A 1993 460F350 175,000 miles. The cylinder on the passenger side rear has 0 compression.
All other cylinders fine and plugs are clean. The plug in the bad cylinder is caked with burnt oil.
Is there a way to determine if the low compression is due to Valves or Rings?
I have been using oil and some smoke from the exhaust.

 

What you can do is pour a table spoon or so of oil into the cyl before the compression test. If the compression goes up, you have problems with the rings, cyl wall, etc... If it doesn't then you know your problems are somewhere else.

If the cylinder has ZERO compression, however, it sounds like you have a valve issue. Take off the valve cover and watch to see if the push rods, rockers, etc, are working properly for that cylinder.

 

It really doesn't matter if it is valves or rings, you have to remove the head anyway, and then you will know. It could be a burnt valve or even the piston. Let us know what you find.

 

I will open her up in about a week and let y'all know. Thanks for the help.

 

With no reading at all I would suspect the piston is not moving at all in the cylinder. I have had this happen once. Another thing could be a hole in the piston.