I have never seen my engine yet except from looking through the side door at the air filter. If you know it is a 1/2 in wrench and yours will not go on it, it might need a 12 point socket or wrench. You might be using a 6 point (hex) head wrench. If you could look at it or feel it you could be sure it it not an allen head bolt. They use a lot of those on engine parts. I have seen a lot of allen head and 12 point bolts used on engines. If I was still working I could look at one. I may have to break down one of these days and try to see my engine.
Here is a thread I found on google.
Caterpillar - Belt question
And another one. Maybe one will have a hint of what you need.