Perhaps more than any other decade, the 1920s represented a step change in motorcycle evolution. As the ’20s began, ‘flat tank’ designs with their origins in the bicycle industry were the norm; by 1930, the public wanted sleek, saddle-tank designs, and manufacturers obliged.
The motorcycle industry’s approach to design and production was changing, too. Separate oil tanks allowed crankcases to become more compact and frames shorter. Fuel and ignition systems became more reliable, and the Amal Type 6 carburettor and Lucas Magdyno, both manufactured in huge numbers, would change little in the next quarter-century. By the end of the decade, motorcycles were more comfortable, easier to control and more reliable – trends which would continue in the years that followed.
Frame Construction
Prior to the introduction of the I-section top frame member in 1930, the BSA Sloper’s frame, like that of most motorcycles, was constructed from steel tubes and malleable cast iron lugs. The lugs were fitted to the tubes and then broached and pinned in position before being brazed in place. The photograph, taken down the saddle tube, shows the pin coming in from the 9 o’clock position, and the curled edges of the hole show that the tube was broached rather than drilled. The bright bar running from top to bottom is the nickel-plated stud on which the gearbox is mounted. Traces of braze can be seen in the photograph showing the lug stamped with the machine’s frame number.
Piston and Bearings
An unusual feature of the main bearings – to modern eyes – is the use of a steel pin pressed into the cheek of the flywheel, which engages in a groove in the bearing’s inner track. Metallurgy and machining tolerances were not what they are today, and this mechanical locking prevented the track spinning on the shaft and leading to rapid wear.
The gudgeon pin is a conventional floating fit in the little end of the piston, but its method of location belongs in the distant past. Instead of the normal arrangement of circlips to retain the gudgeon pin, brass ‘buttons’ are pressed into the open ends, as shown. These prevent the steel pin from damaging the bore in the event of contact.
Sealing
In an era of exposed valve gear, external oil sealing was not a priority! Where sealing was found, it was not to keep oil off the driveway or rider, but was used to preserve pressure, for example between a static feed and a rotating crankshaft, or to prevent the ingress of dirt, as in wheel bearings. On the Sloper, both of these applications use compressed felt material, while the clutch push rod is sealed with leather washers.
Cabling and Connectors
Electrical cable on 1920s vehicles was rubber coated and black in colour. Identification was by coloured rubber sleeves fitted close to the component or switch terminal, and an outer harness, where used, was woven from loose braided fabric. Colour coding of individual cables, to Lucas’ own colour code, began to appear in the mid-30s and took the form of a laquered cotton braid over the rubber core.
The familiar Lucas ‘bullet’ connectors – or, more correctly, ‘snap’ connectors, did not appear until well into the 1930s. Wiring in the 1920s went directly from the component to the battery or switch; connectors were little used and, where they were, took the form of a threaded brass tube within which the bared ends of the wires were clamped, covered by an insulating rubber sleeve. Cables were secured to the frame or mudguard stays by metal ‘buckle’ clips.
While strictly ’30s Technology’, the Lucas H52 headlamp, introduced in 1930, has an unusual feature. Instead of connecting the switch, ammeter and head/pilot bulb terminals with wire, the brass fingers of a ‘spider’ mounted between the back of the switch and the headlamp, bear against the various terminals.
The uppermost blades bear against the terminals of the ammeter in the headlamp shell, while the spring-loaded pins on the back of the bulb holder make contact with terminals on the spider when the lamp is assembled in the headlamp shell.
The design did not prove durable and the DU/MU series of headlamps, introduced in 1934, reverted to conventional wire connections.
Paintwork
In the 1920s, cycle parts were ‘dipped’ – immersed in a bath of paint, usually black enamel, and hung up to dry. There was no evidence on the Sloper of primer or undercoat and the original paintwork, although of good quality and hard wearing, looks ‘thin’ by modern standards. To replicate this, I avoided modern ‘high build’ primers and powder coating; Precision Motorcycle Paintwork applied an acid-etch primer, a light coat of grey primer/surfacer and finished with black two-coat epoxy, which gives a gloss finish close to that of the original enamel.
Decorative Finishes
Bright finished parts were generally nickel plated, which has a warmer lustre than the chromium plating which was beginning to appear and would replace nickel in the following decade. Nickel continued to be used beyond the 1920s as a substrate for chrome, but in the ’20s bright and dull nickels were used as a decorative finish to protect and enhance the appearance of controls, levers and detail parts. Most plated parts on the Sloper are bright nickel plated but items such as the carburettor body and float chamber have been finished in dull nickel, to match traces of the original plating.
The Sloper - a 1929 BSA Motorcycle Returns to the Road 