A joint that was gas-tight when it left the factory was tight because two mating faces were pulled together with enough even load to seat whatever sat between them. Once you undo that joint, the condition is gone. Gasket material that has already been compressed does not spring back, the load holding the faces together is released, and the faces themselves may have picked up a witness mark or slight distortion from the heat cycles they have seen.
That is why a section swap is the moment to treat gaskets and fasteners as service items rather than as reusable parts.
A gasket works by being squeezed into the imperfections between two faces. Composite and fibre materials crush, metal gaskets deform, and multi-layer steel gaskets rely on controlled spring-back. Each behaves differently when a joint is opened and closed again.
Choose the gasket by what the joint sees. A hot joint near the head needs a material that tolerates that heat without losing its structure, while a cooler joint further back can use a simpler one. Where a new length of exhaust pipe is going on, matching the gasket to the flange it will sit against matters as much as the material itself.
Exhaust fasteners live in the worst place on a car. They cycle between hot and cold, they collect road salt and water, and they are usually the smallest parts carrying the largest loads. A stud that has been through many heat cycles will often have stretched slightly, and a corroded nut will not pull down evenly the next time. Both are cheap to replace and expensive to discover later.
Studs and bolts should be renewed when they are visibly corroded, when the threads are damaged, or when they have been removed from a joint that has run hot for a long time. If a fastener has to be forced out, that is a sign it should not go back in. Use hardware matched to the material it clamps, because joining dissimilar metals in a wet, hot environment invites corrosion.
Slip joints and V-band clamps deserve the same examination. A V-band relies on two matching angled faces closing evenly, and once a clamp has been overtightened or distorted it will not pull the faces square again. A slip joint relies on a close fit, often with a sleeve or a sealing band, and a sleeve that has been crushed will not seal on reassembly.
Where a section is replaced with a fabricated piece, the flange or clamp interface is part of that new part's design. Getting the mating face flat, drilling the pattern to match and controlling distortion during welding so the face stays flat after the heat goes in is fabrication work. Titanium fab components are made to match existing patterns for exactly this reason.
Before anything goes back on, look at both faces. A flat, clean, unpitted face is what a gasket needs in order to seal. Scale, pitting, a burnt or dished area around a bolt hole, or a face that has been machined unevenly all reduce the load the gasket can hold. Clean the faces properly rather than relying on sealant to bridge them.
Check the tube for cracks near the joint as well. A crack starting at a weld or at a flange corner will reopen and will look like a gasket failure when it leaks. Cleaning the area back to bare metal is the only reliable way to see it before it becomes a leak.
Reuse is reasonable where the parts are in good condition, the joint is not critical and the metal is sound. A clean stud with undamaged threads and a nut that turns freely can go back with a new gasket. What is not reasonable is reusing a gasket that has already been crushed, or a corroded nut, simply because the part came off intact.
The rule experienced builders follow is simple. Gaskets are consumable. Fasteners are consumable when they show any of the signs above. Clamps are consumable when they have lost their shape. Everything else in the joint can be inspected and returned to service if it is genuinely sound.
