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Sunbeam S7 500

Make Model.	Sunbeam S7 500, S7 500 De Luxe
Year	1946 - 56
Production	7 658 units (inc. De Luxe models)
Engine	Parallel twin, SOHC
Capacity	487 cc / 29.7 cu in
Bore x Stroke	70 x 63.5 mm
Compression Ratio	6.5:1
Cooling System	Air/Oil cooled
Lubrication	Wet sump
Starting	Kick
Electrical system	6V
Max Power	18.6 kW / 25 hp @ 5800 rpm
Clutch	Single plate, dry
Transmission	4-Speed
Final Drive	Shaft
Frame	Twin cradle, steel
Front Suspension	Telescopic, one way damped
Rear Suspension	Plunger, spring loaded saddle
Front Brakes	Drum, leading shoe, 203 mm / 8"
Rear Brakes	Drum, leading shoe, 203 mm / 8"
Wheel, Front	4.50 x 16"
Wheel, Rear	4.75 x 16"
Wheelbase	1400 mm / 57"
Dry Weight	195 kg / 430 lbs
Top Speed	121 km/h / 75 mph
Source	Sump Publishing

WHEN THE 1946 SUNBEAM S7 was unveiled to the post-war biking public, it was one of the most technically advanced motorcycles of its age. With its all-aluminium, "unit-construction", single overhead-camshaft, horizontally split engine, it also boasted shaft drive, a smooth power delivery and numerous other technical refinements that ensured it was going to be a massive hit.

Or so felt BSA which had acquired Sunbeam in 1943.

In fact, the bike turned out to be something of a sales flop and was (arguably) one of the most woefully under-supported acts in the history of British motorcycling. It could have been great. But instead, it was merely adequate.

Other problems with the S7 prototypes included stripping of the underslung worm gear final-drive—that in turn is said to have led to swingeing cuts in engine power, the concomitant of which was a fairly mediocre (and struggling) 75-mph performer in an age when riders were pushing hard for the magic ton.

However, according to David Holyoake of Sunbeam specialist Stewart Engineering, there was never anything wrong with the worm final drive which, over five decades, has proved to be very reliable in service and maintenance light; a claim borne out by the direct testimony of many other Sunbeam S7 and S8 owners who have had no trouble whatsoever in tens of thousand of miles. However, the correct AG140 oil is crucial.

Meanwhile, oil capacity for the wet-sump engine was, at just three pints, always marginal leading to overheating with the very real risk of engine casting distortion and/or seizure (an aftermarket extended oil sump is available from Stewarts and is advised).

But the handling, with its one-way damped telescopic front fork (no damping on early models), plunger rear suspension and 16-inch balloon tyres, left much to be desired. And words such as ‘wallow’ and ‘weave’ were, with some justification, quickly associated with the mount.

Meanwhile, the 8-inch single leading shoe brakes, although not bad for the performance, would never be described as particularly impressive. Moreover, a bulky machine like this was always likely to find both an ally, and an enemy, in ordinary wind resistance.