MLW and Alco 251 parts: variants, sizes and who still supports the engine

7 October 2026

The Alco 251 is a four-stroke diesel with a 9 inch bore and 10.5 inch stroke. Montreal Locomotive Works (MLW) built it into the RS-18, M-420, M-630, M-636 and other units, which is why it matters to Canadian short lines and heritage railways. Alco closed in 1969, but the engine is still supported.

Variants

Development started in 1949 to replace the Alco 244 and 539 engines. The 251 was built as an inline 6 and as a V8, V12, V16 and V18. Ratings run from about 800 hp for the early 251A to between 3,600 and 3,700 hp for the 251F.

When you order, the letter matters. A 251B and a 251F share a bore and stroke but not every part.

Who supports the 251 today

• Fairbanks Morse holds the 251 line and still lists the engine. It is also named as the sole-source contractor for US Coast Guard 251 overhaul kits.
• Independent suppliers such as Hatch and Kirk supply new and remanufactured 251 parts.
• Used and take-out parts come from dismantled units, and the supply shrinks every year.
• India has a large supplier base, because Indian Railways built Alco-design locomotives for decades. Quality varies, so ask for the maker's name and certificates.

Before you ask for a quote

Have these ready:

1. Engine model and serial number from the data plate, for example 12-251B.
2. Locomotive model and road number.
3. The Alco or MLW part number if you have a parts book.
4. A photo of the part and of any number stamped on it.

Searches for MLW parts mostly return model train pages. Real sourcing runs on Alco 251 part numbers, so the engine plate is the place to start.

Browse parts for the MLW and Alco 251 engine, or go straight to the RS-18 and M-420 pages.

Sources

• ALCO 251, Wikipedia
• MLW RS-18, Wikipedia
• MLW M-420, Wikipedia
• ALCO 251 overhaul parts kits, contract notice, HigherGov
• Hatch and Kirk ALCO parts