Keeping tension on your timing chain is imperative for keeping your camshafts turning properly. If some slop develops in your chain because of a poor tensioner, the timing chains can skip a tooth, causing catastrophic engine damage and ruining your day. If you are replacing your lower timing chain and guides, you should also be replacing this.
The lower timing chain moves along on this tensioner/rail and it keeps your chain tight on the oil pump and balance shaft. It makes sure that no teeth are missed and your engine is running as quietly as it can.
