A hanger does not just hold weight. It holds weight while the pipe it supports is expanding, contracting, and being shaken by every bump and every change in engine load. That makes it a fatigue joint, and fatigue joints fail at the fastener long before they fail at the bracket.
This is why a hanger bolted into a thin single-skin panel with a self-tapping screw looks fine for a while and then starts to open the hole. The screw was never the problem. The panel around it was never thick enough to spread the load, so the hole gradually grew into a slot.
The convenient hole under a car is frequently a locating hole for a clip, a drain, or a trim fastener, and behind it is a cavity rather than structure. Others sit in seam sealer, which looks like metal and behaves like nothing at all once a load is applied.
A threaded hole is not automatically a fixing point either. A captive nut for a plastic undertray is sized for a light, static part, and hanging a stainless muffler from it asks a fastener designed for almost nothing to carry a vibrating mass.
Tap the panel around the intended point. A dull, dead sound with no ring means you are over a single skin or a void; a tighter, higher sound means you are near a pressing, a reinforcement or a double panel. Aim for the second wherever the layout allows it.
Then look from the other side if you can reach it. Rust staining, underseal texture and the shape of the pressing all tell you where the structure is. A hole drilled through a reinforced area can take a proper fixing; one drilled through a flat unsupported span will need a spreader plate behind it.
The alternative to drilling the body is to move the fixing point on the pipe. A hanger arm welded further along a straight section, or rotated to reach an existing threaded boss, changes the geometry of the part rather than the car, and that is usually the cheaper and more reversible decision.
Keep the arm short and load it in line with the pipe rather than across it, because a long arm turns vertical load into a bending moment at the weld. Where the arm has to be long, add a gusset and weld it to the hanger and bracket rather than relying on the tube wall.
A hanger needs a compliant element between the pipe and the body, and that element has to stay in the load path. A bracket bolted straight to the shell with no rubber transmits every vibration into the panel and every panel movement into the pipe, which is how you get a crack in one or a drone in the cabin.
If the new fixing lands where the original isolator cannot reach, use a short arm to a proper rubber rather than bolting metal to metal for convenience. The isolator also allows the pipe to grow and shrink, and a rigid fixing removes that allowance, so the movement goes into the nearest weld instead.
Drill only if three things are true: the point is over reinforced metal, the fixing spreads its load over an area larger than the fastener, and there is room for an isolator in the resulting load path. If any of those fails, move the bracket instead.
If you do drill, treat the exposed edge immediately, because a bare cut edge in a wheel arch is where corrosion starts and a hanger that rusts loose is worse than one that was never fitted. A fixing into structure with a spreader and a protected edge will outlast the pipe it holds.
