I haven't had a need to change a conventional tap washer for maybe 30 years, but when the cold-water bath tap needed to be turned off hard to stop the flow I reckoned it was time to replace the washer, or at least check it. The tap/shower combo is very similar to one of these:
The tap came apart without any problem, but although I had spare 3/4" washers handy, surprisingly found that they were far too big. It looked like the washer was about 1/2". I say "about" because a 1/2" washer seems about a mm (sorry about the mix of units!) too wide to fit the headgear. But there is a more awkward issue to get round - although it was easy to remove the domed nut holding the old washer, it seems impossible to get the old washer off. It has become very hard, and looks like it might be recessed in a shallow brass shroud. If it isn't it appears too thick for the thread the domed nut screws onto. See photos:
Headgear showing old washer in position with 1/2" and 3/8" washers for comparison:
Side photo showing old washer on headgear with thread available, domed nut, and new 1/2" washer.
What am I missing? All the internet info pages I have read simply state "remove the old washer and fit the new one". It doesn't seem that simple; do I have to cut the old washer off? Should I immerse the new washer in hot water to soften it before trying to screw on the domed nut?