All vehicles must be full-bodied cars and weigh no less than 2,800 pounds except for six-cylinder cars (2,000) and four-cylinder and rotary-powered cars (1,200).
Opposed, air-cooled four- and six-cylinder piston engines are by far the most common engines used in small general aviation aircraft requiring up to per engine.
Here standardisation was taken a step further with the bodies on 9 hp four-cylinder and 15 hp six-cylinder being almost indistinguishable except for bonnet length.