What is a pop-up drain assembly?
A pop-up drain assembly is commonly used in bathroom sinks. A mechanical plug called a "pop-up stopper" is used to hold water in the sink.

Why is there a gap between the handle and the escutcheon of my one-handle shower faucet?
There is a gap in this location to ensure the screws on the escutcheon do not interfere with the operation of the handle. Depending on the installation, it is acceptable for this gap to be between ½" to an inch.

Can I replace a two-handle lavatory faucet with a single-handle model? Do all faucets fit all bathroom sinks and tub/showers?
Standard-size sink openings and faucet dimensions are used throughout the plumbing industry. However, since there are a few exceptions (such as 4" spread or 8" spread), it is a good idea to check sizes first.
Bathroom Sinks:
Many single- and two-handle faucets are on 4-inch centers, which is the distance between the centers of the hot and cold inlets or mounting bolts. Another popular style for two-handle faucets is the 8" to 16" widespread. These faucets come with flexible hook-ups to allow for adjustments.
The distance between hot and cold water supplies for two-handle faucets is generally 8". The hole opening for single-handle tub/shower combinations may vary. The opening must be smaller than the faceplate of the unit you are installing for proper coverage.

Can I use my existing drain assembly when installing a new faucet?
In most cases, yes. But if you're changing finishes, (i.e.,from chrome to brass), you'll probably want to change the drain assembly to match your new faucet finish.

The temperature on my shower seems to be reversed ? cold water is where hot should be and the hot water is where the cold should be. Is there an easy way to change this?
This can be corrected by removing the handle and turning the stem of the cartridge 180 degrees (either clockwise or counter-clockwise) and reinstalling the handle.

When I first turn on my shower, I get scalding hot water. As I turn the handle counter-clockwise, the water gets cooler. How can I fix this?
The entire cartridge can be removed, rotated 180 degrees, and then reinstalled to correct this temperature issue.
How can I make sure my shower is safe?
To avoid dangerously hot temperatures, make sure your water heater is set at 120º F or below.
Install a pressure balance tub and shower valve.
Set the valve’s adjustable rotational limit stop to prevent the temperature handle from being turned to dangerously hot levels.
Test the tub/shower water with your hand before bathing.

I don't want to change my plumbing. What kind of shower can I get?
If you don’t want to modify your existing plumbing, you can get a standard shower, which you can customize by simply choosing your own configuration options. You can also get a jetted shower, which can give you that spa-like experience without requiring a major overhaul of your entire bathroom.

Why does water come out of my shower head and the spout at the same time?
Most likely you are experiencing "shower-rise," where the water can’t get out of the spout as fast as it flows through the valve. This causes the water to back up and come out the shower head. There can be one of several reasons for this:
The valve could be upside down.
The length from the valve to the tub spout is outside of the 8" to 18" rule.
There is more than one 90-degree angle.
Something other than copper or galvanized pipe going to the spout was used.
The most common reason is that there is something restricting the water flow to the spout (for example, solder or something lodged in the pipe). Something is impeding the flow of water to the spout and causing it to back up and go out of the shower head. If this is the cause of shower-rise, it is possible to remove the blockage by removing the tub spout, and then feeding a plumber’s snake or a speedometer cable up through the tub drop. The act of feeding it causes it to twirl and loosen the blockage. Be sure to flush the valve before reinstalling the tub spout. Be sure to check the tub spout itself for blockage.

If none of these help, you may need to call a plumber. After the shower has been installed and the wall has been closed, there is little that can be done to remedy this situation yourself.