I have a '59 520 with a fitzgibbons boiler. This is what I've done to clean the system
1. From the rear, removed the cleanout door and the stove pipe. Took a brush and ran it through every tube. Wire brushed the tops of the tubes from the cleanout door. Vacumed and blew compressed air to get rid of the dust. Cleaned out the bottom rear where the stove pipe connects.
2. From the front, used an awl and poked through each hole in the pot.
3.Wire brushed all of the metal walls around the pot. Kept the fines handle pulled and blew compressed air through the holes in the grate.
4. Cleaned the sheet metal that separates the bottom of the pot from the ash tub. Man that was loaded with fly ash. I'll have to clean that more often.
5. Disassembled and cleaned all of the stove pipe. I had to take it out this time to install the water coil. Next year I'll just brush and vacumn
6. Cleaned the chimney thimble and vacumed out the cleanout.
Did I miss anything? This is my first time cleaning.