A V-band clamp squeezes two grooved flanges together, and the flat faces between those grooves are where exhaust gas actually gets stopped. That sealing face isn't just "flat" — the surface finish, measured in microinches Ra (roughness average), determines whether you get a leak-free seal or a slow hiss that gets worse every heat cycle.

The physics: exhaust gas at 1,600°F and 15+ psi will find any leak path wider than the gas molecule mean free path. V-band flanges seal metal-to-metal — no gasket, no crush washer. The two mating faces deform microscopically under clamp load and fill each other's valleys. Too rough, and the peaks bottom out before the valleys close. Too smooth, and you get galling as the faces rotate slightly under thermal cycling.

Typical target: 32–63 µin Ra, with machining marks running circumferentially (concentric with the flange bore), not radially. Radial tool marks create tiny leak channels straight from ID to OD — a perfect exhaust escape route. Circumferential marks force any leak path to spiral around the flange, effectively sealing themselves as the metal seats.

Real-world example: Vibrant Performance and Ticon Industries both specify their stainless V-band flanges at 32 µin Ra with a circumferential lay, produced by finish-turning on a lathe with a sharp carbide insert at ~0.005"/rev feed. Cheap eBay flanges are often milled flat (radial lay) at 125+ µin Ra — they'll seal cold but weep exhaust after three heat cycles as the surface work-hardens and stops conforming.

Rule of thumb for surface finish vs. feed rate on a lathe:

• Ra (µin) ≈ (feed rate in inches/rev)² × 32,000 / (tool nose radius in inches)
• Example: 0.005"/rev feed with a 1/32" (0.031") nose radius → (0.005)² × 32,000 / 0.031 ≈ 26 µin Ra — right in the sweet spot.
• Bump feed to 0.010"/rev and you get ~103 µin Ra — leak city.

Two other details matter: flatness (target ≤0.001" across the sealing face) and perpendicularity to the flange bore (≤0.002" TIR). A face that's rough and tilted will never seal, regardless of clamp torque. That's why premium flanges are single-point turned in one setup — the sealing face and bore are cut without re-chucking, guaranteeing they're square to each other.

Diagnostic tip: if a V-band joint smells like exhaust after a few weeks, don't just re-torque the clamp. Pull it apart and look at the sealing face under raking light. You'll see the leak path as a witness mark.