they did not replace them when i went in to the subaru dealer for a big overhaul,what do they do and could they prove to be detrimental to my car down the road?

The guide the belt(s) along the intended path(s).

 

I am assuming that you are refering to the pulleys on the timing belt? They certainly aren't always replaced by all mechanics, and can be a bit expensive. I don't know if Subaru generally replaces them as common practice on a timing belt job. If one does fail, it can cause the timing belt to snap or skip as the belt rubs against the non-rotating pulley. This is of course a problem, and more-so for interference engine owners.

 

When the timing belt is removed you can take the pulleys and test them to get an idea of whether they are showing wear. This is no guarantee of their longevity but I would imagine it was done at the dealer.

 

I prefer to replace the idler pulleys and water pump on every other timing belt (120k) to minimize this risk.

 

-Heikki

thx for the help