The concept of the parallel motorcycle engine has been around practically since the beginning of the Twentieth Century and the very earliest motorcycles; however, it wasn’t until 1937 that Triumph Motorcycle’s chief designer Edward Turner came up with an overhead-valve parallel twin that really worked.  When Triumph debuted the engine the next year in their Speed Twin T100, the effect was dramatic.  At an original price of just 75 pounds Sterling, here was an affordable, reliable, mid-sized cruiser that started easily, handled exceptionally well at just over 360 pounds, and with its new 500cc, 27 British horsepower vertical twin engine, boasted a top speed over 90mph.  In a word, the T100 defined everything a modern motorcycle should be, and soon dozens of British bikes were available with a parallel engine design.

Throughout the 1960s Triumphs, BSAs, Nortons, and other makes set the standard for speed and durability, as well as becoming the stuff of 50s and 60s legends like café racing and the “ton-up” boys.  And there were legendary evolutions of Triumph’s T-100 including 1949’s 650cc Thunderbird – maybe the original Superbike – and perhaps the best-known parallel twin of all, 1959’s T120 Bonneville.  It was also in the 1960s that Japanese bike-builders began copying and improving on British bikes with models like the Honda 450 Black Bomber, but the real bombshell came in 1968 when Honda introduced the CB750 with its astonishing 738cc parallel-4.  Reliable to the point of being bulletproof, this bike was fast, maneuverable, and fun to ride.  With features such as an electric start, a 5-speed gearbox, and a front disc brake, more than 400,000 750s were sold in the following years, and their incredible popularity gave rise to what became known as “UJMs” – that’s Universal Japanese Motorcycles – made by a variety of manufacturers, and all based on British cruiser design and some form of the parallel engine.

Now, before we look inside the parallel engine, let’s get a quick refresher on the other two major bike motor designs.  For big highway cruisers it’s still the V-Twin.  Two pistons inline with the frame connected to a single crankshaft at anything from 45 to 55 degrees, depending on manufacturer.  The slight offset in the piston’s firing sequence not only gives this engine giant torque, but also its characteristic loping, uneven rumble.  An oversized flywheel and large crankshaft counterweights eliminate this vibration and smooth out the ride.  The valves on a V-Twin can be actuated by a camshaft in the crankcase with pushrods that reach into the engines heads, or by a timing chain that operates overhead camshafts.

Another major bike engine design is the boxer, or opposed engine.  Imagine turning the v-twin sideways, then bringing the cylinders down to horizontal.  This engine is naturally balanced since the pistons are always exactly opposite each other in the stroke sequence, so no counterweights are necessary on the crankshaft.  While early boxers used pushrods, independent overhead cams are current valve technology on opposed engines, and most employ driveshafts to get the power to the back tire.  For a better explanation, we turned to expert Chris Reo of Motorcycle Mechanic Institute.

Chris starts to tell us about the crank-and-cam set-up of this type of engine, saying that when it’s offset 180˚ one piston reaches the top as the other reaches the bottom.  This produces less torque, but naturally balances the crankshaft, reducing weight.  In the 360˚-offset configuration, both pistons move together but fire alternately.  This design is more powerful but requires large crankshaft counterweights to defeat vibration.  In the crankcase, the piston rods turn a transverse crank.  The crankshaft operates the timing chain and cams.  The crankshaft is also geared to the clutch input, and when the clutch is engaged, power is sent through the transmission to the output sprocket, which is connected to the rear wheel sprocket by the final drive chain.

Some retro parallel twins still use carburetion, but most modern parallel engines depend on electronic fuel injection (EFI) to get a precise fuel-air mix into the cylinders.  In fact, one of the drawbacks of the parallel engine has been the complexity of some of the carburetor arrays associated with it.

Many of motorcycling’s greatest race events have been dominated by parallel -powered bikes for decades.  Sportbikes, cruisers, retro classics as well as the fastest production bike on the planet, the Suzuki Hayabusa, all share this essential design from motorcycling’s past, a proven powerplant that continues to evolve.

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