I'll throw out some observations based on overhead valve, overhead cam, iron, and aluminum engine experience. Bear with me:
For starters, let's dispense with the "feel" method, since that's so hard to describe in text form. There's a more empirical way; one thousandth of an inch one way or another is not going to trash your engine. The designers know that (we're not talking race engines here, just stock or nearly so). That being said, for any given clearance needed, get a feeler gauge that's one-thou larger, and one-thou smaller. This will be your "go no-go" gauge. If you're shooting for .006", a .005" should be easy to insert, whereas a .007" will not go in. At this point, you have to be pretty close to the desired clearance (less than .001" either side). In fact, I have seen feeler gauge sets with a step ground into them, and labeled something like .005"-.007". This, obviously, is what you'd use for our hypothetical .006". Of course, this is with the follower on the heel of the cam lobe, which is obtained by following the procedure in the FSM. As I've heard said in the past; "Valves are like children; if you can't hear them, they're probably doing damage".
Here's the thing with the Norton, (and Volkswagens); they have aluminum pushrods (expands lots with heat) and iron cylinders (expands little with heat). Let's start with the engine up to temp, since this is where we're concerned about running clearances. You want the absolute minimum clearance, but not so small as to hold the valve open (spoken of in earlier posts). Now, cool those aluminum pushrods and iron cylinder down to room temp; the aluminum shrinks more than the iron, increasing the valve clearance. How can we know that this is happening? Two totally differnt combinations that give virtually identical results. My 78 Yamaha XS650, and a Hardly-Ableson Shovelhead. The Yamy is overhead cam with rockers, while the H-D is all steel pushrods and cast iron jugs. The Yamahahaha runs .002" and .004" and has no pushrods to throw in the mix, while the H-D (get this) runs ZERO clearance when cold! As the engine heats up, the jug will expand a bit more than the pushrods, giving you your desired running clearance. Crazy, but it works.
So, back to the Norton. Since it has aluminum pushrods and cast irons jugs, it needs quite a bit more clearance than a lot of other engines so that it will be correct once up to temp.
