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[cwill144]

I’ve had an issue with one manifold to Down pipe (cat) blowing. I’ve changed the gasket about 3 times now cures the problem and 2-3 months later happens again and gasket has blown. I’ve tried different types of gasket with no luck. One was a genuine GM part.
I’m worried about snapping off a stud if I over tighten it. The flanges look ok but can only assume one isn’t flat as only have the issue on the o/s
An advice would be appreciated

[dave the builder]

If the cat is aftermarket  NON original ,maybe the flange is welded slightly wonky ,
or....
 too much pipe is sticking out the flange when welded ,so the gasket is not compressed  (I had this issue with BM CATALISTS replacement, when some scum nicked my cats ) I trimmed some off with a death wheel 
also ,the manifold to cat bolts need tightening first ,then the support bracket bolt ,then the cat to mid flange ,in that order,

[cwill144]

No they are original GM cats, been on the car since I’ve had it. Only had this issue the last 12 months. And I do tighten the same sequence as you say.