The 228-270 engines all used the same valves except some used a 30º degree seat and some used a 45º seat. And what the book calls for may not be right. It was common to retrofit 45º seats with 30º seats and of course then you need valves to match. It is possible to make 30º valves out of 45º, but not vice a versa. A 51 probably came with 45º but it may have 30º seats now. The valve stems are all the same. If you are familiar at all with valves, you can tell the difference in 30º and 45º at a glance.
