Wiktionary
major diameter
n. The outermost or largest diameter of something that has multiple circular, cylindrical, or helical features (e.g. a screw or gear)
Usage examples of "major diameter".
It was a little over one hundred fifty miles in diameter along its major diameter, and on its surface there existed a population of mobile, self-directing activity patterns measuring, on average, an inch or so tall, who perceived themselves as self-aware, autonomous beings.