Nobody has posted this so figured I would 
Posted by Scott G online a few days ago:
"Much of what has been written about the 4005’s brief time as an oil burner speaks of problems with uneven heating in the firebox and broken staybolts. The Steam Crew has made an important change during the 4014 restoration that was not part of the 4005’s configuration when it burned oil in the 1940’s. When the UP converted a steam locomotive class from coal- to oil burning, they removed the firebrick arch and the circulator tubes that supported it. In the case of the FEF Northerns like the 844, that meant the arch tubes were removed. In the case of the large Challengers like the 3985 and the 4005 Big Boy, they removed the security circulators. Security circulators were large tubes that connected the water spaces along the side of the firebox with the water space above the crown sheet. They had several functions, one of which included promoting water circulation between areas filled with relatively cool water and the area above the crown sheet, where the most vigorous boiling occurred. During the steam era on the UP, oil burning power did not use security circulators in the firebox. This practice was not unique to the UP. Southern Pacific cab forwards also didn’t use circulators (you can see this in the firebox of the SP 4294 cab forward preserved in Sacramento—no circulators in its oil-burning firebox). On the other hand, the Santa Fe did use a type of circulator tubes in its large oil burning Northerns and 2-10-4’s. For the 4014’s conversion from coal to oil firing, the Steam Crew elected to keep the security circulators in the firebox to improve water circulation, and therefore reduce the uneven heating of the crown sheet that can excessively strain the boiler structure and lead to broken staybolts and leaks. You can see the 4014’s circulators in the photo captured during an Ed Dickens update video. Compare it to the photo of the 844’s firebox, which has no circulators or arch tubes."
