The FM/ALCO 251 F engine is universally recognized for its great reliability, high specific output, and low specific fuel consumption. This rugged 4-stroke engine is available in 6-, 8-, 12-, 16-, and 18-cylinder configurations and provides 500 to 2810 kWe of power. All versions feature exceptional commonality of components with 229 mm (9 in) bore and 267 mm (10.5 in) stroke used throughout, forged alloy crankshaft with nitrided bearings, welded counterweights, fabricated steel cylinder block with redesigned camshaft bearing supports, and an under slung crankshaft that is carried in forged steel saddles. The latest version of the FM/ALCO 251 F engine incorporates performance improvements known as the Plus Package. The "Plus" configuration has been engineered utilizing years of actual field experience with the intent to reduce fuel consumption and improve reliability and durability.

The FM/ALCO 251 F "Plus" includes the following improvements:
• A stronger, more durable cylinder head casting increases the life of critical components such as valve seats and reduces thermal distortion and leaks.

• A new precision ground valve lever bushing that improves the reliability and service life of the valve train.

• A newly designed Super Bowl piston crown provides more thorough combustion and subsequent increased fuel efficiency. This provides a 11.5-to-1 compression ratio while using the same ring set and requires no change to the injection timing.

• A redesigned chrome pattern on the cylinder liner improves lubricity between the liner and piston and reduces lube oil consumption.

• Completely redesigned camshafts made of a stronger material to increase life. The new design also incorporates the "unit cam" concept, vastly improving maintainability by allowing replacement one cylinder at a time.

• Forged steel piston crowns are bolted to forged aluminum alloy piston bodies.

• Forged alloy crankshaft with nitrided bearings, welded counterweights. The under slung crankshaft is carried in forged steel saddles.