[krausewich]

Hi Kevin,
I do not have a number on what size the ERA Core is, but I can say your friend should be safe with the 3 or 4mm. It acts as a spacer between the glasswork of the body, and the encompassing glasswork to prevent shock transfer of road debris. An 1/8-3/16 should be plenty safe as long as he doesn't saturate the core material.

Good luck,
Mike.