Hi, new Mini owner, and new to this board. My mini is a 2003 r53 hatchback. I purchased it a few months ago with many upgrades already installed. So far what I've identified :
Eibach springs
adjustable rear lower control arms
borla exhaust
Alta intercooler
Alta intake
Alta crank pulley 0%
MSD ignition
Aftermarket wheels/tires
Aftermarket engine mounts
Aftermarket SC pulley of unknown size.

My serpentine belt fell apart, and I'm trying to figure out what size belt to get.
Can anyone help me identify what size pulley I have on the SC ? Will the stock sized belt work? I can put a caliper on the pulley if that would help. I have yet to find any website that gives dimensions for their SC pulleys.. any help would be appreciated!

 

Odds are it's an Alta 15%.
Have you tried looking at the face of the pulley? Odds are if it's not a 15% it will say what it is on the front.

 

alta SC pulley should say alta on it, and usually they are 15%
most should say on it on what it is

 

Welcome to NAM. If you send me dimensions I should be able to tell you the reduction size on that pulley.

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Odds are with the mods, its an Alta...
I would just go to the local auto parts store...
Buy a belt for a 15%...a 535, and a belt for a 17%, a 532....and put them on...
At about 20$ each...not much risk...then just see of the tensioner is tight enough with one belt, if not, try the other.....