A belt tensioner or/and idler pulley is a system that guides the serpentine belt and the timing belt on a car engine. The function of a pulley and a tensioner is to limit the belt slip. The belt tensioner can be mechanic (spring-loaded) or hydraulic.
Tensioner Assembly Belt Roller - 25281-3E000 is available to buy in increments of 1
A drive belt tensioner is a pulley mounted on a spring mechanism or adjustable pivot point that is used to keep tension on the engine belts. Spring tensioners are designed to tension automatically while the pivot design types can be adjusted manually. A tensioner maintains the correct amount of tension on the belt at all times throughout its duty cycle. It also helps protect other components such as the alternator and water pump from undue stress and premature failure.