Embodying many unique features which will appeal to the sporting rider, this model is produced as the most up-to-date design in high speed single-cylinder machines. A high standard of mechanical silence has been attained, and the subdued exhaust note gives no indication of the power which is available when the throttle is fully opened. The low riding position and the rigid frame inspire a feeling of security, at speed.
Inclined single cylinder, 4.93 h.p., 80 mm. x 98 mm. bore and stroke, with oil sump. Detachable head with two large overhead valves mounted at 90° operated by enclosed roller bearing rockers. Special section alloy steel push-rods working in dust-tight tubes, made telescopic to allow adjustment of return springs.
Two springs per valve with adjustable rocker return springs. Silent timing gear embodying flat base tappets and cams with wide working faces are driven, separately from the crankshaft. Steel fly-wheels with built-in shafts running on generous ball and roller bearings. Double row roller big-end bearing fed with oil direct from pump through holes drilled in mainshaft and fly-wheels. Low compression piston fitted as standard. High compression piston can be fitted if specified, or supplied as an extra with the tool kit. Two spare valve springs are supplied.
Large plated silencer, more than six times the engine capacity, and conforming to Brooklands regulations.
Carburettor and Ignition.
Special Amac carburettor. Lever, or twist grip throttle control, if specified. An air cleaner is fitted. Special racing magneto, gear-driven and mounted behind cylinder on crankcase. Twist grip control is extra.
Oil sump integral with crankcase ; capacity, 3 pints. Submerged gear pump driven by skew gear from main-shaft, with output controllable from saddle by valve on suction side. Oil supplied direct to big-end bearing through special oil-ways. Visible tell-tale is fitted to output side of pump. Surplus oil in crankcase returned to sump by fixed scraper acting on flywheels. An oil-level indicator of dipper type is provided in filler elbow. For long distance racing a seat tube oil tank, ½ gal., capacity, with heel-operated force pump can be supplied at an extra charge.
Special system of primary chain lubrication. Hubs, fork links, etc., are fitted with grease-gun nipples.
Transmission and Clutch.
Front chain, ½ in. x .305 in., totally enclosed in case. Rear chain, 5/8 in. x 3/8 in., protected by an efficient guard. A cam-faced cush drive is fitted to the engine shaft. The clutch is of large size, contained in a large chain wheel. Floating dry-plate type. Controlled by lever on left of handlebar, with a large diameter cable.
B.S.A. three-speed, with special pivot mounting for chain adjustment. All gears constantly in mesh and increased in width to give additional strength. The kick-starter mechanism is enclosed in the gear-box. Inclined change-speed lever on right side of tank. A close, or specially wide, ratio gear-box can be fitted if specified. For gear ratios, see page 69.
Duplex loop frame of great strength and rigidity, built from steel lugs and weldless tubing. Exceptionally low riding position (minimum obtainable 24 in.). Fitted with special lugs to take B.S.A. sidecars. Head fitted with B.S.A,. steering damper. Quickly detachable luggage carrier extra. Ground clearance, 4¾ in.
Spring Fork and Handlebar.
The spring fork is special B.S.A. type with barrel spring and large shock absorbers. A special sporting type handlebar mounted behind steering head with rubber grips.
Saddle type tank for fuel only, with large diameter filling, orifice, supported by brackets brazed to frame. Capacity, 2¼ gals. Flexible tubing to carburettor. Adjustable rubber knee-grips are fixed to tank.
Wheels and Tyres.
Wheels: heavy gauge rims, 21 in. x 2½ in., enamelled. Taper roller bearings to both hubs. Dunlop cord tyres, 26 in. x 3.25 in., wired-on, or 27 in. x 2.75 in., wired-on, can be fitted if specified. 27 in. x 4 in. wired-on balloon tyres, with special front mudguard, can be fitted as an extra.
Both brakes are internal expanding type, 7 in. diameter, front-operated by toe pedal on left of machine (or hand-operated if specified), rear by toe pedal on right of machine. Dual hand and foot operation of front brake, extra. Grease-gun nipples are fitted to the cam spindles.
Saddle and Mudguards.
Terry spring seat, De Luxe type. Leckie super sports or Brooks supple seat supplied if specified. Rear mudguard, 6 in. wide of plain section ; front, plain blade sports type, or with side wings if specified.
1. More pages from this book are available at Craig Howell's Flickr pages. Craig Howell
Source: Book of the BSA by Waysider. Pitmans, 1928.