Reuleaux Prisms

The Reuleaux triangle is a constant width curve based on an equilateral triangle. All points on a side are equidistant from the opposite vertex. This is nerd for: These prisms are based upon the Reuleaux shape, the other shape that has constant width, the first of course being the circle.

Once printed they can be placed between to planes where they will act as normal cylinders, allowing you to roll them back and forth. This was utilised in the uncommon alternative to the standardised four-stroke in the variety known as the Wankel engine