Lots of good info on this elsewhere. This is a nice treatment off the NOC site.
Stephen Hill
Fitting taper roller head bearings
The earlier Norton steering stems suit the adaption to taper rollers much better, because of the fine threads make it easier to adjust the bearing clearance. Also with the early design, since the top nut locks the bearing adjustment nut under the top yoke without altering the clearance, it is inherently a better system to use with taper rollers. The nut on the later system was designed to clamp up rigidly, and its coarse threads are not well suited to adjusting the bearing clearance. With the later stem, you must use a spacer as I describe below.
If you have the earlier style Commando (or Atlas, or Dommi) stem, don't worry about this ... just fit the rollers, and proceed with adjusting the clearance with the nut beneath the upper yoke, and lock it with the top nut. With the later system, you must use a spacer between the inner races, so that you end up with the correct bearing clearance when the lower nut is tightened fully. How you arrange this spacer is a matter for personal ingenuity, but in the past I've made up a tubular steel spacer that is a little too short (i.e. causes the bearings to bind), and then gotten the proper fit by means of adding thin shims between the end of the spacer and an abutment that bears against the top bearing's inner race. You can get as fancy as you like here, but the idea is like the shimming system used in the Norton Isolastic mountings.
Do your initial fitting on the bench, and remember to pack the bearings with grease. However, before you make any final adjustments, ensure the outer races are fully seated in the frame headstock .. no matter how tightly you press them in, they will always 'seat' for the first several hours of use, and become loose. Thus, get the adjustment close, and then go for a ride. While making the final adjustments, keep in mind that the objective is to ensure the rollers can roll and not skid, nor hammer from looseness when loaded under rotational force. That is the main reason for light pre-loading, and this is quite OK with a taper roller, while it is very bad practice with a purely radial bearing such as a ball type. The other reason for pre-loading taper roller bearings (as in Velocette main bearings) is to ensure roller contact even when surrounding alloy (such as the crankcases) expands. With headstock bearings, expansion from heat is not an issue, and the adjustment is much less critical. As long as you take up the clearance so there is no bearing 'shake' when under load, things will be fine.
As things bed down, clearances will open up, and you should be very careful to remove shims to regain a proper fit. Check the fit by rolling the bike forward, and applying the front brake quickly. The feel should be free of shake, but there should be no binding at all in the steering. One last comment: - whereas most Japanese designs used angular contact bearings in the old days, virtually every one of their designs in recent years has a headstock fitted with taper rollers.
Greg Kricorissian (grkricor@ccs.carleton.ca) on NOC-L 26th. Nov 1997
