This aftermarket crankshaft assembly is precision-engineered as a direct replacement for the SYM Symphony 125I ST 125cc scooter, specifically covering model years 2017-2020. Cross-referenced to OEM number 13000-ANL-010, this component is manufactured to meet original equipment dimensional specifications while providing reliable performance for daily commuting and extended touring applications.
The crankshaft serves as the mechanical heart of the engine’s bottom-end assembly, converting the reciprocating linear motion of the piston into the rotational force that drives the transmission and ultimately the rear wheel. In the 125cc 4-stroke single-cylinder architecture, this component rotates at speeds up to 8,500 RPM under normal operation, subjecting it to tremendous centrifugal forces and cyclic bending stresses. The 15mm piston pin diameter is precisely machined to H7 tolerance, ensuring proper wrist pin fitment without excessive clearance that could lead to noise or accelerated wear.
Manufactured from forged 42CrMo4 alloy steel, this crankshaft undergoes comprehensive multi-stage heat treatment. The main bearing journals and crankpin surfaces receive induction hardening to 58-62 HRC, creating a wear-resistant case while maintaining a ductile core with yield strength exceeding 750 MPa. Subsequent precision grinding achieves surface finishes below 0.4 microns Ra, minimizing friction and extending bearing life. Dynamic balancing to within 5g ensures smooth operation across the entire RPM range, reducing vibration transmission to the chassis and improving rider comfort.
Key technical parameters: pin diameter 15mm; material 42CrMo4 forged steel; journal hardness 58-62 HRC; surface finish <0.4μm Ra; dynamic balance <5g. Installation requires complete engine disassembly including crankcase splitting, a procedure best performed by experienced technicians with access to proper torque wrenches and assembly fixtures. The crankshaft must be paired with matching main bearings and connecting rod components to maintain proper oil clearance specifications. Recommended replacement scenarios include: major engine overhaul at 50,000+ km, detection of journal scoring or out-of-round conditions exceeding 0.01mm, connecting rod big-end play exceeding service limits, or following any engine seizure event that may have caused torsional stress damage. Always verify crankshaft runout on V-blocks before installation, and replace the oil pump and screen simultaneously to ensure adequate lubrication for the new bearing surfaces.
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