The Martin truss rod wrench features premium materials for professional-grade instrument setup. Its ergonomic design ensures proper truss rod adjustment on all your prized instruments. Precision ball end tip for easy fit Ergonomic handle Available in 4mm & 5mm 4mm, Item # 18TOOL32 5mm, Item # 18TOOL31 The 4mm truss rod wrench can be used on the following models: DSS and OMSS Models Modern Deluxe Series SC-13E For other models, please use our 5mm truss rod wrench.