When you use helicoils, the expanded coil (once threaded into the body) must be shorter than the body thickness. If any coils extend below the body, they will not be threaded and expanded within the body. This will cause the screws to bind. I used 0.207 inch coils. Also, once you have the piece trimmed and ready to fasten to the body, tape it in place and drill a hole through both the plastic and body together. Then tap the body, install the helicoil, and install a screw. This will serve as an "anchor" point. Then proceed to the next hole and repeat. It is time consuming to take the piece on and off, but If you try to pre-drill the holes, they will most likely not match up. This method ensures all the holes match up and the piece can be easily removed and re-installed after cleaning and waxing.