I've seen some videos on how to fit a damper in a duct but his damper had only one spindle, requiring a single hole. The dampers in my local store, however, have two spindles, at 180 degrees, meaning diametrically opposite. That means I need to know exactly where the point on the duct is 180 degrees from the other hole, which is hard.

Is there a trick to drilling two holes in a duct that are diametrically opposed to one another, so that they are aligned with the spindle shaft of the damper?


1 Answer 1


Say you have an 8 inch diameter duct. Then the circumference is 25.13 inches (you can find the formula online).

Get a thin strip of paper 30 inches long.

Make two dots 12.56 inches apart on it (half the circumference).

Wrap the paper around the duct and tape it.

The dots are where you drill.

Sort of like this: https://www.youtube.com/watch?v=1XsBwEmFO_w

  • Nice answer! Simple, accurate, easy. Great idea.
    – FreeMan
    Jan 25, 2021 at 16:35
  • 1
    If you make a third dot with the same spacing and wrap the full circumference, you can make sure you got the length right before drilling (when the third dot meets the first).
    – Kevin Reid
    Jan 25, 2021 at 16:50
  • If there is insufficient room on one side of the duct to admit a drill (ie, it's already installed and can be accessed only from above or below, but not both) then put the paper guide inside the open end of the duct. Drill through the near side and then the far side of the duct by using a 12" or longer cable installers (or "bellhanger") drill bit.
    – Greg Hill
    Jan 25, 2021 at 17:19

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