Union Pacific Steam Big Boy Differences

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This page was last updated on November 21, 2024.

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Overview

UP 4-8-8-4 "Big Boy" locomotives were delivered in two separate orders. The first order, for UP 4000-4019 (20 locomotives) was delivered in August through December 1941. The second order, for UP 4020-4024 (five locomotives), was delievered in November 1944.

(Read more about UP's Big Boy locomotives)

After a page-by-page review of Kratville's Big Boy book, there is very little mention of the actual differences between the 1941 series and the 1944 series. The differences that are apparent were that heavier metal was used due to restrictions during the war, and that the air compressor after cooloers were changed to make them less troublesome.

(Compare the diagram sheets for both orders)

Heavier Metals

[page 15]
In 1944 the road received authority from the War Production Board to build five additional Big Boys along with more Northerns and Challengers. Few changes were permitted, or really necessary, and the 4020-4024 were delivered in the fall of 1944. The only differences were the use of heavier wartime-available metals in the rods and boilers.

[page 51]
The 1941 Big Boys used alloy steel wherever possible but medium-carbon, quenched and tempered steel had to be substituted on the 1944 series with a slight increase in weight. In 1944 and 1945, new rod replacement designs were drawn up in Omaha using poorer materials which were available in wartime, but the new designs never had to be put to use because there were no rod failures during the war.

After Coolers

[page 56]
Aftercooling was a problem and the first series had the fin type units with welded return ends mounted on the pilot deck, basically because that is where the designers first thought they should be and there was not much room for them elsewhere. In service, the fin coolers were easily maintained, but the frigid weather caused operating problems in the form of condensation and ice.

The 1944 series came with the coolers mounted behind the pump shield and with Wilson after coolers connected in series and also mounted on the deck. The Wilson units were made of cast iron and broke often due to condensation freezing and cracking the body. After the war, the original series had the fin coolers moved to behind the shield and Wilson units were used as replacements as the fin units wore out because it was cheaper to buy a Wilson unit than fabricate fin type units in the shop. The 1944 fin coolers were designed by Westinghouse along the same pattern as the 820-835 Northern's coolers.

[page 58]
If the Big Boys were known for any particular problems, the two most prevalent seemed to be after cooler failures and steam leaks. The original series had been equipped with the fin type coolers with welded returns and mounted on the pilot deck. The pump shield was fitted with louvres which were designed to direct air to the coolers. In practice, however, the openings did not function as designed and the air hit too low. A side shield was applied later which helped somewhat to keep the air in the pump area.

The coolers continually caused gasket headaches and failures, thus allowing a great leakage of air and permitting the coolers to burn up. The cast iron headers also presented problems with condensation freezing and cracking the header. The cooler sump, a tank located beneath the cooler, also was a victim of freezing. A Wilson after cooler was applied to the 4020 series when new and from 1948-1952, the remainder of the type had their coolers moved to behind the pump shields. The original fin type coolers, with all their connecting piping, were expensive to maintain whereas the Wilson units were separate items and so were used as replacements whenever original coolers broke down.

Comparing Diagram Sheets

Comparing data from the diagram sheets:

1941 Series (4000-4019, Class 4-8-8-4-1)

1944 Series (4020-4024, Class 4-8-8-4-2)

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