You can't talk about timing belts and chains without ta […]
You can't talk about timing belts and chains without talking about interference engines. However, you need to aware that not all engines are interference engines. Some car and truck manufacturers have them, some don't. It is totally a hit or miss thing depending on the vehicle. That being said, an interference engine is an engine where the pistons and the valves share the same space in the cylinder but at different times.
This engine design claims to be great for engine efficiency, but it is definitely not ideal for people who procrastinate on timing belt maintenance. That's because on an interference engine, the valves collide with the pistons if the timing belt breaks. On a non-interference engine, you can break the timing belt, and it could never cause internal engine damage because the valves and pistons have their own space to play in. This brings us to the next topic.
When it comes down to it, you really don't want the valves and pistons to meet each other. They can happily share the same space, even multiple times in the same second, but never, ever at the same time.
Timing belts break most often at start up and shut down of the engine, because that is often when the most tension is on the belt. If you have a non-interference engine, a broken belt is not really a big deal. You throw a new timing belt kit on it and keep driving. However, if you have an interference engine, internal engine damage is nearly inevitable, though the amount of damage varies depending on engine speed at the time of failure.
If it were to happen on start up or shut down, it won't cause irreparable damage, maybe some bent valves, broken valve guides, etc. But if a timing belt decides to let go at higher rpm, the inside of the engine becomes an amazing, high speed, metal-chewing blender of destruction. When it isn't your car, it can really be incredible to see. The valves typically break clean off of their stems and bounce around the cylinders multiple times per second. This action typically horrifies the piston, sends aluminum shrapnel into the oil pan, and bends the connecting rod. Once the piston completely disintegrates (which it will), and the rod loses its direction in life, it will proceed to poke holes in the engine block and oil pan until the engine is cut completely in half.
When a timing chain decides to break, it can have varied results as well. If it happens at low speeds, the chain may just slide to the side of the timing cover and not cause any internal engine damage. Simply throw a new V Belt China on and you are on your way. On the other hand, if the chain lets go at high engine rpm, it will likely whip around the timing cover violently, destroying everything in its path. Again, this can be quite incredible to see, but you really don't want it to happen to your car or truck.