These are a set for both the front and rear timing chain tensioners. At some point though the tensioner reaches the end of its travel and then, when the chain stretches just a little more it starts rattling, usually against the spark plug tunnels (tubes) causing the damage. The chain can eventually wear a hole in those tunnels which then fill (slowly) with oil and suddenly the rider notices oil seeping down the side of one cylinder.

These extenders are sold in pairs.