The timing-chain guarantees the exact coordination with the cam timing, at least it does when it's new. There is barely any play at all, because it is guided (picture 2) from both sides. Even if, in the course of time, the chain should stretch a little, the oil pressure operated chain-tensioner compensates for this, thus avoiding oscillations.
The previous, often used duplex-chain has been replaced by the single-chain. Not always are both overhead cams driven by one and the same chain. Picture 1 above shows the more common variation with two chains. These two, or even more, can also run serially (picture4). In this case, two hydraulic tensioners are also necessary. A further advantage over the timing-belt, with which this is not possible because of it's sometimes special width, is shown here.
The last example can also not be achieved using a timing-belt. This is a two-rowed chain combined in such a way with the pinions for the camshafts, that their diameter can be distinctly larger than the distance between the centers of the pinions. The diameter of the large crankshaft timing gear also becomes somewhat larger, thus more favourable. The chain can describe a larger arc and there are more sprocket-teeth engaged.
For a while it looked as though the timing-chain would, in the years to follow, completely replace the timing-belt. This was not completely justified, because the manufacturers prescribe a replacement interval for the timing-belt, but not for the timing-chain, indeed, behind closed doors, it is repeatedly said, that the chain and guide should actually be replaced every approx. 100.000 kms., because then the engine would last longer (e.g., Mercedes).
It's difficult to say whether or not this is still necessary with modern engines. One thing however, is certain, the pretty steep price which is sometimes asked for the replacement. The job used to cost up to DM 1000, nowadays, it's sometimes over €1000 (e.g., Opel). This is definitely more than it costs to replace a timing-belt, and this is more than likely, because of the material cost, and not the amount of work involved. Lately the timing belt catches up considerably in the life time. 03/13