ALCO 251

The Alco 251 diesel prime mover was developed by the American Locomotive Company to replace the 244 and 539.

Development was initiated in 1949, and like the 244 would have a bore (cylinder diameter) of 9 inches (229 mm), and a stroke of 10.5 inches (267 mm). Its designation combines Alco's identifier for that bore and stroke - 2 - with the year its design was approved for laboratory testing - 1951. Chief Engineer of Diesel Engine Design, Paul Vaughan, designed the 251 to improve upon the major weaknesses of the 244. A wet block design was used to address severe thermal differences which existed in the dry block 244. The welded block and base was kept, as was the four-point mounting. New in the 251 was an aftercooler for the turbocharger, which minimized temperature differentials but also benefitted performance. Many small improvements were made to the crankshaft, bearings, pistons, injectors and camshafts.

To avoid the problems caused by rushing the 244 into production, the 251 was put through an extensive testing process.

The 251 engines first installed in locomotives for testing were inline-6 designs of 800 hp (597 kW). A simple, safe design, it also offered a direct replacement for the aged 539 which was still used in switch engines and the ALCO RS-1 road switcher. Like the 539, it was produced in Auburn, New York.

In 1955, the 251 went into production in inline-6 and V-12 versions at Auburn, New York - where the 539 was built - and a V-16 version built at Schenectady, New York, replacing the 244.

Ultimately a refined and successful design, the 251 outlived its designer. For a time it was built in Canada by Montreal Locomotive Works. Today it is still available from Fairbanks-Morse.

North American Locomotive Uses

Engine type	Power (hp)	Locomotive(s)
inline-6 251	800	S-5.
inline-6 251A	900	S-6.
inline-6 251B	900	S-6, T-6.
inline-6 251C	1000	S-13, RS-23.
inline-6 251D	1200	DL535E.
V-8 251F	1500	C415.
V-12 251B	1800	RS-11, RSD-12, RS-18, RS-36, FPA-4, FPB-4, ALCO DL500B.
V-12 251C	2000	RS-32, C420, M420W, HR412.
V-12 251C	2150	DH643.
V-16 251B	2400	RS-27, RSD-15, RSD-17, C424.
V-16 251C	2500	C425.
V-16 251C	2750	C628, C855, C855B.
V-16 251E	3000	C430, C630, M630.
V-16 251E	3600	C636, M636.
V-16 251F	3700	LRC-2, LRC-3.
V-16 251G	3200	HR616.
V-18 251	4000	M640.

External links

• Pinkepank, Jerry A. (1973). The Second Diesel Spotter's Guide. Milwaukee, Wisconsin: Kalmbach Publishing. ISBN 978-0-89024-026-7.
• Steinbrenner, Richard T. (2003). The American Locomotive Company - A Centennial Remembrance. Warren, NJ: On Track Publishers, LLC. ISBN 0911122-07-9