Looking to mount to a metal surface? The RAM® Tough-Mag™ 66 or 88 mm diameter base is the powerful and durable solution you've been searching for. With a B size ball screwed into a 1/4-20" threaded post, simply attach a compatible double socket arm and your desired RAM® device holder. This base features an industrial-strength N35 grade magnet and rubber coating that protects the metal surface youâ??re attaching the RAM® Tough-Mag™ to. Features: Material: Marine-grade aluminum and rubber. Ball size: 1" rubber ball "B" size.