Like like everything else in life there are rules.... Here ya go. F3 -all 4-strokes up to 100cc -air cooled 2-strokes up to 65cc -liquid cooled 2-strokes up to 50cc -auto 4-stroke up to 110cc F2 -air cooled, 2 valve, 4-strokes up to 140cc -air cooled 2 strokes up to 80cc -liquid cooled 2-strokes up to 65cc F1 -air cooled 4-strokes up to 200cc -liquid cooled 4-strokes up to 150cc -all 2-strokes up to 100cc 50cc Stock Modified Class -air cooled 50cc (OEM YSR's) max 57cc (43mm bore) More info on this class visit: http://www.ohioysr.org/rules.htm 50cc Supersport Class (same as above, modified parts are allowed) -air cooled 50cc (YSR's) with modified parts More info on this class visit: http://www.ohioysr.org/rules.htm 4 Strokes Motard (exhibition class) -up to 200cc air cooled -up to 150cc liquid cooled Auto 50 -No change