I don't know about the exhaust port and the head is currently at the 'doctors'.
However, interestingly enough, the guide doesn't come out completely it appears the bottom ring is sufficiently large to prevent this.
I have a set of new guides that I supplied to the engineers, but they will turn up a new guide if this should prove necessary.
On the subject of choice of materials is there any benefit to be gained in choice of different materials?
AOMCC Member 4296
1950 Rigid Ariel VH
1951 Rigid Ariel NH (under construction)
1973 Norton 850 Commando Interstate