding in anger.

Of 499cc capacity, the engine was of the long stroke (62.25 x 82mm) type with its exposed overhead valve gear operated by tubular steel pushrods. The crankshaft was of particularly ingenious design, taking the form of a one-piece forging running on ball races.Its shape was such that the connecting rods could be threaded over the outer webs and then the rollers inserted, held in their semi-circular light alloy cages with grease. The detachable balance weights could then be fitted and locked into position with cotter pins. Some of the very early models used uncaged rollers in the big end assemblies, relying instead on rings riveted through the big end eye to keep them in position. The crankshaft assembly is quite small and yet immensely rigid, a virtual trade mark at the heart of any Douglas machine.
The Engine.
CAPACITY: 496cc. Bore 62.25mm. Stroke 82mm. Compression Ratio with Douglas Pistons 7.5 to l using petrol/benzole fuel. 9 to 1 using PMS2. Castor based lubrication

CARBURETTOR: Choke 15/16ths-inch. Twin carbs operated by twist grip via cable and minute connecting arms.

MAGNETO: BTH or Bosch. Points time to open 45 deg. before TDC.

Inlet. Opens 10 deg. before TDC - Closes 50 deg after BDC
Exhaust. Opens 63 deg before BDC - Closes 20 deg after TDC

PLUGS. 18mm in size with KLG and other make equivalents.

The single camshaft ran above the crankshaft assembly in ball races, the pushrods being arranged above the cylinders. These and the detachable cylinder heads were retained by through bolts direct to the crankcase, the heads being of the hemispherical type with large diameter valves and highly polished ports. Rocker carriers were cast integral with the cylinder heads, the steel rockers running on bearings of a similar material. Each had at its extremity a finned alloy casting containing light grease that soaked into wicks threaded through the centre of each rocker. The engine was mounted longitudinally in the frame, with the gearbox immediately below the nose of the saddle so that the wheelbase could be kept as short as possible.

Carburation was by two 15/l6in racing Amacs, which drew air from a gauze protected airbox on the offside of the machine. Like the case which protected the exposed rocker gear from flying cinders and other debris, the gauze provided an effective screen without unduly impairing free flow of air. To aid synchronisation of the carburetters, their throttle and air slides were joined by flat links attached to either side of a rotating rod that pierced the airbox. This was actuated by a single cable, thereby obviating the need to adjust individual cables.

A racing magneto was mounted on top of the crankcase and lubrication was effected by a hand-operated pump in the tank top. The three speed gearbox had no clutch and acted virtually as a countershaft. there was a gear change gate and lever on the offside of the petrol tank but racing was conducted in top gear giving direct drive. Changes to overall ratio were made by varying the size of the sprockets to suit the requirements of the different tracks, none of which were identical in size. With no clutch, a push start was essential, the machines then having to circulate around the track until they were all in line as they approached the start.

Although the front forks were limited by design to about 3/4inch movement, the Douglas was a really outstanding machine in the steering department. It was not an unheard of thing that after a rider fell off, the bikes would straighten themselves out and continue until the safety fencing stopped them.

FRAME: Duplex cradle Pattern designed to suit and give ample clearance for Dirt Track conditions.
FORKS: Built to suit above Frame with central spring suspension and one-piece steel connecting links fitted with shock absorber and steering damper.
TANK: Cellulose Finnish. Capacity 2 gallons.
WHEELS: Specially built. Steel hubs serrated both sides of back wheel so that the rear sprocket can be fitted when wheel is reversed. Also fitted with heavy axles and taper roller bearings.
GEARBOX: Three speed separate unit with specially stiffened up mainshaft. Gear control by lever and gate attached to frame.

The Dirt Track Douglas reigned supreme for almost three years and such was its success that in 1929 alone something like 1,200 were sold.

Before the Douglas era ended, attempts were made to regain the initiative by developing a short stroke model of 68mm x 68mm with a camshaft timing of 40 deg before TDC. The wheelbase was shortened by 2/3rds of an inch yet the weight was not reduced. The old fashioned gearbox was retained as were the front forks - unlike other makes which were switching to the plunger type.

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