Why stroke is multiplied by two
In one crankshaft revolution the piston travels down and back up, covering twice the stroke. Mean speed is total travel divided by time, rather than instantaneous speed at a particular crank angle. Stroke in millimeters must first be converted to meters.
mean speed(m/s) = 2 × stroke(mm) ÷ 1000 × RPM ÷ 60
Example with an 86 mm stroke
At 6,000 RPM, mean speed is 2 × 0.086 × 6,000 ÷ 60 = 17.2 m/s, or about 3,385.83 ft/min. Doubling RPM doubles mean speed; doubling stroke at unchanged RPM also doubles it.
Limitations
Instantaneous and peak piston speed depend on crank angle and connecting-rod length and are not calculated here. Mean speed alone cannot establish a safe rev limit: component mass, loads, lubrication and engine design also matter. This tool does not prescribe a universal RPM limit.