A resurgence in popularity for this configuration brings an old story right up to date

As an engine configuration, a parallel twin has both advantages and disadvantages. For a start, it is much simpler and therefore cheaper to manufacture than a V-Twin, while still having the benefits of two cylinders in a compact package. However, before the advent of the internal balancer shaft, a parallel twin with a 360° crankshaft vibrated as a consequence of both pistons rising and falling together. Later developments with 270° and 180° crankshafts smoothed this out even further, as well as giving different power-delivery characteristics. The parallel twin story started in the 1930s and defined the British motorcycle industry for the next 40 years before falling out of favor as the four-cylinder engine took a hold, only for a resurgence of interest in the parallel twin to occur in the 2000s.

10 Triumph – 1938

Triumph Speed Twin in Amaranth Red, facing right

It wasn’t the first parallel twin engine: it wasn’t even Triumph’s first parallel twin. That accolade goes to an engine designed in the early 1930s by chief designer Val Page. Then, in 1938, Edward Turner designed what was to become immortalized as the definitive British parallel twin, which first saw service in the new Speed Twin model. After the Second World War, Triumph had a six-year lead over their rivals, who scrambled to catch up. The Triumph parallel twin, despite initial worries that it wouldn’t be as adaptable as the single-cylinder engines that had been so popular before the war, was adapted to every conceivable type of use, from road racing, off-road/scrambling competition, trials and record-breaking, not to mention a wide variety of Triumph models, both sporting and workaday.

9 BSA – 1946

Side-view shot of a BSA A65 Lightning from the late 1960s, finished in red and chrome

Despite war work taking precedence over civilian motorcycle developments, BSA was ready to launch its Triumph-rivaling parallel twin engine in 1946, having been designed immediately before the war. Ironically, one of the designers was Val Page, who had designed Triumph’s first parallel twin engine in 1932. The A7, as it was known, had a displacement of 495cc and then BSA beat Triumph to 650cc by one year, the BSA A10 arriving in 1949. Then, in 1962, the engine was completely redesigned to incorporate unit construction – the crankcase and gearbox case all one casting – and the crankcases and covers given a distinctive egg shape. As with all British motorcycles o the time, vibration was a problem, but the engines were powerful and helped BSA become the highest-selling motorcycle manufacturer around the world.

8 Norton – 1949

Norton Dominator with parallel twin engine

Norton relied on its single-cylinder engines for longer than Triumph or BSA but, by 1949, the parallel-twin engined Dominator model had arrived. As with BSA, its designer, Bert Hopwood, had been involved with the design of Triumph’s parallel twin engine before the war. The Dominator started as the 500cc Model 7, then was enlarged to the 600cc Model 77, the 650cc Dominator 650SS and, from 1962, to 745cc, by this time being called the Atlas, even though it was essentially the same engine. To combat vibration, which was severe, Norton developed rubber ‘isolastic’ engine and gearbox mounts for the 1967 Commando.

7 Honda – 1966

Honda RC116 50cc Grand Prix racer

Not a production engine but it deserves to be mentioned here because it represented the absolute pinnacle of the vastly expensive motorcycle Grand Prix wars that characterized the Japanese efforts in the 1960s. The Japanese philosophy towards performance was more power = more speed. The way to more power for Honda was not via two-stroke technology as espoused by Yamaha and Suzuki, but through more cylinders in their four-stroke engines. Not only did this culminate in the six-cylinder 250cc engine but also the incredible 50cc parallel twin, as fitted to the RC116 racing model. The crank, con rods and piston assembly would fit into the palm of your hand and the engine produced 15 horsepower at 22,500rpm, with a nine-speed gearbox. Top speed was around 110mph. After this, the governing body laid down rules for the number of cylinders permitted for any given displacement.

