Piston pin Ø15x39mm SYM

This aftermarket wrist pin is precision-manufactured for the SYM Symphony 125I ST 125cc engine, fitting 2017-2020 model years and cross-referenced to OEM number 13111-ARA-000. Though small in size, the wrist pin is absolutely critical to engine reliability, serving as the pivot point that transfers the full force of combustion from the piston to the connecting rod.

During every power stroke, the wrist pin experiences alternating shear and bending stresses as the combustion pressure—exceeding 80 bar in the 125cc engine—pushes the piston downward. At 8,500 RPM, these load cycles occur 141 times per second, demanding exceptional fatigue resistance and dimensional stability. The 15mm diameter and 39mm length are precisely maintained to ensure proper fitment in both the piston boss and connecting rod small-end, with clearances tight enough to prevent hammering noise yet loose enough to allow the full-floating rotation that distributes wear evenly.

Manufactured from 20CrMnTi alloy steel, this wrist pin undergoes carburizing to achieve a case hardness of 58-62 HRC with a case depth of 0.8-1.2mm. This surface treatment provides exceptional wear resistance where the pin contacts the piston boss and connecting rod bore, while the core maintains a softer 30-35 HRC structure that resists brittle fracture under impact loading. The outer diameter is ground and polished to a surface finish below 0.2μm Ra, minimizing friction and promoting oil film retention. The ends are chamfered at 15° to ease installation and prevent damage to the piston boss during assembly.

Technical specifications: diameter Ø15mm; length 39mm; material 20CrMnTi; surface hardness 58-62 HRC; core hardness 30-35 HRC; surface finish <0.2μm Ra; weight 45g. Replacement is essential during any piston replacement procedure, as the original pin will have developed wear patterns that match the old piston boss. Reusing an old pin in a new piston can cause accelerated wear and noise. Additionally, replace the pin if blue discoloration indicates overheating, if dimensional wear exceeds 0.01mm, or if surface pitting from oil contamination is visible. Always install new circlips and verify they seat fully in the piston boss grooves before engine assembly.