Well, I've recently noticed some belt slippage, and after looking at my serpentine belt, I noticed it was very glazed and the ribs were cracked & gouged.

So, knowing that I run a Helix 15% pulley and stock crank pulley, I bought the NAPA 25-060535 (1379mm) belt that seems to be the standard for those with my same set-up. I went to install it and it immediately became clear that there was no way in Hell this belt was going to fit. I had the tensioner fully compressed (second hole braced with a screwdriver), and the belt couldn't even get halfway around the bottom idler pulley. Triple checked the belt routing, belt part#, tensioner position, etc. I even had a friend compress the tensioner until it interfered with the bracket above it (truly "fully compressed") and the belt was still not even close to going over the idler pulley.

So, I ended up grabbing a NAPA 25-060547 (1404mm) and it went on beautifully. After releasing the tensioner, I have one full hole, plus ~1/8" exposed on the tensioner rod. While I'm happy with fitment and performance, this just doesn't make sense: I'm using a stock sized belt on a pullied car, yet have the correct amount of holes exposed on the tensioner. Does anyone know if an aftermarket tensioner or idler pulley exists that would alter fitment as I'm describing? Everything besides my supercharger pulley seems to be stock, yet I can't explain this fitment.

 

Bet your crank damper is a +2%......

 

That was my first thought, but it appears to be stock. Aftermarket guys love labeling their stuff, and while I can't recall any labeling, it was just a plain 'ole solid-bodied black unit.

Edit: Supporting the stock crank pulley / harmonic balancer theory: I remember that the only writing on the crank pulley was operating frequencies (harmonics). Had to be a stock unit...

 

Stock unit is largely rubber....metal center...and out...rubber middle...no freq markings...stock..so it doesn't need specs on it...
Sounds like the non-ati unit..SFI UNIT...think waymotorworks only sell stock...but think rmw sells a +2%...

 

The stock unit does have frequencies on it:



This is what I have. So, again, this belt thing makes no sense unless A)I have a replacement tensioner that is built slightly different OR B)I have an idler pulley that is sized differently.

 

Mine was so rusted...guess I missed it when I removed it (still in a box in the garage)
2 diffent designs on an 2005...both early and late s (depending on production date...the switchover has never really been pinned down)...but both have a WIDE strip of rubber...
The ones with a narrow strip is for the non-s.

 

I'm not sure when the change was made, but regardless, none of this explains the belt sizing issue. My crank pulley / balancer is identical to what I've linked to above and in good shape.

 

Well, because I like to be thorough, I wanted to update this thread:

Just yesterday, my idler pulley finally "gave up the ghost" and pretty much ate itself and my serp belt at the car wash. Anyhow, I got towed to a friends garage, brought along my new idler pulley and bought a new 060539 belt. I figured I would try the 539, seeing as it's longer than the 535 (previously tried and was too short, inexplicably) but a bit shorter than the 547 (stock, and what I was running when the idler failed).

I removed the idler pulley and noticed something quite unexpected: There are two threaded holes behind the idler pulley which allow it to be mounted to the engine block! These holes are only about 1/2" away from each other, and both appeared to be factory. Whether it happened at the factory or not, my failed idler pulley was mounted in the hole that apparently most are not, thus, explaining the odd disparity between the belt lengths I was trying based on info on these boards. The 060539 was still too short, and I once again used a stock 060547 belt, and kept the idler in it's original position. I have 1+ full hole on the tensioner showing. In case anyone else has belt sizing issues, I hope this is helpful. Model Year 2005, Mini Cooper S.