belts

I have 2 6hp yamahas, one for parts, the other is in good shape, I needed a new auger belt but the one off the parts machine is smaller ( perhaps by an inch in length)  and when I put it on the one I use the auger turns all the time even when not engaged. It took a lot of muscle to get it on...My question is over the years did the belt size( lenght) change or is there some adjustment I don't know about and does anyone know where I can get a new belt