Where to Begin …

Weighing up what you’ve got is often the starting point for a restoration, and detective work and research are as important as spanners. It was evident that most of the rear end of SM7741 was not original, nor were the magdyno or headlamp; everything else, though, looked correct and untouched – even down to the cables and exhaust system. Some areas were brush painted black, and once-plated surfaces had been given a thick coat of silver paint – but this probably saved what lay underneath.

The frame number P10975 and engine number J633 are correct for a 1929 model, according to machine dating information published on the BSA Owners’ Club website. So, with solid fundamentals to work with, planning the task could begin.

A second question, then – what did I want to achieve? Usable ‘oily rag’; restored and rideable, ‘as it left the factory’ or a concours showpiece? My objective has been to build something which is as close as possible to the machine despatched from BSA’s Smallheath factory in June 1929, while using original parts wherever I could. Motorcycles are built to be ridden, though, so I have used a pattern tail lamp which includes a stop light – these only became an obligatory fitment from January 1936 in the UK. Not entirely original, but probably easier than explaining to your insurance company why the car behind ran into your back…

A nice feature of BSAs of this era is that almost every part is stamped with the stacked rifles logo and the model type, in this case an ‘S’. Fork links, gear wheels, petrol cap, even mudguard brackets – if it was big enough to stamp, BSA did so. One has to feel sorry for the employee given the job …

Frame, Forks & Wheels

Although SM7741 has clearly pulled a sidecar for much of its life, the frame and forks proved to be straight and undamaged. The wheel bearings, however, were completely worn out with cracked outer tracks, and some simply fell apart on removal. The original bearings incorporate a wide collar and replacements were unavailable, but I sourced new tapered roller bearings of the same internal and external diameters and width, and turned up collars in the lathe to give the correct spacing.

The steering head bearings are conventional loose ball races but the upper track is a vintage-style ‘top hat’ arrangement which clamps in the girder top clip. All four races were worn but I found good replacements at an autojumble and reassembled them with new balls. The girders themselves were fitted with new friction discs and rebuilt with new bushes by Jake Robbins Vintage Engineering – which cured the half inch of fore-and-aft movement at the wheel spindle!

At the back end of the machine, the rear stand crossbar had been broken at some time in its life and a previous owner had simply bent the two legs inwards and welded it back. This might have worked – but the bent leg now fouled the nuts securing the rear sprocket…   A replacement crossbar was welded into place. Finally, one of the snail cam adjusters for the rear wheel spindle was broken, so a new one was carefully shaped with a bench grinder and files.

With the paint blasted off the wheel rims, ‘Dunlop 1928’ was exposed to the light of day. While the spokes were beyond use, the original rims came up beautifully and were laced back onto the hubs with new black-painted spokes and nickel-plated spoke nipples.

BSA used a curious design of long tubular grease/oil nipples. A few were missing and I could not find replacements; fortunately a skilled watchmaker friend turned some up for me in his watchmaker’s lathe, complete with internal balls and springs!


The Sloper’s side-valve engine is a traditional British design of the period, with the crank supported in substantial roller and ball races, geared timing drive and oil carried in integral compartments in the front and rear of the crankcase. The lubrication system, though, was advanced for the 1920s – the integral oil pump is driven off a skew gear from the timing side mainshaft and is fully immersed, with oil fed to the big end via a quill into the timing side mainshaft – BSA claimed that this gave the big end ‘up to 250 times as much oil’ as a conventional system. Piston lubrication is by oil splash from the crankshaft. Less advanced was the use of a ‘blind’ barrel, where the head and barrel are a single casting. The magneto – or magdyno where lights were fitted – is driven by a train of gears on the timing side of the engine, and a cam-type shock absorber on the drive side mainshaft smooths out the power delivery. A quaint feature of the cylinder head is the brass priming tap, which allows a little petrol from the tank to be dribbled into the cylinder head, as an aid to kicking over and starting on winter mornings.

The main bearing tracks were corroded, so all three bearings were replaced. The big end had also suffered, so the flywheels were separated, the crankpin reground to 0.010” undersize and new oversize rollers fitted by T&L Engineering. Damaged areas and stripped threads were repaired by alloy welding and thread inserts. The piston was in surprisingly good condition, so I was able to get away with new piston rings and a light hone to assist bedding in. There is no provision for lubrication of the valve stems on side-valve models and, inevitably, these were worn. New valves, guides and springs were fitted and the seats lightly re-cut. The original priming tap was damaged but eventually the correct item turned up on ebay!


