TDC flanges need to match level and alignment before hardware starts. Drift pins can help line up close corner holes and C-clamps can hold the flange faces, but neither one should be used to force a twisted or unsupported run together.
Do not use bolts, corners, cleats, screws, drift pins, or C-clamps to drag a badly misaligned TDC joint into place.
Check
Check whether both duct sections are supported evenly before hardware starts.
Verify gasket is on one flange face only and has not rolled at the corners.
Look for flange offset, twist, bowed duct, or holes that do not line up naturally.
Re-support and align the duct faces before tightening corners, cleats, screws, drift pins, or clamps.
Use drift pins only for close hole alignment.
Use C-clamps/locking clamps only as controlled holding pressure and protect the flange/gasket from crushing.
If cleats are missing, ask before using 5/16 self-tappers as the fallback.
Steps
Check whether the duct sections are supported evenly before starting hardware.
Look for flange offset, twist, bowed duct, or bolt holes that do not line up naturally.
Loosen or reposition the section before the gasket gets crushed unevenly.
Start hardware only after the flange faces are sitting square and the gasket is in place.
Say this to your foreman
The TDC flanges are close but not sitting clean. Do you want me to re-support and align first, then use drift pins/C-clamps to hold it while I start the hardware?