There is a "collision" and "non-collision" type of valve gear configuration. "Collision" type engines, the intake and exhaust valves are open only slightly when the piston is at top dead center during valve overlap. The timing of the camshaft and the crankshaft is therefore critical because if the valves are allowed to open more during valve overlap, they will collide with the rising piston, thereby causing extensive damage to the engine.

However, there are also engines that are of the non-collision type. Usually, these are the older or non-performance type OHC engines with lower compression ratios. In these engines, the valves have enough clearance to open fully without hitting the piston in it's highest position in the cylinder. If your timing belt breaks in one of these engines, all you suffer is the inconvenience of a breakdown but nothing more. Once the broken belt is replaced, the engine will be just as it was before, without any damage whatsoever.

When replacing the timing belt, it's standard procedure to also have the front crankshaft oil seal replaced. It's an opportune time to do so, considering the trouble one has to go through in accessing it 10-20,000 kms down the road, after having replaced the timing belt.