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.
Optimal solutions for Rubik's Cube The maximum number of face turns needed to solve any instance of the Rubik's Cube is 20,[2] and the maximum number of quarter turns is 26.[3] These numbers are also the diameters of the corresponding Cayley graphs of the Rubik's Cube group. In STM (slice turn metric) it's unknown.