A flange that misses its pattern badly is obvious on the bench and goes back in the box. A flange that is close enough for two bolts to start threading looks like a fit, and that is the one that gets welded onto a pipe before anyone discovers it will not pull up square.
The cost is not the flange. It is the section already cut to length, the weld already made, and the finish already applied to a part that now has to be cut apart. Treat any pattern that needs the bolts to pull it together as a fail, not as a fit with tight tolerances.
Three things define a pattern: the circle the holes sit on, the angle between them, and the diameter of the holes themselves. Two flanges can share a pitch circle exactly and still differ in how the holes are spaced around it.
The third dimension is the one most often missed. A hole that is generous lets a bolt pass at an angle, which feels like the pattern is close when actually the clearance is doing the work and the flange is being held off its face. Generous holes hide a pattern error rather than correct it.
A correct bolted flange carries load in friction and clamp, with the bolts in tension and the faces flat against each other. A mismatched pair carries the same load with the bolts bent, because each bolt is being asked to sit at an angle through two holes that do not line up.
Bending in a bolt is a fatigue load, not a static one, and it concentrates at the thread runout. The joint holds at first because the clamp force is still there, then loosens as the bent bolts relax, and eventually the faces part on one side. The symptom is a leak that keeps returning at the same clock position.
Do not test with all the fasteners. Push two bolts through opposite holes, or through the two holes furthest apart, and check that both drop in by hand with the faces touching. If either needs a drift or a clamp to enter, the pattern is wrong.
Then check the faces with the two bolts snug rather than tight. A gap that opens on the far side as you draw the pair up means the two flanges are rotating about the bolt axis, which is what an angular error in the pattern looks like once it is assembled.
Enlarging one or two holes is a legitimate correction when the error is small and the flange has material to spare around the hole. The hole should grow in the direction the joint needs, not in every direction, and only enough to let the bolt sit straight and unstressed.
What you must not do is enlarge holes until the washers no longer cover them, or slot a hole so far that the flange can slide while the joint is being tightened. A slot removes the register that keeps the two faces concentric, and a joint that can slide under load will always find a way to slide.
Re-drill when the error is angular but small, when there is a full ring of material round each hole, and when the joint is a low-demand one such as a bracket or an inspection cover. Those are forgiving joints, and a corrected hole will outlive the car.
Remake when the flange carries a gasket face, when it has to stay concentric with a bore, or when the correction would put a hole close to the weld. On a welded assembly a hole near the TIG welded seam concentrates stress exactly where the heat already softened the metal, and that is where the flange will crack.
