Ronstan's Core Blocks have a 2-stage bearing system which provides excellent performance across the full working load range. Under light and moderate loads, ball bearings minimize friction and also act as side-load/thrust bearings when the lead isn't fair. At loads that would deform the ball bearings, a solid bushing takes over, maintaining low friction performance. Use these blocks for sheets, halyards and control line applications for either cruising and racing. Versatile - easily handles high dynamic loads, as well as heavy static loads Shackle head lockable at 0 or 90 degrees Acetal (grooved for wire) sheave options Aluminum alloy cheek plates shaped for maximum strength, light weight, ease of freshwater rinsing, minimal chafe Beckets have removable pins to allow fitting of pre-spliced lines On non-becket models, the central hub hole can be used as a becket take-off or tie-up point