1 1/8" outside diameter, 5/8" inside (for the longer arm, anyway). 85 cents @ Abauchon Hardware. It looks too large but fits perfectly once you get it in there (I use that line a lot). It's much easier to install with the arm off (your other plastic grommet is about to break anyway - buy two per problematic side). I replaced both on the passenger side about 50 miles ago - so far no problems after repeated up and down cycles. If either end drops off I'll report back in, but it seems pretty snug. I may safety wire the arm (or try a creatively knotted rubber band) just be safe.

 

How about posting the part number? And I think you mean Aubuchon Hardware: http://www.hardwarestore.com/

Thanks,
Rich

 

I don't know if there even is a part number for these things. Any hardware store that sells loose nuts, bolts, washers, etc should have a selection of rubber grommets - all you need to know is the inside/outside diameters.