Upgrade your Hermle clock with mainspring #21/50, designed for reliability and performance. #21/50 are both the same size. This mainspring measures 43/64″ (17mm) in width, .0155″ (.40mm) in strength, and 47-1/4″ (1200mm) in length. Ideal for ensuring smooth operation and longevity, it’s an essential replacement for clock enthusiasts and restorers.