6Yamaha – 1969

Yamaha XS650 in red and white, publicity shot

While Honda was busy developing its four-cylinder engine, Yamaha was being more conservative and looked to the British for inspiration. The Yamaha twin engine arrived in 1969 in the new XS650, and it was immediately clear that the parallel twin had a lot of life left in it, as long as development wasn’t left to the British. The Yamaha engine had horizontally-split crankcases which not only aided assembly but also helped prevent oil leaks. Surprisingly, Yamaha chose to stick with a 360° crank, in which both pistons rise and fall together, rather than a smoother-running 270° crank, but that development was a few years away at the time. The XS650 continued until 1985 as one of the last of the traditional parallel twin engines.

5Triumph – 2002

Triumph Speed Twin 1200 in red, facing right

For a decade after being resurrected, Triumph resisted the temptation to return to its roots and produce facsimile models, although model names were seen as an essential link to the past. Then, in 2002, the first of the ‘newBonneville models appeared, powered by a brand new, counterbalance-equipped parallel twin engine that – outwardly at least – was a direct copy of the original. Displacement was 790cc initially, rising later to 865cc, then 900cc and, finally, 1200cc in either high power or high torque versions. Smooth, powerful and full of engaging character, the Bonneville line of models, including roadster, scrambler and café racer would come to dominate the sales charts of ‘newTriumph, providing a tangible link to the original company and pricing the parallel twin wasn’t dead just yet.

4 BMW – 2018

Motorcycle Parked On rough road During Sunset

In the 1980s, BMW started to consider alternative engine configurations with a view to retiring the venerable boxer-twin engine. The first to appear was the lay-down inline four-cylinder as found in the K100. Then, in the 2000s, another four cylinder appeared but so did a brand new parallel twin. Confusingly, the same engine displacement – 798cc – was to be found in the F650, F700 and F800, each model having different power outputs – 71 to 85 horsepower – and equipment levels. Neat and compact, the engine has a 360° crank which makes it surprisingly characterless. The following F750/850 parallel twin engine, introduced in 2018, featured a 270° crank, giving better traction on off-road surfaces as well as a lot more characterful feel and exhaust note.

3 Kawasaki – 2018

Kawasaki Ninja 400 in ice white, facing right

As a strong argument in favour of small-displacement parallel twins, it would be hard to think of a more suitable engine than the 399cc twin in the Kawasaki Ninja 400. A large single cylinder engine would have been OK, but the little twin has character, smoothness, power (48 horsepower) and torque (24.6 foot pounds), not to mention being compact for its output and so easily fitted into a physically smaller motorcycle, avoiding adding excess weight. Not as sporty as the same company’s ZX4RR, with its diminutive inline four-cylinder engine, but so much more accessible and still possessing sufficient performance and chassis capability to keep the rider entertained.

2 KTM – 2018

A front left shot of a 2023 KTM 790 DUKE

For many years, KTM was known for its large displacement single cylinder and V-Twin engines powering its road and adventure bikes. Then, in 2017, the 790 Duke appeared, featuring a brand-new parallel twin engine. The advantages of the twin were beginning to be appreciated again: smoother and producing more power than a single and much simpler and cheaper to manufacture than a V-Twin. To combat vibration and to give a power delivery similar to a V-Twin, KTM utilized a 285° crankshaft and the engine produced 105 horsepower and 60.7 foot pounds of torque. The 790 Duke was highly praised, not least for the punchy and compact engine.

1Yamaha – 2020

Action shot of a Yamaha MT-03

The rise in popularity of the parallel twin engine continues with this jewel-like beauty from Yamaha. Found in the YZF-R3 sports bike and MT-03 naked roadster, the 321cc parallel twin develops a very healthy 41 horsepower and 21.8 foot pounds of torque, giving excellent, smooth performance. The Yamaha parallel twin engine confirms the theories of Edward Turner of Triumph, that said the smaller the twin, the smoother the running, as anyone who ever rode the 350cc version of the Triumph engine will attest. In spite of this, the Yamaha twin features a balancer shaft to smooth things out even further and Yamaha also offer the 700cc parallel twin-engined YZF-R7/MT-07.

Source: topspeed.com