If you pressure test....close off the ends of headers and inject a high
pressure source of air, nearly all commercial manufactured headers will leak at
the
slip joint. This is not a problem in that the slip joint is a tightly fit
piece and when the inside piece expands the joint becomes tighter. After tens
of
thousands of both new and used headers we have yet to have a single incidence
of exhaust leaking as your concern describes. Where we find leaks is at the
point where two pipes meet before going into the collector. This usually occurs
on the up side of the junction at the "V." To get a slip joint to leak exhaust
the positive pressure inside the exhaust would have to be fairly significant.
Not many years ago you could see collector slip joints that were held
together by two springs, both aftermarket and on some factory vehicles.
In the coating process pin hole leaks become very noticeable after the first
application of the outer coating. If the coater coats the inside of the header
with a thermal barrier (it's a different color) the results if a hole is
there is having to start all over after repairing the hole. Your coater should
be
one that is not a mass producer (like but I won't mention names like hot and
jet) any problem that may exist should be taken care of at the time of coating.
Moonstone
Good luck
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