I used a dremel with a wire wheel to clean those very fine threads and then doused them in PB Blaster before attempting to pull them off. I only broke one. If you break them you have to drill out the rivets that hold the grille together, replace the studs (you can get at your local nut/bolt store) or from
http://www.restorationspecialties.com
nice rims, are those sbp or lbp?