on the C5 tank. i tried the heat/cool cycling along with soaking it in kroil and then trying to drift it out. wouldn't come out. finally decided to drill it out. unfortunately the center of the bit didn't exactly coincide with the center of the broken screw. not too far off though. anyways, after using progressively larger bits - and the OEM screw - i got enough out to accept the OEM screw. it is slightly offset toward the center of the tank but only by about 1/3 to 1/2 the diameter of the screw. mocked it up using screws in the upper and bottom right corners then put the screw in. took the two corner screws out just to see if there was any bind left in the new screw hole; nope. the new hole allows the screw to draw the cover down as supposed to. thanks, guys for all the help. gonna try to get the tank together this weekend, let it cure for about a week, and see if there's any leaks. my plan is the tighten the screws down to what i feel is reasonable torque, let it cure for about a day, and retighten. we'll see. i'll let you know.