Before starting, I cannot stress that you should do this in a very well- ventilated area, outdoors if possible. Chlorine gas is very dangerous and should be taken seriously.
Lubricate the new o-ring using a non-petroleum based lubricant, and begin installing it by pressing it into the groove. The lubricant will help prevent the o-ring from getting “pinched” when the cover is threaded back on. The application of lubricant will also help protect the o-ring from corrosion caused by the chlorine.
Once the lid has been installed onto the chlorinator, you can turn on the pump and filter to check for leaks. If there are leaks present, turn off the pump and filter and review the steps above.