Have been listening to a squeaking engine noise for too long, hopefully this info will help others. There seems to be some question as to exactly which belt to order, I measured the new belt against the old, and it is an exact fit.
For those that have the squeaking, sounds like a bad water pump sound, it is likely not the water pump, in my case it was a worn bearing on the serpentine belt tensioner. The bearing was also worn on the idler pulley, so I changed that as well. I mainly changed the belt to confirm that I have the correct spare, however, there was a lot of small cracks on the tread side of the belt. After changing these components the belt system is quiet again.
2004 National Dolphin 5320
2003 WorkHorse W-22 Chassis
GM 8.1L Engine
Mileage: 32,000 miles
I sourced all of these parts from rockauto, the Gates parts were not expensive and are of good quality.
Belt Tensioner: Gates #38258
Idler Pulley: Gates #36299
Serpentine Belt: Gates #K061080
There is a 3/8" square drive hole in the belt tensioner arm, insert a 3/8" ratchet there and rotate clockwise while pushing down to unload the spring so that you can remove the belt. I had no room to rotate the ratchet and had to apply strong downward force to counter the force of the spring.
I then replaced the Idler pulley and the tensioner assembly and torqued the tensioner bolt at the required 37 ft lb.
When installing the belt, refer to the diagram, you do not need to route the belt around the fan. The easiest way to get the belt back on is to apply downward pressure on the tensioner arm while you push the belt under the idler pulley.
Thanks to Oemy's Web Site -