How far should a thermostat open?

I have a 98 Chevy s10 vortek and I am rebuilding it, I am not getting coolent flow and was checking out the thermostat first before ripping off the water pump. I tested the therm. in boiling water and it opens only about a less then 1/16", is the water pump out or is the thermostat out?

Thermostat. It should open almost a full inch. 1/16" is not nearly enough.