They are more commonly referred to as a tensioner... even though they aren't technically a tensioner at all!

You can get the OE parts here. Parts 17, 19, 20, 21, 22 make up the lower roller/tensioner, 17 is possibly the only bit you need to replace.

Alternatively you can get a better quality tensioner with a proper roller bearing (rather than just a plain bush) from here for less money. These are the roller/tensioners that I use because they actually roll, the OE ones just don't spin freely enough which is why they wear out quickly.

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