The Stiletto under reamer provides a simple and reliable solution to the drilling inefficiencies and out-of-gauge wellbores that can increase hole enlargement costs exponentially. Whether enlarging an existing wellbore or reaming-while-drilling new hole, the Stiletto delivers the utmost in durability regardless of the formation or operating parameters. This versatile tool comprises a single-piece body engineered with Finite Element Analysis (FEA) modeling to ensure a robust low-mass design. This concentric symmetric mass-balanced design provides optimum hole opening ratios up to 25%, with increased flows around the tool for enhanced hole cleaning and reduced downhole vibration.