20C979 Counterbores with Built-In Pilots

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High-speed steel counterbores are the most common choice for creating a flat-bottomed cavity at the top of the hole so that a bolt, nut, or other part is hidden beneath or sits flush with the top of the hole when installed into it. High-speed steel is less brittle than cobalt and can better withstand vibration and chattering. Sometimes known as solid pilot counterbores, these counterbores have a built-in pilot that cannot be removed and which keeps the counterbore centered in the hole to ensure a level seat.