The diverter for our guest bath leaks like a sieve, diverting very little water to the shower. The tub/shower set is American Standard, about 20 years old. I have seen SpeedBall1's picture in response to a post by EJJ88 showing that if there is a slot near the wall, you can remove the spout using an allen wrench.

However, there is just smooth copper pipe above this slot on my spout. I am rather afraid to try twisting the thing off, although I know where the whole-house water turn off is! Any help would be appreciated.

Oh, for what it's worth, the bottom of the spout is marked with "AS" and "92".

Thanks.

You are fortunate that you have the smooth copper pipe. I also do not like twisting these off but will try to ever so gently with my hands only. If I feel that it will twist the pipe in the wall or the exposed pipe I will put an old towel inside the tub and use my torch to heat up with direct heat on the tip of the spout. This of course will ruin the spout but you are replacing it anyway. One of 2 things will happen with the torch, 1- the spout will loosen and easily unscrew or 2- the spout will melt off and the thread adapter on the end of the copper will be exposed so you can secure it while unscrewing the remnant of the spout. If you put 12 wraps of teflon tape on the end thread you can screw the spout on hand tight and be able to stop short without it leaking. Many twist the pipe trying to finish that last turn to get the spout straight.