the minimum number of rings in a piston is
Piston ring Most automotive pistons have three rings: The top two, while also controlling oil, are primarily for compression sealing (compression rings). Meanwhile, the lower ring is for controlling the supply of oil to the liner, which lubricates the piston skirt and the compression rings (oil control rings). At least two piston rings are found on most piston and cylinder combinations. Typical compression ring designs will have an essentially rectangular cross section or a keystone (right angled trapezoidal) cross section. The periphery will then have either a barrel profile (for the top compression rings) or a taper napier form (for the second compression ring or scraper ring). There are some taper-faced top rings as well, and on some old engines simple plain-faced rings were used.
Worm drive A gearbox designed using a worm and worm-wheel is considerably smaller than one made from plain spur gears, and has its drive axes at 90° to each other. With a single start worm, for each 360° turn of the worm, the worm-wheel advances only one tooth of the gear wheel. Therefore, regardless of the worm's size (sensible engineering limits notwithstanding), the gear ratio is the "size of the worm wheel - to - 1". Given a single start worm, a 20 tooth worm wheel reduces the speed by the ratio of 20:1. With spur gears, a gear of 12 teeth must match with a 240 tooth gear to achieve the same 20:1 ratio. Therefore, if the diametrical pitch (DP) of each gear is the same, then, in terms of the physical size of the 240 tooth gear to that of the 20 tooth gear, the worm arrangement is considerably smaller in volume.