What sort of issues might arise if a threaded headset is used without using the keyed washer? Also, does the thickness of the washer matter? Thanks for any info!

Without the keyed washer, you must take extra care that tightening the top lock nut doesn't affect the headset adjustment.

That's it.

I'll add that to trust the locking tab to stay in the steerer's slot, while adjusting the headset bearing, is asking a lot of many bikes. I long ago filed off said tabs on my bikes and use two wrenches when adjusting threaded headsets. Andy

Thickness matters. Make sure the lock nut doesn't bottom out on the top of the steerer threads before contacting the adjusting nut.

Yea it becomes a 2 headset wrench project ..

washer / spacers will adjust to the steerer length ..

so the top nut engages the threads, fully, but not so far as to bottom out on the top of the steering tube..

so as to not secure the adjustable headset cup .. and bearing adjustment ...







.....

OK - I think I get it. The washer mainly serves as a device so the top lock nut can be secured byusing one wrench? I always use two wrenches anyway. I just wanted to make sure the top lock nut would not come loose without the washer. Thanks!

The keyed washer may also help keep the lock nut from loosening. You can use a drop of Loctite on the lock nut.

I've seen a few threaded steerers without a keyway. Maybe they were custom made.

At this point, I always hope someone asks, "What's a keyway?" Then I can respond, "About an ounce."

I always thought it was 2.2#

Actually (there's that word/claim) a keyway is a negative weight addition. Meaning that a keyway is material removal so less resulting weight. Andy (who sculpted down a pair of drop outs to loose a big 8 grams...)