(De Luxe 7.70 below)
Although the present trend is to make all large twin-cylinder machines almost solely passenger vehicles, the light 7.70 h.p. model provides a sweet, easily-handled solo mount. Riders will find a fascination in the big reserve of power and feeling of road supremacy coupled with the smooth running that can only be obtained from, machines of the twin type. It is perfectly adapted for use with a full touring outfit which may be required to take heavy loads and yet maintain a high average speed.
Twin-cylinder, 7.70 h.p. (770 c.c.), 76 mm. x 85 mm. bore and stroke. Mainshaft mounted on ball bearings. Aluminium alloy pistons. Big-end bearings fitted with rollers.
Large capacity of efficient design, mounted on chain-stay.
Carburettor and Ignition.
Amac carburettor. A special carburettor fitted with air filter can be fitted as an extra. Magneto driven by adjustable chain in dust-proof case.
Gravity feed to mechanical pump, then to sight feed on tank, and feeding to crankcase. Hand pump is also fitted for emergency use. Oil is supplied to front chain by depressing spring by-pass valve on timing case. Hubs, fork links, etc., fitted with grease-gun nipples.
Front chain, ½ in. x .305 in. Rear chain, 5/8 in. x 3/8 in. Front drive totally enclosed in chain case, and rear drive protected by an efficient guard. Cam-faced cush drive fitted to engine shaft. For gear ratios see page 69.
Clutch and Gear-box.
The clutch is contained in large chain wheel. Floating dry-plate type, indestructible. Controlled by lever on left handlebar with large diameter cable. The gear-box is B.S.A. three-speed. All gears constantly in mesh and increased in width to give additional strength. Kick-starter mechanism enclosed in gear-box. Screw adjustment of box position for accurately setting chain tension. Change-speed lever on right side of tank.
Suitable for solo or sidecar work, with integral sidecar lugs. Head fitted with B.S.A,. steering damper. Weldless steel tubing and forged steel lugs are used throughout. Head lug of continuous type giving great strength. Strong carrier fitted.
Spring Fork and Handlebar.
B.S.A. type spring fork, fitted with barrel spring and shock absorbers. Adjustable handlebar, mounted behind steering head with forward bend at centre, and back sweep at ends. Long rubber grips.
Tapered type, supported by brackets brazed on to the frame. Filler caps, unleakable, secured with bayonet joint and chain. Combined priming cock and petrol tap. Capacity: petrol, 17/8 gals. ; oil, 3 pints.
Wheels and Tyres.
The wheels have heavy gauge rims, 19 in. x in., enamelled. Taper roller bearings to both hubs. Dunlop cord tyres, 26 in. x 3.25 in. wired-on, or 27 in. x 4 in. wired-on balloon tyres with special front mudguard can be fitted as an extra.
Both internal expanding type, 7 in. diameter. Front operated by lever on right handlebar, rear by heel pedal on independent fulcrum on right side of machine. Grease-gun nipples fitted to the cam spindles.
Leckie spring seat. Terry spring seat, De Luxe type, or Brooks supple seat B. 190/2, if specified.
Rear, 7 in. wide of plain section. Front fitted with side wings of ample dimensions.
Pyramid rubber covered, set at comfortable angle; or footrests if specified.
The big reserve of power, perfect running and absence of mechanical fuss which are features of this machine, have made it extremely popular with tourists and utility riders alike.
The road performance, when fitted with a big touring sidecar, compares very favourably with that of the average light car.
This model is designed expressly for heavy sidecars (such as the B.S.A. tandem and 2/3 seater models), and is fitted with quickly detachable and interchangeable wheels. Only those parts of the specification which differ from the preceding model are here dealt with.
5/8 in. x 3/8 in. roller chains front and rear. Front drive totally enclosed in chain case, rear drive protected by an efficient guard. Cast aluminium chain cases extra. A cam-faced cush drive is fitted to engine shaft. For gear ratios, see page 70.
Touring or sporting pattern, both types adjustable.
Petrol, 2½ gals. Oil, 3 pints.
Heavy gauge rims, 19 in. x 3 in., enamelled. They are quickly detachable and interchangeable. Taper roller bearings to hubs.
Dunlop cord, 26 in. x 3.25 in. wired-on, or 27 in. x 4 in. wired-on balloon tyres can be fitted at an extra charge.
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.