Brucey wrote:if the threads on the steerer are worn at either the locknut or adjusting race location, it is best if you can hold that part static as the other is tightened against it. If the two are already locked against one another, it is a very bad idea to turn them both together; this can mangle the threads and it (of course) is guaranteed to knacker things if there is a tabbed washer between the two.
Nope, the only guarantee to make damage is by using brute force.
If you read what I've said above, is exactly what I don't advise to do. In fact, I've said to tighten
the locknut not to lock it against the top cup
One thing that has to be checked beforehand, is the health of the threads in both the steerer and the top cup&locknut. The former, is often chased by the manufacturer or in a shop, the latter option can give an incredible amount of variation in the thread itself: from a die that's not been cleaned and picks up swarf and damages the thread, to an incorrect die; all this can turn the original threads to a very low value of engagement.
Bear in mind that most of the bicycle parts are made to fit easy, so they are made with a relatively low percentage of thread engagement for ease of fit to the operator. If you reduce this value by chasing over and over the threads, or by chasing them wrong, then it's easy to damage the locknut or top cup.
The fork's steerer, being the male thread, will be the last to be damaged especially when the headset locknut and top cup are made out of alluminium.
The same applies for the tabbed washer: a sloppy fit can start to rotate altogether with the headset no matter what technique you are using. Best to file down the tab and use it as plain washer than use it as a thread-wrecker.
A threaded headset that constantly keeps working itself loose, needs particular attention in the state of its threads: it's not easy to judge the percentage of engagement of the threads, but certainly you don't want to see the top cup or locknut been able to rock up&down the thread when you're freely screwing them on the forks.
Brucey wrote:One mechanic I know adjusts headsets by screwing the adjusting race down finger tight, then screwing the locknut down finger tight. Next he holds the locknut with one spanner and backs the adjusting race upwards into it using a second. If this is done correctly this gives an initial setting that is quite close to the final setting, because of the way the parts bear against the thread flanks.
If you see the above, this is no silver bullet.
A thread that is not decently snug, will have a noticeable variation in the preload once you are to tighten the two preloading parts. Sometimes you can unscrew the top cup (with the locknut tight) up to 1/2 turn which is guarantee of a loose headset right away or pretty soon.
That ought to be the reason why Mtbs gave up with threaded headsets and never looked back