This insert has a higher volume / lower compression ratio, meaning better over-rev characteristics and more top speed. However, keep in mind that this compromises bottom power reaction. The combustion chamber fits the VHM cylinder head perfectly and is easy to change. The last numbers of the part number are engraved on the insert for identification. Use in combination with VHM head. More top. Best for hard ground / enduro. Good resistance to detonation. Note: This insert does NOT fit into an O.E.M. head, only a VHM cylinder head.