Two suppliers can both call a section the same size and both be honest, because the label describes what the part is for rather than what it measures. One quotes the outside of the tube, another the bore it was formed from, a third the system it replaces. None of those is the number your caliper will show.
That is why a buyer ordering by name and a fabricator ordering by measurement end up with parts that will not meet. Before comparing anything, decide which dimension you actually need: the outside diameter where a hose or a clamp grips, or the inside diameter where flow and slip engagement are decided.
Exhaust tube is usually described by its outside diameter, because that is the surface everything else clamps onto. Hose and some pipe is described by bore. A part sold against a bore reads as oversize on the outside by twice the wall, even when it is exactly what was ordered.
The trap is that both descriptions sound identical in a listing. Ask for the outside diameter at the end, and ask whether that figure is nominal or measured. A supplier who can answer the second question is describing a real part; one who cannot is repeating a catalogue heading.
Two sections with the same outside diameter and different walls are not interchangeable at a slip joint. The thinner one enters the thicker one with clearance and leaves the clamp to close a gap. The thicker one will not enter at all, and forcing it distorts the very end that was supposed to seal.
This is also why a brushed exhaust pipe can measure correct at the cut face and still be tight further in. Surface finishing removes material unevenly, and a bend thins the wall on the outside of the radius while pulling it in on the inside. Measure where the parts engage.
A caliper across one axis tells you almost nothing about a used or roughly handled section. Take two readings at right angles and compare them. The difference between the two is ovality, and ovality is what makes a joint leak while both parts measure correct on the bench.
Do the same on the part you are joining to it. A joint is decided by the worst pair of readings rather than the average, and the small axis of one part against the large axis of the other is where clearance disappears. If an end has been clamped before, its small axis is the one that was crushed.
Sealing depends on overlap, interference and how far the clamp can deform the parts. A slip joint needs enough engagement for the band to sit fully on both sections, and enough interference that the band closes metal onto metal rather than closing a gap that was never there.
A butt joint is a different problem. It cares about squareness and wall match, not diameter, so two sections of identical size but different wall will weld with a step at the root, and a step is where a crack begins. Decide which joint type you are building before judging the sizes at all.
Lay both parts on a flat bench, push them together as far as they will go without force, and mark where they stop. Compare that with the engagement your layout assumes. If the marks disagree with the drawing, the parts disagree with each other, and clamp load will not repair a fit that never existed.
Where the difference is small and the joint is a slip, resizing one end is a reasonable correction, and a resized pipe section should be checked for cracking along the new edge before it goes on the car. Where the difference is in wall, stop and order the matching section.
