Putting them back in the heads, threads clean - should some sort of thread locker be used? Or just put them in without anything?

 

I think they are the type of fitting that is tapered or bevelled -- probably not "by the book here" but -- I have always put a little of the silver antiseize on the threads, since they can be such a beast to remove. Never had an issue. Never had an issue with leaks there either (ie .. no, I have not used thread locker)

 

I didn't realize until after cleaning them and reading the RAVE that they have a 9mm hex in the center, haha, way easier to install/remove now knowing that.