Quote Originally Posted by deannos View Post
Every car i have had has always got some form of tensioner on the timing belt, in most cases its a idler pulley under spring tension (auto belt tensioner).
Not everything does though, and I normally deal with vehicles that don't.
You could argue this just keeps the belt tight against the pulleys, but i have noticed a slight difference in length when i have changed them in the past. But whether you would get this much on a router to cause major problems, who knows.
The normal difference you'll notice between new/old is flexibility. If you were to put them on a proper belt measurement jig, there would be neglible difference in length.