The Ronstan Series 15 Ball Bearing Single Block is a tiny, light and versatile block with a 5/8" diameter sheave and a very high breaking load.Ronstan's Series 15 Ball Bearing Utility Blocks are the tiniest ball bearing blocks anywhere, running on sheaves only 5/8" (15 mm) in diameter.They're so light, strong and inexpensive that you can use them in lots of creative applications, like shock cord retractors for your control lines, purchase systems for your foiling Moth, or outfitting your kid's El Toro, Sabot or Opti. Use them for cunninghams, vangs and tweakers, or even for non-boating applications.Key FeaturesSingle loop head block includes an O-ring to separate running line from head lashing or other attachment hardwareThe sheave is precisely-molded, UV-stabilized acetal running on stainless steel ball bearings, so it delivers high performance with low frictionCheeks, headpost, shackle and ball bearings are all 316 grade stainless steel for maximum corrosion resistance in saltwater sailingSmallest and lightest ball bearing block availableSheave Diameter: 5/8" (15 mm)Max. Line Diameter: 3/16" (5 mm)Max. Working Load (MWL): 260 lb. (120 kg)Breaking Load: 1210 lb. (550 kg)Weight: 0.3 oz. (7 g)