What is a balance shaft belt?

The balance shaft belt (known as timing belt B to the dealership) is a small auxiliary belt running right next to the main timing belt. It is about 10 mm wide, while the main timing belt is almost 30 mm wide. It is normally called the balance shaft belt because it operates a small, asymmetrical shaft inside the engine called a balance shaft. It can also be referred to as the 'silent shaft belt', since 'silent shaft' is synonymous with 'balance shaft'.

The function of the balance/silent shaft is to smooth out unwanted engine vibrations. The shaft is unevenly weighted. If you were to take a roll of paper towel and cut off half of the 'towel' part without cutting the cardboard center tube, you would have the general shape - more mass on one side than on the other.

The balance shaft belt spins this offset weighted shaft inside the engine block. As it spins, the shaft tends to pull the engine towards the side with the weight on it. As the engine moves forwards, the shaft pulls it backwards, and vice versa. In this way, the engine shake is reduced. Since the shaft helps 'balance' the engine, it may be called a balance shaft. It can also be called a silent shaft, since it not only doesn't drive anything but it also helps quiet the engine down.

There are actually two balance shafts on a DSM engine. One is separate, and is run by the balance shaft belt, while the second shaft is integrated into the oil pump. Both spin in phase with each other.

Balance shafts are not strictly necessary to engine operation, and many engines lack balance shafts altogether. Removing them can increase engine output, as the engine doesn't waste energy spinning the shafts. The engine will run rougher as a result, but most people find the change bearable. Most people also agree that the engine must be properly balanced if you want to remove the balance shafts, to eliminate the possibility of long-term engine damage.

The balance shaft belt is often a culprit in major engine failures. See Why is it so important to change the balance shaft belt on a [DSM]?