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The Harken HR D Stainless Steel shackles used on Harken blocks are available separately.
The breaking strengths on the Harken HR D Stainless Steel Shackles are derived from tests that support 80% of the length of the screw pin that is unsupported, which is similar to the area of a post in a block. Safe working loads are no more than half the minimum breaking strength.
The high-resistance shackles are marked "HR."