Also make sure the flanges are straight AND all the welds on the tubes at the flanges are straight, level and on the same plane. Esay enough done by placing the headers on a smooth flat surface like the concrete floor and getting a mirror and looking for daylight under the welds. If you see some, just file the welds down appropriately. A lot of people think you can just sling a set of headers on, but they are like any other aftermarket part and often require mods to make work correctly. Once everything is straight and flat, you'll be hard pressed to MAKE a set of headers leak. Even a cheap set.
