... 3. No noise, high efficiency NOVO's timing pulley:1. Plain bore: MXL, XL, L, H; T2.5, T5, T10; AT5 ... 5M, 8M,14M.3. Material: Aluminium, Cast Iron, Steel.4. Surface Treatment: Black, Phosphate, Zinc Plating, Hard anodized, Nitriding, Dichromate.5 ... qualityCertification:We've been ISO9001:2000 certified since 2003. HTD plain bore pulleys