Based on zinc alloy which is extremely durable. Ideal for medium to high risk of theft, the padlocks are recommended for a wide range of security requirements. All models in this series have a par acentric keyway, zinc alloy plated cylinder cores and hardened steel shackle. The lock is built to resists being picked by a shim (along the sides of the shackle), and also has a unique internal mechanism that is more difficult to pick. And, because there is a metal barrier that blocks the shackle, it is also more difficult to clip with bolt cutters.Variations vary in sizes where small is 40mm Medium is 50mm and Large is 60mm.