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Apprentice answer

Why do TDC flanges bind on a tight back-to-back 45-degree offset

2nd YearDuct Assembly, Seams & JointsStandard Correction

Close back-to-back flanged fittings can run out of corner clearance. Align the faces, protect the gasket, and get approval before trimming or using alternate hardware.

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Plain-English answer

A tight S-offset can put two companion-flange corners so close together that the corner lips or brackets fight each other. Forcing the bolts can twist the fitting and open a leak path.

Loosen the line, square the faces, and verify what is actually overlapping. Minor corner cleanup may be acceptable if it does not weaken the flange or violate the shop standard, but get direction before grinding factory hardware.

Ask Foreman

The back-to-back 45s are so tight that the TDC corners are binding before the gasket faces seat. I can clamp and align it, but the corner lip may need approved cleanup. How do you want it handled?

Verify before acting

Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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