This is the exhaust seat insert we use in all of our rebuilt cylinder heads and engines.  Unlike low grade hardened cast iron seats, or slightly better chromoly bar stock seats, we only use OEM specification powdered (sintered) metal hard seats.  These powdered metal seats are extremely heavily alloyed with tough durable elements like tungsten.  The seats will cut with a beautiful finish and the seat further work hardens in use, both reasons that OEM cylinder heads today feature powdered metal valve seats.
Priced individually, sold only in sets of 6.