They screw into a brass bushing pressed into the plastic manifold. They msy be seized into the brass bushing from corrodion, or if overtightened in the past the bushingsmay be spinning in their bores.
Try placing a lever beteeen the COP body and the mounting boss and applying a bit of upward forve while attempting to remove the mounting screw. If you get lucky the COP, screw and bushing will come out all together. You can then grab the bushing with curved jaw vice-grips and usually remove the screw.
Use JB-Weld to secure the bushing back into the manifold. I have repaired dozens of these in this manner...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq