Found this mate

The standard cylinder is very good. It can support up to 350bhp without modification. For 350-500bhp the valves can remain standard but the ports need to be enlarged to 22.5mm on the intake side and 23mm on the exhaust. For 500bhp+ the valves can still remain standard but the ports need to be made even larger, 25mm inlet and 24mm exhaust and this works well with BD16 inlet cam and BD15 exhaust or BD15 inlet and BD14 exhaust. For 600bhp+ port need to be taken to 26.5mm inlet and 25.5mm exhaust. At this level the valves are becoming restrictive so bigger item need to be fitted. For bigger valves, bigger cams are required and this means further machining of the head