It won't be an O ring, it will be a paper gasket and yes, you will have to clean the surface thoroughly. You can use a tool called a gasket scraper. We use an air operated buffer to buff off all of the old gasket. Make sure you get every bit as any left will lead to a leak. I generally use a spray adhesive on the side that attaches to the water pump and rubber silicone on the side that hits the motor. That way if it's ever removed, the entire gasket will stay on the pump and the scraping won't be necessary.
Edited by Tech501 on January 20 2005 at 5:45 PM