I just got another old bike. It's got a threaded headset lock nut. Talking 90's UK Raleigh. The lock nut measures exactly 30mm. With wrenches does this mean I need 30mm wrench or a 32mm wrench? I guess I'm asking a basic question, the size stamped on a wrench is the size of nut it expects to turn, right? So a 30mm should be the one? I know that they typically come in a 30/32 set (double ended). Just wondered.

Yes, a 30mm nut needs a 30mm wrench.

You could use a metric adjustable wrench.

I agree about a adjustable wrench. I gave up on headset wrenches. The wrenches and the nuts are anything but consistent, so an adjustable one works great, for the top nut that is. The adjustable nut you do need a HS wrench. I've got two, one for when 32mm doesn't =32mm , and one for when it does!

A good adjustable wrench is a Stanley Max Jaw 12" from Walmart. It doesn't loosen as easily as Crescent brand ones.

By HS you mean one of those "thin" wrenches to hold it while the lock nuts tightened?

Yes, one of the those thin ones. (HS is short for Headset BTW) Park is your best bet, but beware they can be slightly irregular. It may help to buy one from a real bike shop so you can try to get one that really does fit your adjustable cup well. Like I said, I have one that fits most of my adjustable cups well, and a newer one that is too tight for all but one. Both 32mm Parks.

... my 17" bikes (MTB UK Raleigh) stem is not long enough, and I was going to switch it with another. I measured the I have and one is 22.5mm the other 21mm. I'm guessing that means that 22.5 is actually 22.2 (INCH STANDARD), whereas the 21 is actally 21.15 (bmx size)? Strange because the 21.15 bike is not a BMX, it's a 17" MTB UK Raleigh (1990's). Anyone know if older Raleigh's are likely to use 21.15 stems?