Bowser Manufacturing

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HO Discontinued Union Pacific Big Boy 4-8-8-4

We have discontinued production of these kits.

Most repair parts are available. When we exhaust these repair parts we do not plan to restock.

The Union Pacific 4-8-8-4 "Big Boy" are considered to be the longest and heaviest locomotives built by Alco. They were designed in 1941 and 1944 to conquer the grade between Ogden and Evanston through the Wasatch Mountains.

This design utilized lateral motion devices, and were able to run as fast as 80 miles per hour, but their best horsepower was at 30 miles per hour. The engines had 23 3/4" x 32" cylinders, 68" drivers, used 300 pounds of boiler pressure and weighed 772,000 pounds. Twenty engines were originally ordered in 1941. These were engine numbers 4000 to 4019. Five more engines were eventually ordered in 1944. These were numbered 4020 to 4024.

Around September 10, 1941 the 4000 was tested on the grade that it had been built to beat. It was hauling a 3800 ton train and made the 65 mile plus trip into Wahsatch and on to Evanston ahead of the photographer.

Fittings were added, including twin stacks hidden behind a sheet-metal fence and equipped with side-folding smoke hoods. Feedwater was heated with an Elesco exhaust injector combined with a centrifugal pump. The engines were used as coal-burners. Big Boys first used air cooling piping supported lengthwise on each side of the pilot deck and later used Wilson after-cooled radiators as on the Challenger series.

The Big Boys were the King of steam for eighteen years, but they might not have ever been built, for while the 4000 series were in the design stage, diesels were being tested experimentally throughout the nation.

This loco kit is not a beginner kit. We suggest that you to build at least 2 other Bowser kits before you try the Big Boy. This kit requires filling and filing to smooth a joining seam in the body. We make the body by cutting two Challenger boilers and soldering them together.

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Prototype Specifications
Builder Alco
Tractive Force 135,375 lbs.
Drivers 68"
Total Engine Weight 772,000 lbs.
Total Engine wheel base 72' 5-1/2"
Steam Pressure 300 lbs.
Height 16' 2-1/2"
Length 85' 9-1/2"

Model Specifications

Drivers 69" Box Pok
Gear Ratio 31 - 1
Power DC-71 motor
Boiler length 12-1/4"
Engine height 2-1/4"
Overall length
w/ centipede tender 18-5/8"

Engine Overhang

(From inside of Outside Rail)
18"Radius Curve 1.688"
20"Radius Curve 1.437"
22"Radius Curve 1.375"
24"Radius Curve 1.250"
NOTE: If curves are given the clearance as outlined in NMRA standards, no difficulty with overhang will be encountered.


#100350 Locomotive Kit Sold Out
#100375 Super Detail Kit Sold Out
#100321 Painted Engineer & Fireman


Tenders for the Big Boy
#150001 SP-1 Semi-Vanderbilt Tender Kit
#150650 Long Haul Tender Kit
#150681 Electrical Pickup Kit for Con-Cor tenders

UP Big Boy Instructions
Reference Manual Pages