I've used several big valve Commando heads with bronze valve seats. The only thing I've noticed is that they seem to wear better than the stock iron seats. Various alloys of bronze have been used for valve seats, with nickel aluminum bronze being pretty popular. One of the phosphor bronzes was popular for seats and guides fro a while, but I haven't heard much about it for a couple of decades now. I've run beryllium copper alloy seats in my twin-cam Rotax singles racer in combination with titanium valves, and they seem to last forever, but never tried them in Norton heads. They are more pricey than the nickel aluminum bronze, however, and there are fewer suppliers. And people worry about the potential health hazards from grinding and machining them. I don't know how much hazard there is with the beryllium copper alloys, compared to the known hazards of working pure beryllium. I've been told it's not really an issue with the alloys, but I'm not a toxic metals health expert by any means. In any case, it worries some engine builders enough that they won't use it. Like DWS said, the thermal expansion rate of the bronze and beryllium copper alloys is closer to that of the aluminum head than iron or steel seats, so less likely to come loose, and they definitely conduct the heat away from the valve into the head better.
If you want to try them in a Commando head, KPMI sells the seats in what they call a proprietary nickel aluminum bronze alloy, and for a reasonable price.
I think the first Commando head I saw with bronze seats was a Dunstall big valve head, where they used it for the intake valve seat.
Ken
