After carrying out twelve to sixteen days of continuous work, British Rail's Class 5 locomotive no. 73020 is booked for an external examination and washout. This involves the fire box, tubes and boiler being cleaned out and the engine being given a thorough wash and brush up.

Inside the boiler, a pile-up of ash in the fire box prevents the fire burning properly, as hot gases cannot pass freely through the tubes, scale in the boiler and water spaces prevents heat from getting to the water, so the engine steams poorly and burns more coal than is necessary. However, the cleaner the inside of the engine, the more steam it will produce for each shovelful of coal.

73020 is booked into the cleaning shed at 7am. The shed turners move the locomotive to the water crane, and the tender tank is filled by a fireman. The locomotive is then positioned so that its smoke box is directly over the ash pit and the hopper doors are opened, clearing lumps of ash that are too large to pass through the boiler screens. The fire is then dropped into the hopper, giving the engine a chance to cool down.

The locomotive is moved to the shed, where the boiler smith and his mate remove the smoke box screens to test for leaks in the steam pipe and other joins. The examining fitter carries out a steam test prior to climbing inside the smoke box to hammer-test the large pipes for corrosion.

At 10am, the cooler-down takes charge of the engine, filling up the boiler from the tender tank with a live steam injector. The engine then stands for three hours, after which a water regulating valve is connected to the wash-out hydrant and a hose is run to the large steam injector exhaust pipe. The water is turned on, and water flows through the boiler at 300 gallons per hour, slowly cooling it down, for four hours.

At 3.20pm, the engine is cool enough for the boiler smith and his mate to be able to remove the baffle plates from the smoke box, allowing the tubes to be swept by the tube sweeper using a steam lance to blast out the ash. When the large tubes have been cleaned, a rod and cloth are used to clean out the small ones. If a tube is completely blocked, the cloth can be removed to reveal a screw at the end of the rod, which will cut through the blockage.

By this stage the engine is cool enough for the bar-man to get into the fire box to clean it out. He shovels the ash off the top of the brick arch and down onto the fire bars, where it falls through into the ash pan.

5.20pm: the engine has cooled right down, and the washer-out can empty 1,500 gallons of water from the boiler. At the same time, the wash-out plugs are removed: 41 in total. The washer-out then removes the scale and deposit from the top of the firebox, and uses a different nozzle to wash the inside of the boiler backplate. The process is repeated for each washer plug, and then the smoke box end of the boiler is washed out.

The boiler smith inspects the work done prior to reassembling the parts. Repairs identified earlier are carried out. The last of the screens is replaced, and the washer-out fills the boiler with water. The engine is now ready for steam-raising, and a fire is lit at the bottom of the fire box. By 8.35 the following morning, enough steam has been generated for vacuum tests. These passed, the locomotive is in good time for its first booking at 10.50am.