An exhaust is a long tube hung on rubber, and rubber is a spring. Release one joint and the system redistributes its weight towards whatever still holds it, which is usually one hanger at the rear and the flex section at the front. Nothing about that movement is violent, but it is enough to misalign every joint you have not yet touched.
The movement matters because the parts you are about to fit were made to line up in the position the system sits in service, not in the position it droops to when a section is missing. Support restores the service position, and every measurement taken from it then means something.
Support at the hangers, not in the middle of a span. A jack under the centre of a long tube lifts the tube but does not restore the angle the hangers were designed to hold, and the joint faces will still be at the wrong attitude.
Put support under the heaviest item first, which is normally the muffler or the section carrying the resonator, then add a second support at the far end of the run you are working on. Two supports placed at existing hanger points hold the geometry far better than one placed for convenience.
A bare trolley jack cup on a tube will dent it, and a dent in an exhaust pipe creates a restriction and a fatigue point at the same time. Use a block of softwood between the jack and the tube, shaped so it cradles the pipe rather than pressing on one line of it.
Straps are better than jacks for anything overhead or anything that must be held at an angle. A strap around the tube and up to a beam holds the system at the height you set, leaves both hands free, and does not creep the way a hydraulic jack can over a long job.
The flex section is the part most easily damaged by bad support. It is designed to absorb small movement, not to carry the weight of the system, and a flex that is asked to hold a drooping section will fatigue its braid and eventually crack at the collar.
If the flex is taking load, the support is in the wrong place. Move it until the flex hangs neutral with the braid straight and no visible stretch. That is the check, and it can be done by eye before any joint is released.
With a section out, the two open ends will drift apart or past each other as the remaining system settles. A temporary spacer cut to the length of the missing section, taped between the ends, keeps the gap where it was and stops you measuring the same gap twice and getting different answers.
Keep the hangers in their rubbers while the section is out. A system left hanging on bare hooks will sit lower than it should, and the new section will then be aligned to a position that does not exist once the rubbers are back in place.
Before the new section goes up, check that the open ends are parallel and that the gap matches the section you are fitting. Then check every clamp and mounting part you intend to reuse: a stretched clamp will not seal and a hardened rubber will not isolate.
If those checks pass, the section can go in supported and be tightened from the middle outwards. If they fail, adjust the support rather than the part. Alignment fixed with force comes back later as a crack at the joint that was forced.
