For what it is worth, and that's not much, what follows is my take on the ongoing headset lockwasher "controversy."
I'm with Dale... pretty much.... and not just because he's the listmeister. I've seen cases where people have had to forego the spacer and then have had trouble keeping their headset in adjustment. Now, these folks may have just failed to exert enough torque when tightening the locknut and adjustable race together, but what is relevant, to my mind, is that these same people had not previously been running into the same problem with their headsets that did have the spacer. So, yes, maybe if they had used two proper wrenches and really cranked the daylights out of things, the adjustmant would hold, but using their normal tools and methods, and having prevously been successful at the task, they ran into trouble without the spacer. From this I conclude that at the very least, the spacer has some purpose other than taking up space.
Here is where I don't agree with Dale: Dale says the key (or the flat, on frenchy bikes) is important, and I say it is not. I think the key/flat is there to allow tightening the locknut against the adjustable race without having the adjustble race turn. Nice in theory, but in practice encouraging the one-wrench approach (attack) is what does in the key/slot or flat/flat. The almost inevitable slippage causes damage to the very parts intended to stop slippage. The better approach is to use two wrenches. Taking this one more step, there is a super cool pro-dude way of locking in a headset adjustment that a friend showed me years ago, and which may be the most important pro-bike specific tip I've ever been given. It is especially relevant to pro bikes because it is on those bikes where scratched hardware is most offensive, and also most liklely, given that delicate alloy units only appear on better bikes. The trick is to think of the locknut as the part that sets the adjustment, and the race as the thing that locks it all together. It's like this: you use your knees to hold the front wheel, one wrench to stabllize the locknut, and the other wrench to back the adj. snugly up against the spacer and overlying locknut. Now, a lot of people will assume this makes it harder to nail the adjustment, but it doesn't. Either way you do it, the pressure between the parts influences the final adjustment. If you do it the normal way, you need to sart with the adj race too loose and let the pressure of the locknut push the race into the right position at the same time that the correct pressure between the parts is accomplished (bearing clearance and thread tension being two different things, both of which are critical here). With the pro-dude way, you just turn it all upside-down, setting the race a tad tight, and backing it off to achieve the right bearing clearance just as the thread tension comes to the right level. Either way, it takes a few iterations for even the most experienced mechanic... and proper frame prep and assembly are required, of cources.
The question you might ask is "what is the difference?" That's a fair question. My experince has been that the wrench is far less likely to slip off the flats of the adjustable race than it would be to mar the locknut if you were using the normal technique.
Now, as for why the spacer-less headset is more likely to have problems, I'm not sure. Here's one idea that should be taken as the conjecture that it is. The spacer creates a section of steerer across which the locknut-to-race thead tension is borne. This section acts like a spring. Without this springy section of steerer, there is no spring to maintain tension when the whole system gets instantaneaously unloaded during cycles of vibration.
One last thing: If the key was necessary, would it not also be necessary on a hub axle? We know in the case of hubs that some are keyed and some are not, and to my knowledge thay all hold their adjustment.
Tom Dalton Bethlehem PA USA
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