Like starbuck said, a new belt will stretch see'ing as it's never been on a car before! What i did was got my mate to push as hard as he could on the alternater with a low entry jack handle (as they're really long), and as he was putting all his weight on it i tightened the bolts, simple. Don't worry about it being 'too tight'.. Don't think it will be tighter than mine is haha.