Your flat tank Norton will be fitted with a Sturmey Archer three speed gearbox similar to the one in the pics, which is the 3-speed CS type. Dismantling and reassembly is fairly straightforward. Mine had to be dismantled because of 'funny noises' that could be traced to a broken bush that is meant to dislodge the kickstarter ratchet. While I was at it I found that a few bearings needed replacement. Most notably, the sleeve gear mainshaft bearing that consists of 'two cups and a double cone' was worn out.
Reassembly starts with fitting the new sleeve gear mainshaft bearing; thoroughly heat the gearbox casing and insert the 'double cone'. Then stick the balls (20 for each cup) in each cup using a lot of grease. Some juggling will result in fitting the whole set including the sleeve gear in the gearbox casing. (My bearing was brand new and a modern replica that does not need adjustment; traditionally these bearings would contain shims between the cups to get the adjustment right and the recommended way to compensate for wear was to remove a shim. Often discarded, some boxes may still contain a thin pressed steel cover which pushes on to the outer end of the bearing and is secured by centre punching it into the three dimples in the outside of the bearing)
Then fit the chainwheel, first fitting a felt washer saturated with heavy oil, to keep the dirt out and the oil in. Fit the large nut and pull really tight; don't be tempted to slacken this nut in order to be able to fit the retaining plate on the outside of the chainwheel; this nut also retains the correct play on the sleeve gear mainshaft bearing and a loose nut will wreck the bearing!
Then temporarily fit the layshaft into the box. Your box may contain a spring washer to limit end float of this shaft. Otherwise fit shims or a spacer to limit layshaft end float. The shaft should have about 0.1 mm play once the endcover is fitted. The spring washer or the shims should be next to the bearing in the endcover.
Remove the layshaft again. Fit the bronze thrust washer that needs to be 7/32" thick or close to it and temporarily fit the mainshaft and endcover. Float is adjusted by placing shims between the clutchworm and the bearing; pushing the bearing 'in' results in less float; about 0.1 mm float is required.
Remove the mainshaft, fit all the gearwheels to the shafts and simply insert the lot in the box, making sure the gear plate bosses engage with the shift fork ends. Fit the endcover and the small cover nuts that should have washers underneath to prevent the hexagon heads chewing the cover.
My box had a crack in the alloy boss that supports the kickstart. I solved this by making a steel bush that fits tightly over the boss. Make sure it's very thin otherwise it will break the kickstart return spring.
Click this link for the manual.
Click this link for the manual.