That was bad luck Mr.Burch as the same thing happened to me too!
So I have cut the offending 'J' head into pieces to find out what went wrong?
The main cause of porosity is internal rusting, this is a symptom of leaving heads on the engine, but dry.
They seem to condensate/corrode more than taking the heads off and storing in a dry place.
Then they go thin just under the seat area by the intake port where they pair up.
John
View attachment 1715290947
View attachment 1715290948