Item #879884

Ronstan 75 mm Series 75 Core Mast Base Block - RF74108A

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Ronstan 75 mm Series 75 Core Mast Base Block - RF74108A

Item # 879884


The Ronstan Series 75 Core Block with Clevis Pin Head (RF74108) is a true all around solution. A tuned 2-stage bearing system provides excellent performance across the full working load range with an integrated thrust bearing feature and the resilience to handle high dynamic or static loads. Acetal sheave, alloy cheeks and stainless fixings complete the durable package. For reliability and a long service life, these blocks are the right choice for sheets, halyards and control line applications for cruising and racing.


  • Class leading performance and sleek, contemporary styling
  • Excellent dynamic and high static load performance
  • Less than half the friction of the nearest competitor
  • Durable aluminum alloy cheek plates
  • Central hub hole can be used as a becket take-off or tie-up point
  • Versatile trunnion snap shackle
  • Provides quick & simple attachment and removal
  • 360 degree relation with side-to-side articulation
  • Integrated becket through the hub of the lower sheave


  • Ideal for mainsheet, halyard and spinnaker sheet applications on boats to 14 m (45')
  • Various control line applications on larger yachts
  • Alloy sheave models are available for use with wire


  • Sheave Diameter: 3" (75 mm)
  • Max. Line Size: 9/16" (14 mm)
  • Safe Working Load: 3300 lbs (1500 kg)
  • Breaking Load: 6600 lbs (3000 kg)
  • Weight: 8.6 oz. (234 g)
  • Materials:
    • Sheave: U.V. stabilized Acetal or anodized aluminum (AW models)
    • Cheek plates: Aluminum alloy
    • Ball Bearings: High compression strength carbon black Acetal

Universal Bearing

The Ronstan Universal Bearing incorporates proven 2-stage bearing system with proven dynamic and high static load performance.

Stage 1 - Under moderate loads, Acetal ball bearings ensure minimum friction.

Stage 2 - Under heavy loads, where deformation of ball bearings alone would result in increased friction, a sliding Acetal bearing on a polished stainless steel race takes over, maintaining low friction performance.

Additionally the ball bearings are configured to act as a thrust bearing between the sheave and cheeks, preventing the sheave from rubbing on the cheeks and causing friction when the line lead in and out of the block isn't perfect. Particularly important for cheek blocks.

Alloy cheek plates

Block cheeks are manufactured from the highest quality alloy for maximum strength. Material optimization and the cut-away design minimizes weight and allows easy fresh water rinsing of salt and debris from the bearings. Cheek design has been further styled with flaring at the block throat and reduced gap between the cheek and sheave, minimizing rope wear.

Part Numbers for Ronstan 75 mm Series 75 Core Mast Base Block - RF74108A

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