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Holley® jets are broached, flowed, and stamped according to flow rate. NEVER drill jets, as this seriously alters flow characteristics. Stamped numbers are reference numbers and DO NOT indicate drill size.

The above is taken directly from the instruction sheet for a Holley DP carburetor.

My experience with QF jets is the number stamped on the jet coincides favorably with a measurement made in thousands of an inch; a jet stamped 70 has a hole size of .070 inch and the entrance and exit openings are coned. The only exception is the exit opening of the Power Valve Restriction, it is not coned.