###### Abstract  
The optimal amplitude of packing motion during nickel-titanium rotary root canal instrumentation remains unclear. This study evaluated how different pecking amplitudes influence the force/torque generation, canal-centering ability, and cyclic fatigue resistance of the Reciproc Blue R25 instrument. Thirty J-shaped resin canals were instrumented with Reciproc Blue using automated instrumentation and a torque/force sensing device with an up-and-down speed of 50 mm/min and varying pecking amplitudes: Group S (1 s apically, 0.5 s coronally), Group M (2 s apically, 1 s coronally), and Group L (4 s apically, 2 s coronally) (\(n\) = 10 each). Force/torque were recorded, and canal-centering ratios were determined. In a dynamic cyclic fatigue test, the Reciproc Blue instruments were rotated in a curved metal canal with 1 mm or 3 mm back-and-forth motions (\(n\) = 10 each). Group L exhibited significantly greater maximum downward force and cumulative upward (screw-in) force than Group S (\(p\) < 0.05), but torque was not significantly different across groups. Groups M and L exhibited a significantly smaller canal-centering ratio than Group S (\(p\) < 0.05). Pecking at 3 mm led to a significantly longer time to fracture and shorter fragment lengths than 1 mm pecking (\(p\) < 0.05). In the Reciproc Blue instrumentation, higher pecking amplitude improved canal-centering ability and cyclic fatigue resistance while increasing upward and downward forces. c 2020 2020 26 December Nickel-titanium (NiTi) rotary instruments are increasingly preferred over stainless steel hand instruments primarily because of their flexibility. This feature reduces the risk of deviation in curved root canals [5] and potentially improves the outcome of root canal treatment [6]. NiTi rotary instruments can be classified based on several factors: their usage, including shaping, orifice opening, and glide path creation; the metallurgy of the NiTi alloy, categorized as heat-treated or conventional (non-heat-treated); their design, such as taper (constant or variable) and cross-sectional shapes; and their motion types, including continuous rotation and reciprocation [7]. However, unexpected instrument fractures, caused mainly by cyclic and torsional fatigue, are a major concern in NiTi rotary instrumentation [8]. Cyclic fatigue fracture occurs when the instrument undergoes repeated tensile and compressive stresses while rotating in curved root canals [9; 10]. Conversely, torsional fractures happen when the instrument's tip becomes lodged in the root canal while the rest of the instrument continues to rotate [11; 12]. These fractures occur suddenly and are challenging to anticipate, potentially complicating treatment. Consequently, improving the fracture resistance of NiTi instruments is critically important. To achieve this, various strategies have been implemented, including the use of heat-treated alloys [13; 14], modifications of instrument geometry [8; 15; 16; 17], and the implementation of different rotational modes such as reciprocating motion [18] and Optimum Torque Reverse motion [19]. Reciprocating motion involves periodical loading and unloading, resulting in a more even stress distribution, reducing peak stress and improving cyclic fatigue resistance over continuous rotation [18; 20]. The Reciproc Blue system (VDW, Munich, Germany) is a representative and extensively investigated reciprocating single-file NiTi system characterized by an S-shaped cross-section and variable taper. This system is made from a proprietary heat-treated NiTi alloy, offering superior flexibility [21], higher cyclic fatigue resistance [21; 22; 23], reduced screw-in force [24], and improved canal-centering ability [24] compared with its M-wire predecessor, the Reciproc (VDW). Several operator-dependent factors, such as pecking speed, pecking amplitude, and insertion pressure, can affect the outcomes of NiTi rotary instrumentation [25; 26; 27].