Big pistons like these handle big forces; peak combustion pressure sends more than five tons of force down each fracture-split steel con-rod to the crank. Smaller boxers have two main bearings, but evidently when engineers mapped the stresses for an engine this big there was too much “red” in the false-color stress maps. Therefore, there is a third main bearing, located between the 180-degree crankpins, increasing cylinder offset from the approximately 45mm of the smaller engines to more like 62mm. This offset—the left cylinder is offset forward—causes the engine to oscillate around a vertical axis to the left as pistons decelerate to TDC and to the right as they approach BDC. In BMW’s early 500cc engines, a moderate peak rpm of 5,800 plus the light weight of small pistons made this oscillation minor. But as the flat motors grew to 1,000 and 1,100cc, turning higher revs with bigger pistons, a balance shaft became necessary.