Support both sections, check square and alignment, then use proper clamps/duct stretcher if appropriate. If it still will not mate, stop and ask before forcing or cutting.
Confirm piece mark, size, airflow, and connector sides
Seat the S and drive cleanly
Check corners, support, gaps, and sharp edges
Ask foreman
In Rectangular Duct Assembly: S-and-Drive, I’m looking at: What do I do when the two duct sections will not pull together? What should I verify before I cut, drill, seal, cover, move, or install?
Do not
Do not use force, screws, tape, or mastic to hide a bad mechanical fit.
