The studs in the exhaust manifold rust and no longer hold the turbo to the exhaust manifold completely and then you hear exhaust leaks. To fix this you can remove the original studs and then tap the manifold with a 7/16 NC 14 threads per inch bottom tap. You will need to enlarge the holes in the turbo mounting flange to maintain the necessary space for the relative movement of the two pieces. Then insert the new stud and new correct fastener to attach the turbo and you are done. However the original studs and fasteners are a very special type of fastener. The fastener should be a trilobate prevailing torque nut. This is a mechanical type of fastener that does not loosen by itself. It looks sort of squished. Any of the Stainless Steel commonly available as fasteners would be OK. But some of the ferritic Stainless Steel is rather soft and the temperature might have some effect on the other alloys. But the 300 series would be satisfactory---say 308.

Update 10-26-03.