You've learned about V-band clamps and their retention rings, but here's the part that separates a header that seals for 100,000 miles from one that leaks after three heat cycles: how the flange gets welded to the tube. A V-band flange is a precision-machined stainless ring with a 20-degree conical sealing surface. Weld it wrong, and that cone distorts into a potato chip.

The distortion problem: When you TIG-weld a flange to a tube, the weld pool reaches ~2,500°F while the opposite side of the flange stays near ambient. That thermal gradient causes the flange to pull toward the weld as it cools. On a 2.5" V-band flange, uncontrolled welding routinely produces 0.010"–0.030" of coning distortion — enough that the V-band clamp can't pull the two flanges into concentric contact, and you get an exhaust leak that whistles at 4,000 RPM.

Weld prep techniques that actually work:

• Stitch welding first: Four tacks at 12, 3, 6, and 9 o'clock, then four more at 45-degree offsets. Never lay a continuous bead on cold metal.
• Back-purge with argon: Prevents sugaring (chromium carbide precipitation) on the inside of the joint. Sugared welds crack within a few heat cycles.
• Fixture the flange to a sacrificial mandrel: A machined aluminum or copper plug bolted through the flange face holds the sealing cone true while heat flows out.
• Skip-weld the final bead: Weld 1" segments, jump 180 degrees, weld another 1", and so on. Lets each segment cool before the next input.

Real-world example: Ticket Fabrication (a well-known race header shop) publishes their V-band welding procedure — they use water-cooled copper mandrels for the flange fixture on their Cup Car headers. Their spec is 0.003" maximum flatness deviation measured on a granite plate after welding. That's tighter than most cylinder head deck tolerances.

Rule of thumb — the "quarter-turn" flatness check: After welding, place the flange face-down on a granite surface plate with a 0.002" feeler gauge. Rotate the flange a quarter turn at each of four positions. If the feeler slips under at any position, the flange is coned and needs to be machined flat before it'll seal reliably. Most shops just face them on a lathe by 0.005"–0.010" post-weld as standard practice.

The clamp can only pull the flanges together — it cannot flatten a warped one.