The transmission was modified with a different tooth profile between 2001 and 2002. The easiest way to determine how the engine was built is to look at the counter balancers' drive shaft. The early shaft has a straight surface and the later one has a diameter change between the position for the timing sprocket and the drive gear (tapered).
The following gears are not interchangeable between 2001 and 2002/2003: Counter balancer, balance drive shaft, drive gears on balancer drive shaft and crank shaft and also the outer clutch basket.
This is a direct quote from the 03 workshop manual. The difference in tooth profile may be subtle. Many have replaced their 01-02 counter balancer with the factory updated part (double row bearings), did not check the tooth profile and ordered the mid 02 and up part or just ordered the 03 part thinking it was the updated part they needed. The result was a loud gear whine. Husaberg makes a special updated double row bearing counter balancer specifically for the 01 and early 02 models. You could probably find a good used clutch basket from a 01, hopefully from a 400. The 01's had a defective torque limiter which would damage the clutch basket if a backfire occurred during starting. Mostly the 650's were affected. A warranty replacement torque limiter was given to all 01 model purchasers.
