I am a Clean Burn dist. and I will share with you how I do burner rebuilds.
I remove the preheat block from the burner, remove all plugs and elements then I use solvent and gun cleaning brushes to clean all passages. A couple of years ago I bought a ultrasonic cleaner and this has made the process easier. Dremel with a brush can be helpful in some areas.
When you go back together use blue medium strength Loctite for the plugs, elements and nozzle holder. Also do not forget you put a new O-ring on the nozzle holder.