A joint that only seals when the fasteners are pulled far tighter than it needs is not being sealed by the gasket. It is being closed by bending the flanges until they touch. Bolt load is going into deflection instead of into compressing the gasket, and the figure the owner has settled on is simply where the deflection ran out of travel.
That is worth naming plainly, because the instinct is to add more. More torque does close the joint, briefly. It also puts the flange and fasteners into a stress state they were not made for, and the failure that follows is a cracked flange or a stretched bolt rather than a leak.
A flange pair should behave as two flat rings pressed together with a gasket between them. The load path is short: fastener, flange, gasket, flange, fastener. When a joint needs excessive torque, part of that path has become a spring. Either the flange is thin enough to dish under the fastener heads, or the faces touch on a narrow band and the rest of the gasket carries nothing.
You can feel it at the wrench. A correct joint takes up steadily and then stops. A joint with a spring in it takes up, feels tight, and then keeps accepting more as the flange bends, with no clear stop and no change in the gap you are trying to close.
Welding is the usual cause. A flange welded to tube shrinks as the weld cools, and the shrinkage pulls the face away from flat. The distortion tends to be conical rather than wavy, so a straight edge laid across one diameter looks acceptable while the face is in fact dished. A plate flange can also arrive bowed from cutting, or be machined flat and distorted by the weld after.
Thickness matters for the same reason. A thin flange bends before it loads the gasket, so the joint needs torque to make up the bend. Wide fastener spacing, or fasteners grouped on one side of the bore, produces the same result on a stainless exhaust assembly that is otherwise correctly made.
A gasket that is too soft, or one that has already collapsed, also gives a joint that will not hold. The difference is behavioural. A collapsed gasket keeps taking torque as it crushes and then leaks again once it has taken a set. A distorted flange takes torque without the gap closing evenly, and the leak appears at one point on the perimeter.
The other case to rule out is a joint that could never seal because the bore is stepped or the gasket is the wrong shape for the face. That leaks from the first start, where the step is, and no torque changes it. Torque that worked and then stopped working points at deflection. Torque that never worked at all points at geometry.
Bolt the joint up dry, with no gasket, and look for contact. A thin film of grease or a marker line on one face, assembled and separated, shows whether contact is a full ring or a crescent. A crescent means the faces are not parallel and the joint is being closed by force at one edge.
Check the fasteners as well. If a stud runs out of thread before the faces meet, the joint will never pull down, and the symptom is identical to a soft gasket. Check that washers are not bridging a relief, and that the flange is not being pulled onto a shoulder of weld standing proud of the face.
Every bit of torque that goes into bending a flange is stored in the assembly. When the joint heats and the parts expand, that stored load changes, and a joint closed cold with excessive torque opens as they move. The gasket then has to seal against a face that cycles open and shut, which destroys it quickly.
The repair is to restore the face, not increase the load. Face the flange flat after welding, check it against a straight edge before it goes on, and use a flange stiff enough that fastener load reaches the gasket. The flange and the exhaust pipe behind it decide whether the joint can seal at sane torque.
