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Apprentice Q&A · #343

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

2nd YearGREEN · Standard CorrectionTDC/Ductmate Close Offset

Short answer

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.

Field 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.

What to check first

Do not do this

Do not force TDC/Ductmate corners together with a drift pin so hard that the flanges twist or the gasket gaps.

Why it matters

Twisted companion flanges leak, whistle, and make the tight offset harder to service or seal.

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?

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Related Field Rescue route

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Final direction belongs to the foreman, approved drawings/specs, manufacturer instructions, pressure/material schedule, employer policy, and AHJ/code requirements.