2026-07-07
You've machined your V-band flanges flat, held the 20-degree angle, and controlled the surface finish. But there's one more geometric error that will haunt you at 30 psi of boost: radial runout. This is the wobble of the sealing surface as the flange rotates about its true center. Even a perfect angle and mirror finish won't seal if the sealing cone is off-center relative to the pipe bore.
What runout actually is: Radial runout measures how much the sealing surface deviates from a perfect circle concentric with the pipe centerline. Mount the flange on a rotary fixture, sweep an indicator across the 20-degree cone, and rotate 360 degrees. The Total Indicator Reading (TIR) is your runout. A flange with 0.010" TIR means one side of the cone sits 0.010" further from the axis than the opposite side — the clamp will pull unevenly, loading one arc of the retention ring while the opposite arc floats.
Why it leaks: A V-band clamp exerts uniform radial force around its circumference. When the flange sealing cone is eccentric, the clamp's V-groove contacts the high side first and never fully seats on the low side. You get a crescent-shaped gap on the opposite side — invisible to the eye, but at 1200°F and 30 psi, exhaust pulses jet through it. The classic symptom: a persistent tick or hiss from one specific clock position on the flange that doesn't respond to retorquing.
Sources of runout:
Real-world example: A Garrett G30-770 turbo hot-side flange on a 3-inch downpipe developed a chronic leak at the 8 o'clock position. Runout check on a mill rotary table showed 0.015" TIR — the fabricator had tack-welded the flange without a chill block, and the second-pass root weld shrunk the material asymmetrically. Reworking with a piloted weld fixture brought TIR to 0.002" and the leak vanished.
Rule of thumb: Keep sealing-surface radial runout under 0.003" TIR for 3-inch and smaller V-bands, 0.005" TIR for 3.5-inch and larger. Above 0.008" TIR, no torque spec will save the joint.