|  |  Why Do Headers Sometimes Leak?
The stock WRX manifold is a cast iron design. Cast iron isn't that smooth inside, but it is thermally stable, meaning it does not shrink or grow very much when it gets hot or cold.
Stainless steel, on the other hand, has a pretty HIGH thermal expansion coefficient, meaning it WILL shrink and grow as it changes temperature. And what are aftermarket headers made of? Yep, stainless steel.
The long tubes of a set of stainless steel headers for Subaru engines will change length as the headers warm up and cool down. This usually isn't a problem, but it can sometimes result small leaks at the flanges, most commonly where the header connects to the up pipe. A leak in this area will often sound like a ticking under the hood, and the sound may go away as the engine warms up. Get the car on a lift and look for sooty deposits at the flanges to visually check for this kind of leak.
Sometimes a fresh gasket and some good snug bolts will fix the problem. Othertimes it may take more work. In any case, although a well-made header will be less likely to leak than a poorly-made one, there is no way to get around the high levels of thermal expansion/contraction that stainless steel has. That means that ANY stainless steel header MAY eventually develop leaks at its flanges. It's just part of doing business with these parts.
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