A bolted exhaust joint holds by clamping force, and clamping force is stored in the fastener as stretch. When the joint gets hot, the flange and gasket grow more than the bolt does. When it cools, the gasket has been crushed a little flatter, and the stretch left in the bolt is less than it was.
That is why a joint that was correct on the bench can feel loose a day later without ever having leaked. It did not unwind. The stack of material under the bolt got thinner. Re-torquing is the correction for that, and it is a normal part of commissioning a system.
Two failures look identical at the spanner and need opposite responses. Relaxation is the gasket and the flange settling: the fastener is sound and the joint simply needs its clamping load restored. Stretching is the fastener itself giving up, and a bolt asked for more than it can hold comes back longer, so tightening it again only takes it closer to breaking.
The test is to look at the thread where the nut was sitting. A bolt that has stretched shows a narrowed waist and thread that no longer runs cleanly through the nut by hand. If the nut will not spin on freely once it is off, the fastener is finished, and re-torquing it is exactly how people snap studs out of a head flange.
Most owners worry about how tight. More joints are lost to the order they were tightened in. Bringing one bolt of a pair to full load pulls the flange out of square, and the opposite bolt then clamps a gap that is already closed along one edge. The joint feels tight and seals on a single side.
Work in stages and in opposition: bring the pair up together, move to the next pair across the flange, then come back round. On a flange with more than a pair, do the whole circuit twice. Even loading keeps the gasket flat and stops the flange warping next time it gets hot.
Not every joint does. A flange with a compressible gasket and a pair of bolts will almost always relax once and deserves the second pass. A bolted clamp with a machined mating face usually does not, because there is no gasket to crush. A slip joint on an exhaust pipe does not rely on fastener stretch at all, so judge that one by movement rather than by load.
The joints that earn a second look are the ones carrying a gasket, the ones at the front where the heat swing is widest, and any joint where two different materials meet, since they grow by different amounts. Everything else can be left alone unless it makes a noise or leaves a soot trail.
A fastener that is doing its job turns smoothly and then stops turning. A fastener that has given up turns smoothly and then keeps turning with almost no change in effort, which is the feel of thread pulling out or a nut running down a stretched stud. Stop at that point and take it apart instead of adding more.
The other feel worth learning is galling, a dry gritty drag that appears partway through the turn. It happens most on stainless fasteners assembled with nothing on the thread. Fighting through it welds the nut to the stud, and the joint then has to be cut off rather than undone.
If a joint has been corrected once and is loose again, the problem is not the fastener load. Either the flange faces are not parallel, the gasket is the wrong thickness for the gap, or the pipe is under a preload that keeps peeling the joint open. Every one of those gets worse, not better, with more clamping force.
The decision rule is one correction per joint. After that, undo it, check the faces against a straight edge, look at the gasket for a crush pattern that only touches on one side, and check that a muffler or the rest of the run is not holding the pipe out of line. A joint that needs a third pass needs a repair.
