Originally Posted by wingracer

All this stock and modified stuff comes from the early days of RC. Back then you only really had one motor available, a 27t Mabuchi. So stock was for just that, an unmodified 27t motor. If you pulled the motor apart and rewound the arm with your own wire, that was modified. As things evolved, the two classes became stock for 27 turn, fixed timing, non-rebuildable motors (though rebuildables became a thing later) and mod for any wind and any timing you wanted. This class structure has remained even though the names mean nothing with today's motors.

Better names would be spec for motors that have to comply with ROAR's regulations for "stock" motors and open for open motor rules.