The Idler pulley and the tensioner pulley are the EXACT same pulley from the factory. I went through this when I was looking for a new tensioner pulley. I had to buy a new tensioner assy and then found that the pulley is identical to the Idler pulley.......

The factory belt is 114.5 inches long, inside to inside, and 115.3 inches on the outside. There was a lengthy discussion on this a while back. It is confusing because Gates makes a 114.5" belt and a 115.3" belt. I used the 115.3" with my 181mm crank pulley to relieve some of the tension on my tensioner and all the pulley's....