A V-band joint works because a clamp with angled flanks draws two matching flanges into each other. The angle, the width of the band and the outer profile of the flange pair are designed against each other, so a clamp and a flange bought separately are two halves of an assembly that was never specified as one.
Suppliers who sell both usually sell them as a pair for that reason. Buying the clamp from one source and the flange from another is not automatically wrong, but it removes the assumption that the two were drawn to the same profile, and that assumption is doing a lot of work in a joint like this.
Three surfaces interact. The two flange faces have to meet flat, the two outer cones have to present a continuous surface for the band to slide over, and the band has to be able to close far enough to reach both cones without touching anything else.
Any one of those failing gives a joint that looks assembled. The band sits on the outside, the fastener takes load, and nothing tells you that the band is bearing on the flange rim rather than on the cone, until the faces part under load and the joint starts to whistle.
If the band closes until its own ends meet, or until it contacts the outer diameter of the flanges, it has run out of travel. The fastener can still be turned, and it will feel tight, but the band is no longer squeezing anything. All the load is going into the clamp body instead of the joint.
That condition is common when a clamp is slightly too large for the flange pair. The tell is that the fastener reaches its stop long before the joint feels solid, and that the band can be rocked by hand. A clamp that is correct still has travel left when the faces touch.
After one heat cycle, take the clamp off and look at the flange cones. A correct joint leaves a continuous polished band on both cones at the same height, of similar width, running the full circumference. That band is the record of where the clamp actually pressed.
An incomplete band, a band that stops and starts, or a band sitting lower on one flange than the other all say the two parts are not matched. A V-band connection that has been running like that will have been fretting, and fretting on a cone is how the joint starts to seize and then to leak.
The safe way to mix sources is to keep each joint within one supplier's set. The flange pair and the clamp that closes it should come from the same place, and the transition to another supplier's pipe should happen at a different joint, welded or slipped, not at the V-band itself.
Where you must combine them, check the flange outer profile against the clamp flank by eye with the parts together and unclamped, then check that the band still has travel once the faces are touching. Both checks take a minute and they catch almost every mismatch before it is welded in.
Tack the flange to the pipe, fit the clamp and the mating flange, and draw the joint up. If the two faces meet with the band still free to move inward, the geometry is sound. If the band is already hard against its own stop, stop and change one of the three parts.
Do this before the seam is finished, because a flange and clamp set that has to be changed after the weld means cutting the pipe back and losing the length you planned. The test fit is the only point where the choice is still cheap.
