Matt --One of your biggest enemies is the paint, dirt, and corrosion that's on the exposed portion of those studs. A "rethreading die" of the appropriate size, which you can get from just about any auto supply store, used to clean those exposed threads will make a world of difference. But even a rethreading die won't clean out all the crud that's right at the juncture of the stud and outer flat of the nut. A stainless "toothbrush" type wire brush (or the itty-bitty wire wheel on a Dremel) will get most of that. Then, while you've got the wire brush out, clean out the area where the nut meets the washer and the washer rests on the exhaust pipe retaining ring. Once that's done, fire up the engine and go have a cup of coffee. When you finish the coffee, shut off the engine and soak the washers, nuts, and exposed portions of the studs with PBlaster. Wait five minutes, and spray them again. A few gentle taps on the ends of the studs would be a good idea; you don't want to hit them hard enough to break anything or deform the studs . . . just tap, tap, tap. Another five minutes of waiting and another soaking with PBlaster and you put your tools away and allow the engine to cool. Come wrench time, use whatever socket or box-end wrench fits the nuts the most snugly. Don't worry about "metric or Imperial??"; just use what fits best. (Arithmetic says that 5/8 inch is 0.0049 inch smaller than 16 millimeter, but with manufacturing allowances and tolerances it's possible for a 16 millimeter wrench opening to be smaller than a 5/8 inch wrench opening.) Six-point wrench openings are preferred if nut-to-wrench fits are equal, but I'd rather use a tight 12-point than a sloppy 6-point. A few gentle hammer-taps may allow a socket that's just a skosh too small to fit by hand to seat on the nut. Spray more PBlaster. Be sure that the wrench is FULLY seated on the nut, NOT half-way engaged or angled. Use gentle force on the wrench to "wiggle" the nut tight-then-loose, tight-then-loose, tight-then-loose for a few moments before concentrating on the loose. It's ok if the fastener moans and groans a little, but loud squeaking is a request for more oil. If the stud wants to unscrew from the manifold instead of the nut unscrewing from the stud . . . let it. A new stud is easy to install. Then, for your new installation, use brass (even better, if you can find them, bronze) nuts and a high-temperature anti-seize compound. John
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