Definitely DO NOT remove the bolts. Just loosen. The "half moon" plates will drop in the well if you undo the bolts all the way.take a hammer and tap firmly on it (cover) first,the get some CRC Freeze off and spray on it and down bolt holes also providing you can get them out if not tap or pound on bolts to remove them.when attempting to remve use a large pipe wrench and use a twisting motion back and fourth,, a air zip gun with a blunt tip may also work to crack seal (vibration),,good luck.