Hi click,
As mentioned on the previous page by TinTin, I would definitely be giving the oil drain passage some serious attention. Maybe you can devise a way to pressure test it when the head is off. Porosity or a crack between it and the inlet port would possibly explain the 3-4 minute delay before the smoking starts.
A general question for everyone - and I may be way off here, but if it’s the rings, wouldn’t it smoke immediately?
A lot of jap bikes have a hollow dowel between the head and barrel on oil passages which reduces the risk of a head gasket leak. Has anyone ever done that on a Norton I wonder?
While you are going to have it stripped anyway, maybe it’s worth considering the new “waisted” head and cylinder bolts now available. Apparently they are very good at stopping head gasket leaks.
Watching with great interest, ( as are many others it seems)
Cheers and good luck,
cliffa.