Jaknight and several others have asked about this, so here's my history with them.
There are some Japanese-made "kits" available for the steering head, with typically use NKK, KOYO, DAIDO, or other Nippon-related bearings. Most come in a green box with a spacer washer that fits on the bottom of the triple tree's post, UNDER the dust seal (if you still have one!) so as to push the dust seal up into the steering head collar upon assembly. These "kits" usually have a top bearing that sits too high in the race, for the purpose of providing an extra .040" of clearance to nestle into the bottom of the steering crown (there's a cast-in recession under there for the old ball race). (I think M3Racing and PartsNMore carry these kits, among others).
This extra height causes some troubles, which in the long run can result in shorter life and not-as-solid front end stability. Biggest trouble: the top bearing, when tightened as proper, distorts the roller cage and the bearings are forced into a slightly unnatural location, high in the race. The result is a hard-to-feel torque-in point when you're reassembling, and some users don't get them set right as a result.
There are 2 solutions. First step for both is GREASE THE BEARINGS. Don't forget that, wheel bearing grease is fine. Also, the races get driven into the steering head, the bearings sit on the post.
1. If you use the Japanese "kits", then you must follow this procedure to "set" the bearings, and do it again after 100-200 miles of riding. You might find you need to do it again next year, but then it will settle in:
A. C-stand the bike, then either have someone sit on and balance back the bike so the front tire just rests on the ground, or jack up the engine to the same point. There should be about 50 lbs or so on the front wheel.
B. With the upper steering crown raised about 1/4" (or removed altogether) so you can get to the spanner nut, tighten the nut while constantly turning the forks, stop-to-stop. Tighten until the drag is real heavy. It probably won't stop altogether, but it will be close to that when tight.
C. Back off the spanner nut about 1/2 turn from this point and wiggle stop-to-stop several more times to redistribute the grease. See how the "drag" feels. If it is real smooth, tighten it back down about 1/4 turn until it starts to drag again. You want some drag at this point, which will dissipate with some riding.
The "hard tightening" here will distort the roller cage to match the partial-race mesh, without your having to ride & retighten for the next 12 months while it settles itself in.
This is my preferred method:
2. Use Timken bearings. You will need to get your own spacer washer for the bottom, and another one for the top (dimensions below). [NOTE: circa 2002, you must grind Timkens' bearings to fit: probably not practical now. Edited 10/2009]
A. Install as noted above, big washer on the bottom, etc. On the top, the bearing will not stick up high enough to engage the recess in the bottom of the steering crown, because these are correctly-matched bearing sets. This is where the other washer comes in: put it on last, then the crown.
B. Tighten as above, but then just back off 1/4 turn when done. It will be butter-smooth and fully meshed. Retighten in 100-200 miles, because you probably didn't drive in the lower race like you thought you did, and it will move a bit and loosen the stack.
The Timken bearing numbers are:
Lower: 07100
Upper: L45449
The lower washer is: O.D.: 1.750", I.D.: 1.245" (1.250" will do nicely, too). Thickness can range from 0.100" to 0.125" .
The upper washer is: O.D.: 1.500", I.D.: 1.000" to 1.020". The thickness can range from 0.040" to 0.060" and it will fit.
The Timken numbers I have are circa 1972, just removed from my bike. If they have changed, and I can find the new numbers, I will edit this post later. I will be trying to locate some of these bearings, because I just had to install one of those "green kits" in mine, if only to get it back on the road before Fall...
Update 10/2009:Here's the 'word' from Timken: circa 2002, these bearings are only available when ordered in large quantities, like 500 or more sets, at which time they will grind the OD and ID of these two to any spec. The O7100 and L45449 are now only available in 3 standard sizes (each), none of which will drop into the SOHC4 bikes.
Currently, KML is marketing properly-ground bearings to fill in the market's void. These are made of harder materials than the "green box" bearings mentioned above. I found that I can engrave them with my buzzbox engraver, which is VERY difficult to do on the Timkens, so they are not quite as hard. But, the "green box" bearings I have can be scratched with a hard drag of a file, which makes them considerably softer than either the KML or the Timkens.
