Hi - I have a 15% Alta pulley on my 2003 MC-S. I replaced the belt and tensioner (with the Gates unit) when I changed out the pulley (pulley was bought second hand, looked barely used). For the life of me, I couldn't get the undersized belt on, and had to use the standard length belt. I do have the tensioner tool and was pulling the tensioner back when I was trying to install the belt on the last pulley.

Has anyone else had this problem? It still bugs me...

The cynical old coot in me is wondering if someone is selling bogus undersized pulleys...

 

They don't go on easy I'd give it another go.