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Apprentice answer

Do I put the butyl gasket tape on the inside or outside of the bolt holes on a TDC flange

1st YearDuct Assembly, Seams & JointsCheck first

Always run the butyl gasket tape on the inside of the bolt holes, tracking directly along the center point of the mating flange faces. This ensures the air pressure pushes the gasket into the seam instead of escaping through the bolt openings.

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Plain-English answer

Always run the butyl gasket tape on the inside of the bolt holes, tracking directly along the center point of the mating flange faces. This ensures the air pressure pushes the gasket into the seam instead of escaping through the bolt openings.

Field checklist

Ask Foreman

Hey boss, I’m checking duct assembly: Do I put the butyl gasket tape on the inside or outside of the bolt holes on a TDC flange? I found the likely issue and want to verify the next step before I lock it in. Do you want me to adjust it now or check the drawing/detail first?

Verify before acting

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

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