Number drill bit #21. A #21 drill bit is 0.159 inches in diameter, which converts to approximately 4.04 millimeters, a common size used for tapping #10-32 screws and for general drilling applications where a small pilot hole is needed.
Key Details:
Decimal Equivalent: 0.159 inches.
Metric Equivalent: 4.04 mm.
Common Use: Often the perfect size for drilling holes for #10-32 threaded fasteners.
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