There are three main crankshaft configurations for this engine: 360°, 180°, and 270°. There are minor differences in the applications for four stroke and for two stroke engines, largely pertaining to ignition intervals. For example, the 360 twin is the natural configuration for a two cylinder four-stroke engine, since four piston strokes add up to 720°. The below will mostly be concerned with four-stroke engines.
In a 360° engine, both pistons rise and fall together. The dynamic balance is identical to that of a single-cylinder engine, but with twice the number of ignition pulses. The firing order is offset, so that cylinder 2 fires 360 degrees after cylinder 1, and 360 degrees later cylinder 1 fires again at 720 total degrees, the beginning of another four-stroke cycle.
In a 180° engine, one piston rises as the other falls. This gives good primary balance, albeit with a rocking couple; but results in irregular ignition pulses. This is because cylinder 2 fires 180 degrees after cylinder 1, and cylinder 1 doesn't fire again for another 540 degrees - always adding up to the 720 degrees of rotation for a four-stroke cycle.
In a 270° engine, one piston follows three quarters of a rotation behind the other. This results in a mixture of the imbalances in the first two types and yields firing intervals identical to a 90° V-twin. Firing order here is that cylinder 2 fires 270 degrees (3/4 of a rotation) after cylinder 1, and cylinder 1 fires again 450 degrees (one and a quarter rotations) after cylinder two, again at a total 720 degrees and the beginning of the next cycle.
360° and 180°
From the 1930s, following the work of Val Page, most British four-stroke parallel-twin motorcycles used a crank angle of 360°, which allowed the use of a single carburettor because 180° and 270° twins need twin carburettors, as did an early Meguro was a copy of the 360° British BSA A7. However, in the 1960s Japanese manufacturers favoured the 180° whose smoothness allowed higher rpm and thus more power. For example, the 1966 Honda 450 cc dohc 180° parallel-twin “Black Bomber" could challenge contemporary British 650 cc 360° twins.
Many small motorcycles of less than 250 cc use a 360° crankshaft as the vibration issue was less significant; examples includes Honda's CB92, CB160, CA72, CA77s, and CM185. Larger twins over 500 cc, such as the Yamaha's XS650 and TX750, have used 360° crankshafts, but such parallel twins tend to feature balance shafts. The Honda CB-series in the 250 to 500 cc range used 180° crankshafts. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 featured a 180° crankshaft and a balance shaft, while the 1974 Kawasaki KZ400 used a 360° crankshaft and a balance shaft.
A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, as displacement in the crankcase stays roughly constant. However, a 180° engine requires a separate ignition system, points or otherwise, for each cylinder. The 360° twins can have a single ignition system for both cylinders, with a wasted spark on each cylinder's exhaust stroke. The BMW F800 parallel twin motorcycle is a 360° design. Inherent vibration in the BMW F800 means its engine is limited to 9,000 rpm. BMW reduced the vibration using a third "vestigial" connecting rod to act as a counterbalance.
A modern development of the straight-two engine is the 270° crank, which imitates the sound and feel of a 90° V-twin, but requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise which allows a more regular firing pattern than a 180° crank and less vibration than a 360° crank. As with a 90° V-twin, the pistons in a 270° inline twin engine are never both stationary at the same time, thereby reducing the net momentum exchange between the crank and pistons during a full rotation. The oscillating momentum manifests itself as an oscillating crank rotation speed, which, when paired with a driven-wheel rotating at the more steady road speed, will introduce an oscillating torque in the drivetrain and at the tyre contact patch.
Phil Irving undertook to minimise this oscillating torque, and for one particular connecting rod to stroke ratio, arrived at an optimal separation of 76° (294°), instead of the 90° (270°) described above. The optimum for two pistons is found when one piston is travelling fastest at the same time the other has stopped; maximum piston speed occurs when the connecting rod and crank throw are at right angles, not when the crank throw is at 90° to the cylinder bore. This minimisation of so-called inertial torque was also one of the goals Yamaha achieved with its "cross-plane" R1 engine. Note that in neither case was the oscillation completely eliminated, only reduced significantly.