All dirt bike motor sizes

Answer 1

The main FIM dirt bike race classes are the MX1 class, 450cc class where 450cc four strokes and 250cc two-strokes are eligible to race, the other main class is the MX2 250cc class where 250cc four strokes and 125cc(now up to 150cc) two strokes compete.

Less well known is the MX3 class where 650cc four strokes race with 500cc two strokes.

Most production dirtbikes fit into one of these classes, the exceptions being open class bikes like the BMW GS's and HP2's and KTM 990 Adventures and Super Enduros among other manufacturers big touring trailies, and various enduro, trail and fun bikes e.g. the KTM 300EXC, Honda CRF230F, etc.