James Watt’s balls. An important addition to the steam engine.
Ahhh… no I won’t make a bad joke. Very cool . Another one for the Dave R wonderful mechanical bits library
If only James had known beforehand, he would not have invented the darn thing.
(I’m not above a bad joke)
It gave rise to the expression (initially about steam engines, where the mechanism acted as a speed governor) “going balls out”.
That explains why I always get in trouble when I try to do something Balls Out!
You must be referring to @DaveR’s neck for his caption.