The engine’s vital organ
The fact our motorcycle, car, truck et al engine pistons move up and down in their cylinders is why all such units are called ‘reciprocating engines.’ Pistons serve many vital roles, including offering a thrust face to accept the expansion forces meted out on them on burning fuel gases and the pistons, by their connection via connecting rods to crankshafts, are vital in converting the thrust force into rotational power. Additionally, pistons carry their piston rings which act as compression seals and (excluding most veteran and many vintage engines) oil control rings acting as oil seals. In two-stroke engines, they serve as valves, and all deal with the expansion problems created by the timed burning of vaporised fuels.
Add to this early pistons were shaped to serve oil control and most aluminium/ silicone alloy pistons are shaped to handle the differing heat values (hotter at top than bottom) they reach at operational temperatures. Then some are shaped to minimise drag (slipper pistons for example) and also shaped as tuning device to increase port opening times with two-stroke engines. For multi cylinder engines that their weight is identical to that of their mates is vital as an aid minimising vibration, and, likewise, once a maker, mechanic, engineer, engine tuner etc has balanced a crankshaft in combination with its piston/s it is vital replacement pistons match the original/s with regard to weight, even if other dimensions have changed to accommodate cylinder rebores to maintain an engine’s smooth running, as we don’t want to rebalance crankshafts each time we
