On the 400s, Carb is different. Exhaust is different. Piston is different (I would have thought cylinder porting would be different and piston the same, but here, piston is different and cylinders are the same). At least, accoding to parts listings for 1979. Later on, with the 490 series, it appears the exhaust, cylinders and carbs were different on the IT490s and YZ490s.