So the Idler pulley does not have to be changed from stock? Just wondering why people are changing or upgrading them when they put on a smaller pulley? thanx for the info sorry the last thread turned a little ugly. Thanx for the pulley/belt information.
the 03/04 stock idlers' bearings are notorious for failing, essp. when a you install a larger lower pulley. Not a must, but if you do plan on going the lower route, then they start to become a must. Plus, they look damn good!
Let me add on to Dan's statement. Back when the first 03's hit the streets and companies started making smaller blower pulleys there was a lot of problems with belt slip. The initial train of thought was to increase belt wrap around the pulley and this did help in many instances. This was done by adding a larger upper idler, or repositioning the upper idler closer and a little further under the blower snout. Then companies started releasing non-slip pulleys which drastically reduced the problem by cross cutting grooves on the pulleys surface which allowed the pulley to get a better bite on the belt. 9 times out of 10 if you're only going with a 2.93 or larger upper that's a non-slip design you won't need a larger upper idler. Some folks when moving to a 2.80 or 2.76 upper still have problems with slip though.
As Dan stated above the stock idlers (there's 4 total) have seen more than a few failures once you start spinning the assembly faster than designed (like when adding a larger crank pulley). The aftermarket idlers have better bearings and seem to hold up to this stress compared to the stock Ford pieces.