Hex Head Bolt for Armrest Bracket

The reason that your fold-down VANAGON armrests are coming loose from the seatback is because the OEM manufacturer of the seat used screws with different metric threads than the mounting backplates are tapped for. The OEM screws (recessed hex-socket machine screws) can be tightened, but because of the mis-match of screw threads, the OEM screws are still "loose" and allow the armrest to work free of the backplate, over time.

Trial-fit any aftermarket attachment screw for the correct thread "pitch". If the armrest screw is able to be wiggled after being screwed-in wit your fingers (trial fit without armrest in-place), the thread pitch isn't correct. If you want to do this 2-person job just once, check that the screws will not wiggle when tight.