The primary reason a timing chain guide rail will need to be replaced is from general wear. With few exceptions, they are made from varying amounts of metal and plastic. As the chain rubs on the guide, which is its intended purpose, the friction creates wear on the plastic portion of it. This will often lead to breaking or cracking at some point in the vehicle's lifetime. For most vehicles, this is part is replaced around the 100,000 mile mark. In addition, some vehicles have inherent flaws or inefficiencies that cause the wear to appear and need attention sooner. With correct service intervals, however, they can last a long time and would need replacing very seldom. If the timing chain guide rails are not checked often enough, a broken one can cause significantly more damage to the engine if it happens to break during operation. If these parts need to be replaced at any point, it's very wise to replace them immediately. These are critical engine parts and as such, If they fail, it can become a safety issue on the road. You don't want your engine to seize on the road, potentially cause you to panic, and risk more damage to the vehicle.
The timing chain guide rails are designed to keep the chain in place and direct it to the next cog as it travels around the timing case area. Most vehicles have several of these, but there are some vehicles with only one, or even none, depending on the way the engine is set up. When the internal combustion engine was designed, the creators needed something to keep the valve timing in the same rhythm as the engine so the valves would open and close at the correct moments. A chain link was devised to control the camshafts and the chain required small pieces to hold it in place as it revolved. Thus the timing guide rail was born.
Changing the timing chain guide rails is no easy matter. Below I've provided just a few basic instructions for replacing them.