The 16 mm is Harkens smallest block. It is perfect for small synthetic control lines found on high-tech dinghies. The patented bearing system uses stainless steel balls in a captive grooved race and has a 250 lb (113 kg) maximum working load. The 404 uses an O-ring to keep the line in the sheave during intermittent loading.SpecificationsSheave diameter: 5/8″(16 mm)Length: 1 1/8″(29 mm)Weight: 0.33 oz.(10 g)Maximum line diameter: 7/32″(5 mm)Maximum working load: 250 lb(113 kg)Breaking load: 1200lb (544 kg)Spare PartsHSB340: O-Ring – Fits 404 Block