You cant really remove whatever has hardened .
The best I can suggest is to flush the system.
Pull the top hose off the radiator and hang it off to the side .
put a garden hose in the radiator top hose hole .
Run the water enough so that it pours out the sides of the garden hose and radiator hole slightly.
You will need to secure the garden hose in the radiator hole and secure the top hose off out of the way.
Run the motor for 20 minutes after the engine has heated up.
Once the engine heats up and the manual thermostat opens, the (hot) water out of the block will be flushed out the top hose.
Once done reconnect the top hose to the radiator .
Then pull the radiator drain plug let the water out of the radiator .
Put the radiator plug back in and fill with anti-freeze
