I am getting no hot water from my Moen single handle shower valve. I have plenty of pressure from cold water side. I removed the main handle to expose the valve. The hot water pipe has a small valve with a common head valve stem which I turned, hoping to unblock the valve, but this had no effect. Can you offer advice? Is it possible to remove the center of the main valve to access the inside of the pipe leading from the hot water pipe?

You might not be flushing your water heater often enough to prevent crude from building up, maybe even the dip tub has deteriorated. Shut off you main and open another faucet to blow off pressure. Remove the entire cartridge and inspect the inlet ports. I'm betting you'll find small white bits, the color of the dip tub.

If your dip tube is deteriorated you will have very very short hot water supply. Seen as you have to turn the water off to install your taps, there is a good chance that the cartridge could be plugged off from sediment. The sediment could be corroion of the water lines or from rust of the waterheater. Copper pipes will often build up sediment from hard water and can also build salt debries from softners that haven't completely desolved all salt. I am a journeyman plumber and found this to be most common. Newer waterlines, such as pex piping, will show water color through them and allow you to see rust coloring and such. Hopefully this helped.