Pulling a sidecar can give a motorcycle’s transmission a hard life. In the gearbox, worn bushes, bearings and a broken tooth on one of the pinions told a story of hard use, while worn drive sprockets and an assortment of unmatched springs in the clutch all pointed to infrequent maintenance. The primary chain was stamped ‘BSA’ on every link – could it really be the original, in use since 1929? Gearchanging was made worse by a corroded and bent clutch pushrod, so a replacement was made from 1/4” silver steel rod. A new ball bearing was fitted to the gearbox output shaft and a donor gearbox from an autojumble provided a source of good pinions and bushes. These gearboxes are lubricated by semi-fluid grease – or, often, a mixture of oil and grease – and once the gearbox was correctly positioned in the frame, lubricant could be added until it reached the level plug.

Damaged gear pinions are not unusual in early Sloper gearboxes. The gears were case-hardened and prone to breakage; later four-speed gearboxes used better steel which overcame the problem.

There is no provision for spring adjustment of the clutch pressure plate on these machines, so it is vital that the springs are of equal compressed length. New springs of the correct type were sourced and fitted. I retained the original gearbox and rear wheel sprockets, with their stamped BSA logos, but had them re-toothed by John Hemming Engineering before fitting new chains front and rear.

Fuel and Sparks

When I bought SM7741, a 1950s Lucas MO1 Magdyno was fitted. These are a superior instrument to the earlier MBD three-brush type but, as my aim is originality, I sourced the correct MDB1 magdyno and had it overhauled. Unlike the later magdynos which have a large fibre gear with a friction clutch and a small steel dynamo gear, on early instruments the smaller dynamo drive gear is made of fibre and is prone to stripping. A popular mod at the time was to cut through the alloy cover so that the gear could be replaced without removing the the complete magdyno and losing the ignition timing. My MDB1 had suffered this fate, so some careful alloy welding was required to restore the cover plate.

Like most engines of its era, the side-valve Sloper is fitted with an 18mm spark plug. These plugs can be dismantled; they were prone to fouling and regular cleaning was necessary – usually at the roadside! A period ‘Lodge’ spark plug cap completes the picture.

The brass Type 6 Amal carburettor is a simple design which remained essentially unchanged through to the mid-1950s. Wear on the slide was dealt with by Autocycle Engineering, who bored the body and supplied a matching oversize slide. New needles, jets and gaskets were fitted to complete the mechanical restoration, while a combination of dull and bright nickel plating was carried out by CE Design to restore its appearance.

Lights and Wires

Motorcycles of the 1920s did not have much in the way of wiring. In fact, where fitted with a bulb horn and acetylene lamps – and therefore with no need of a dynamo or battery – they had none at all! A ‘lighting set’, comprising a battery and battery carrier, a magdyno in place of the basic magneto, cabling, a rear lamp and a headlamp and switch, usually with an ammeter to keep a check on things, was a very expensive extra. At £5-15-0 (£5.75) it added 10% to the cost of a new top-of-the-range BSA in 1929. At the other end of the scale, for a ride-to-work 250cc machine costing around £36, a further £5-15-0 for a lighting set was a huge additional cost and explains why some workaday machines retained gas lighting into the 1930s.

Lighting sets, where specified, could be factory fitted or were often fitted by the dealer. Machines might be bought without lights or with cheaper acetylene lamps, and an electric lighting set fitted later when the owner could afford it. As a result, the machine and its lights could often be from different years and the headlamp on SM7741 today is a 1930 Lucas H52 type. When I bought the Sloper it was fitted with a very early dipswitch and later ’30s style wiring (see ’20s Technology), which suggests that lighting was probably fitted during the 1930s.

There was no obvious place to fit a battery on SM7741. Lucas supplied an adjustable bracket with their battery carriers, but the popular location on the saddle tube obstructed the gear linkage on this machine. I have fitted the battery carrier where it looks right, using the sidecar mounting hole in the frame below the saddle, and have added a simple bracket on the frame tube for support.