The inline-six engine is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balance, resulting in fewer vibrations than other designs of six or fewer cylinders.
Engine block of a BMW M20 straight-six engine (top view)
3890-litre MAN B&W 6S60MC marine diesel engine
Crankshaft with four main bearings
1978-1981 BMW M88 engine
Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and balance shafts, to prevent unpleasant and potentially damaging vibration. The strongest inertial forces occur at crankshaft speed and balance is mandatory, while forces at twice crankshaft speed can become significant in some cases.
Harmonic damper for a 1937 Pontiac engine
Balance shaft system: 1922 design by the Lanchester Motor Company
Fork-and-blade connecting rods
BMW R50/2 flat-twin engine viewed from above, showing the offset between the left & right cylinders