A SHORT HISTORY OF AN ENGINE
BSA’S parallel twin was both inspired by and intended to compete with Triumph’s Speed Twin. The latter was – as you know – famously designed by Edward Turner. What you may not know is that the same Mr Turner worked for a while on the design of BSA twins while employed by that company for a couple of years from 1942. If you’re feeling enthusiastically anorak, Signs Of Turner can be found here and there in the early 500 twin engine, the A7, including Triumph-style screw-in rocker adjustment covers in the rocker boxes. And the original engine had two rocker boxes, like the Triumph, but unlike the A10. Happily, the fiddly round covers over the rockers were soon replaced with a wider window – a move which Triumph twins failed to manage until 1972.
The development history of the pre-unit BSA twin engines would fill an article all on its own, but Hopwood’s systematic and neat work on the original engine was excellent, despite sticking pretty closely to the original basic idea. Like Turner’s Triumph, the BSA engine is a 360° parallel twin with a pair of overhead valves doing the gassy stuff up top. Unlike the
Triumph, which has a pair of camshafts, one for inlet, the other for exhaust, the BSA uses a single shaft to perform both duties, an arrangement which is also familiar to owners of Hopwood’s Norton twins. In the BSA engine, the camshaft lives behind the cylinders, whereas on the Norton it’s in front. Given that part of the reasoning behind the single cam was to remove potential cooling problems caused by the Triumph’s external exhaust pushrod tubes, the Norton cam placement seems a little perverse. However, the BSA block has the pushrod tunnel cast-in at the back, with careful thought given to air flows.
The head is very neat too, although generations of BSA
owners have cursed the onepiece rocker box and its fiddly pushrod location.
BSA wanted their engine to be mechanically quiet, so while the camshaft is driven by gears, the dynamo is spun into charging the battery by a short chain, running in its own compartment and lubed by grease. This works well, although the costing cannot have been improved by the fact that the engine has two timing cover castings.
Inside is where the horror story lives. At least … it’s a horror story if you believe in such things. Your humble scribe has ridden far too many miles on far too many limply maintained Beezers to really worry about the timing side
main bearing, but has long accepted that other views are both available and popular. The problem? The horror? As was common at the time, the early – and indeed all – versions of the BSA twin rely on a long bush to both support the timing side end of the crankshaft and to supply oil to the big ends. The horror story goes that the bearing can wear prematurely, and as clearance between shaft and bush increases, so the oil supply and pressure to the big ends fall. The SRM fix has been available for many, many years, of course, should an owner wish it. It’s not a small job.
The other end of the
crank is supported by a hefty roller bearing (replacing a ball race in the earlier engines) which is apparently bombproof. So far, so well-known BSA. Look at the engine in the plungersprung machine from the primary drive side – the left. Observe how the chaincase appears shorter than usual for a pre-unit engine. This is because although the design is indeed non-unit – in that the crankcase and gearbox shell are entirely separate castings – the gearbox is rigidly bolted to the rear of the crankcase. As well as being commendably compact, this also does away with the primitive preunit method of adjusting the chains; none of that fiddling with oily bolts to slacken the gearbox in the engine plates so it can be pushed backwards to tighten the primary chain … while at the same time slackening the main chain, which in turn demands grubbing about with more worn out and filthy bolts to yank the rear wheel backwards to tension the main chain again…
Happily, the plunger twins bask in the benefit of a slipper tensioner for the duplex primary chain, a bonus feature which disappeared with the apparently retrograde return to full non-unit construction.
In my long, long time mis-spent fiddling with old clunkers, I have often wondered why BSA replaced their neat semi-unit plunger twin with a seemingly less advanced and more conventional layout. I’ve seen several suggestions: the use of a more conventional BSA box allowed a wide range of alternative gearsets, like the apparently legendary RRT2, is possibly the most common. My own theory is that BSA had decided to use their A10 engine in the Ariel Huntmaster, suitably if a little unconvincingly disguised, and wished to use the Burman gearbox as in other Ariels, along with the distinctive and indeed handsome primary chaincase. To accomplish this, they needed to do away with the plunger engine’s crankcases and their gearbox mounting points.
Whatever the reason, the redesigned twin continued in production until replaced by the A65 in 1962. The A65 twin has a sensible slipper adjuster for its triplex primary chain, too…