It's nearly a week since I dismantled the "Norton" gearbox, so I hope that I can remember the details correctly…. The first thing to mention is the drive shaft where I had misunderstood the French comments in the manual. There is a collar on the shaft which can be released by loosening a set screw, it can then be slid to the right along the shaft, revealing a collar with a slot turned on its periphery. This collar can be unscrewed (you'll need a pair of pipe grips) releasing the shaft from the gearbox. Be careful, there is a damper spring and cone beneath the end of the shaft. The gearbox can then be removed from the lathe, leaving both the leadscrew and drive-shaft in the saddle (I removed the drive-shaft with the gearbox and had a lot of trouble getting it back in place). Except for a large quantity of swarf, the inside of my gearbox was surprisingly clean -  the slot for the gear-change lever being a real swarf trap. The box was heavily greased, not oiled, and there were no signs of wear particles in the grease, either from the gears or the bronze bushes supporting the shafts. There is a label on the front of the box showing the ratios in the different positions of the gear-change lever. To the right of this is a knob which permits 3 positions: C which is threading by the change gears, D in which the lead screw is disengaged and E where the threading is performed by the drive shaft, driven (if my translation is correct) at 1/10th of the spindle speed divided by the gear ratio selected (the French text says "advance = 1/10 of the step" or "1/10th of the thread", the words are the same for both, so translation is not clear.
Now to what I found within: (pictures below)
"Another view of Norton showing the speed change knob indexing adjustment screw lower left", shows the interior of the "Norton" gearbox before removing anything, and shows the only adjustment in the gearbox which is that of the indexing screw/spring/ball mechanism to adjust the speed doubling knob tension so that it will not jump out of engagement under load. The tension is increased by screwing in the slot-headed screw shown in the casting at the bottom left of the picture. I adjusted it so that the knob operated with an audible "click" as the ball bearing dropped into the hole drilled in the shaft( after cleaning the grease off everything) but not so tight as to make turning the knob difficult, It's a "feel" thing and quite easy.
I carefully I removed all the gears, carefully noting the position of everything (There is nothing special in the removal process, it is totally
intuitive), Cleaned off all the old grease and swarf and, after geasing everything with a good quality grease, reassembled it all, after cleaning the surface rust off the gearchange lever. Replacement on the lathe is the reverse of removal, after sliding the box back on to the two shafts, it lines up very well and can be set to align the screw holes with no great difficulty (it's not that heavy). Don't forget to screw on the retaining ring for the drive shaft.
I then removed the spindle gearbox in order to make a gasket to stop an oil leak from the bottom of the headstock. I found it impossible to remove the split pins holding the gearchange levers in place so I decided to remove the spindle. This is not difficult, just ungainly as the various parts drop off the shaft as it moves out of the headstock to the right. It is essential to note the positions of the various parts as they come off.The order (from left to right) is as follows: Split screwed collar (on the outside of the headstock casting), the thrust bearing outer ring, ball race, inner ring, split collar (two halves, held in place by a mortise mating with a tenon on the left-hand adjusting nut, then the left-hand adjusting nut, the right-hand nut, a fibre washer, the back-gear damping ring with four spigots mating with four holes in the chain sprocket, each hole containing a spring. Then the small, sliding back gear and finally the large back gear.
The spindle bearings are adjusted as follows:
The front bearing is adjusted using the split ring on the outer end of the spindle. I use the South Bend recommended method, I place a DTI on the spindle nose to measure vertical displacement, then, with a round bar about 80 cm long in the spindle (penetrating at least half way), I depress the spindle while watching the dial gauge. It will show a change and then stop. Release the force, the gauge returns part way, note the change, now lift the spindle, the gauge deflects in the opposite direction. release the force, the gauge returns but not back to the original reading, the difference between the two "at rest" readings is the play in the bearing, say 0.015" (we'll take a bad case to demonstrate!). Now expand the split nut on the left-hand end of the spindle (you may have to expand it by driving a wedge (= screwdriver) into the split) and turn it so that it is quite tight. Repeat the bearing play measurements. If the play is less than 0.001", slacken the adjusting nut off slightly and measure the play again. You are looking for about 0.001"-0.002" to leave enough space for the oil film. Now the rear bearing: repeat the test procedure with the bar in the left end of the spindle and the Dial Indicator set up to measure the vertical movement of the spindle (there is not a convenient surface to fix a magnetic DTI holder to, I had to rig up a system from the bed, which made vibration a problem on the dial gauge). With the left-hand adjusting nut, pull in the bearing cone into the bearing to remove play and measure the play. Again repeat adjustment and measurement to achieve about 1 to 2 thou of play. Once the adjustment is correct, hold the left-hand adjustment nut with a pin
spanner and lock it in place with the right-hand nut using a second pin spanner. NOW CHECK FRONT AND REAR BEARING ADJUSTMENTS AGAIN to be sure they did not move. Repeat if necessary.
There are adjustment screws for the gear levers to prevent jumping out of gear under load:
For the rearmost of the two levers, the adjustment is inside the headstock about 6" below the rocker gear for the reversing switch. It's easy to see with a torch and I used a socket on a "t" wrench with an extension to reach it. Screw it in to tighten the pressure on the locating ball, unscrew it for less tension. If it is very hard to move the rear lever, it's too tight. If it jumps out of gear under load, it's too loose.
The adjuster for the front lever is shown in picture "The Geabox showing the front lever adjustment screw" the same adjustment procedure is used as for the
rear lever. These two adjustments were the most useful I was able to do on my lathe!
The next most useful change I made was to install a Quick Change toolpost. I bought an AXA style toolpost for my South Bend 9" model #415 and I found
that the deVallière toolpost fixing bolt was about 0.001" to big, so I attacked it with some 800 grade emery cloth strip until it was a good sliding fit into the AXA QCTP, It had no effect on the fit of the original and I now had a much better toolpost with the use of all my indexable tools. It was a different lathe afterwards!..