a mod motor refers to the bearing allowed by governing bodies as well as adjustable timing and any number of turns.
a stock motor is built to a specific set of rules limiting a motors timing and the style ,wire gauge,number of winding around the armature as well as the size and number of cooling vents and the thickness of the can and may also include the use of bushings in stead of bearings.in brushed motors this can also include the compound of the brushes and the use of no internal fans attached to the armature as well as the total length and width of the motor and the magnet type(bonded or sinterd)
we can thank bob novak for the use of sintered magnets .