Flat Headed Pivot Bar Fasteners or Rivets
Those rivets appear to be pop rivets. You would need a pop rivet gun like our 75-012, while you can get the rivets themselves at a local hardware store. To remove the old ones, you'll have to drill them out.
I'll never understand why manufacturers used rivets instead of screws. I guess they never intended you to replace the pivot bar if it ever broke. Honestly, I highly recommend considering installing a new replacement pivot bar using a truss head screw like our 33-073. That way, if it ever breaks in the future, it'll be an easy project.
If this is the case, is there a recommended rivet size that fits the 26-377 / 8254 bar? My preference would be to switch to screws as suggested here, but the rivet option would be a backup if that's not viable on our windows.
Thank you!
It looks like your pivot bar is similarly installed via rivets, though I cannot tell you if it's the same configuration as the above poster. All I can tell you is that the 26-377 is compatible with a #8 screw like the 33-073, as Paul mentioned back in 2018.
I unfortunately do not know what rivet size you would need. I've never installed a pivot bar using rivets. Screws are almost always a viable choice and I use them whenever possible.