BMW used the reverse of this idea on their air heads: Externally threaded spigot with an alu nut with fins. The exhaust pipe can still reach inside the head as far as the port is straight and the finned nut adds a bit of cooling. If you use a liberal amount of a high-temp antiseize, removal was no problem.Funny you should say that, my next post (now this one) was going to address that very issue.
I'm going to digress here and give a little background that lays the basis for what I am going to describe.
What is our immediate goal? To get rid of waste heat. More specifically, to get rid of waste heat from areas where it can cause malfunction. For example, we really don't care, within reason, how hot our exhaust pipes get. They're usually steel or stainless and can stand the heat without losing their functionality. We are concerned about how hot the cylinder head, and more specifically some areas of the cylinder head, get. The areas of greatest concern are the EX valve, cracking between adjacent parts, for example valve seats that have different thermal profiles, EX valve seat and the EX valve guide. Taking these in order.
EX valves are not much of an issue in themselves anymore. A normally aspirated, gasoline burning engine is hard put to do any damage to a stainless steel or Inconel valve. You can hurt one over time but you almost have to try. Not to say that valve selection isn't important, it's just that if everything else is good, valves are the least areas of concern.
Cracking between valve seats or valve seats and spark plug holes is best taken care of by careful design and ensuring that the heat gradient between hotter and relatively cooler parts (EX seats and IN seats) is kept as low as possible.
EX valve seat and the EX valve guide; keeping these cool is the prerequisite for having a successful engine.
There are two ways of keeping the EX cool. The first, and the way most everyone focuses on, is to extract heat from these areas and discharge it to the air, either aluminum --> air with fins or aluminum --> water --> air with a radiator. The second way is to keep the heat away from those areas to start with. This is a design philosophy that the Japanese have used with great success.
After all that build up it's actually simple. Make the EX port as short as humanly possible. And then make it shorter. Get the hot gases into the exhaust system pipes as soon as possible. The longer the hot gases are in contact with the aluminum cylinder head the more they heat it up. Less contact, less heat build up , less heat to remove with our fins. This post is getting long and I still have more to cover but I can give examples of successful use of this design philosophy if anyone is interested.
Next, and this addresses wsimpso1 concerns about fins near EX ports; do away with the exhaust manifold flange. Thread the inside of the port and screw in an exhaust spigot. Triumph has been doing this with their 650 and 750 twins for decades. The EX manifold flange is a large mass of AL. It must be thick enough to provide threaded holes for the EX pipe and big enough around to provide an adequate sealing surface. By doing away with it you can cut fins right up to the outside of the EX port, exactly where they are needed. This also gets your fins much closer to the EX valve guide boss. Not many people that I have spoken with realize that this is the way that R4360 EX ports are made. The R4360 head has a threaded insert cast into the head and short EX spigots with square flanges screwed into the head. My belief is that they did it for the same reason, to put more fins closer to the EX port.
Attached is a kind of low res (didn't know what the rules were about pics) section view of an actual cylinder head designed and built for a customer. This engine is successfully meeting all of its' specs and is used in a research environment. The EX port is in red. The reason it looks bigger than the IN port is that this is really a 3 valve engine and only one IN valve is shown. To put the size in perspective, the cam bearing bore at the top is approx 1.0 inch (25mm). The recessed area at the right hand end of the EX port is the area where the exhaust spigot is threaded.
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