Thanks Jon, but I am not sure that I have any tube thin enough. What size finished tp should I be aiming for please?
My initial thought was to drill and tap the end plug to take an M8 threaded insert, then to make a new insert from apiece of 8mm mild steel rod, threaded with the die only closed enough to let the insert fit tightly into the end plug and the protruding stud reduced to 7mm dia as the existing one. Insert from the inside to get the projection right, then cut off the excess and file flush with the plug. Does this sound sensible, or am I un-necessarily complicating things?