Measuring against the linchpin was a method of determining which enemy civilians would be beheaded, used by the Mongols and other tribes that lived in the Mongolian plateau.
Genghis Khan used this brutal method against Jamukha's coalition of tribes in 1202. All male captives were forced to walk beside a wagon wheel. If their heads were higher than the linchpin (a pin inserted at the end of the axle) they were immediately executed. The wagon wheel was a large wheel used to transport yurt and other goods. This technique was probably used to preempt against revenge attacks that were common between tribes at this time. If one tribe were to attack another, there was always the possibility that there would be a revenge attack soon after. By eliminating the older males, there was less chance of a counter attack from tribes that were in perpetual conflict due to centuries of distrust and robbery.
Cummins, Joseph (2006) History's Great Untold Stories: Obscure events of lasting importance, Murdoch Books, ISBN 9781740458085