You may want to review your bolt-up method as the result is repeating... the
rear
headers should be attached loosely prior to fitting muffler then front headers;
give it all a jiggle and wiggle when loosely attach and tighten all the headers
gradually after fitting springs giving an occasional jiggle and wiggle
as the headers are tightened; this de-stresses the exhaust system and prevents
symptomatic cracking and leaking seats at the cones; a very thin smear of
copperseal
anti-seize on the sealing face of each cone can aid alignment and will
burn off fairly quickly on startup (not always required), copperseal each header
stud and nut too (it is a high temperature paste and will remain intact to
aid periodic service removal)(the stud nuts are reusable and require a lite
re-crush
in a vice applied to the lock ring)(studs should have a thread die run
over them before refitting nuts). A thin smear of Maniseal or similar exhaust
sealer in the muffler union pipes can aid gas sealing and will provide enough
lead time to position exhaust system before setting; dags can be brushed off
after drying. Grinding off the spring loop and TIG welding a strap over the
crack
repair area before welding the loop back on should add sufficient material
strength to prevent further cracking.
Occasionally a stud will screw out of the head leaving nut attached; fit a new
stud or secure stud in a large vice between copper sandwich and carefully work
seized nut off the stud with Wurth Rostoff. Clean the stud with a thread die
and refit and re-torque into the cylinder head. Run an intermediate thread tap
through the nut and re-crush locking ring before refitting.... Sometimes the
nut just won't budge; put a flame on it until its blood red then unscrew while
hot and keep it hot as its being unscrewed.
That's the whole gamut of working with and servicing exhaust systems.
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