How to Identify a Ford 391

by Floyd Drake III
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truck image by Andrew Breeden from

Ford's 391 cubic inch big-block, manufactured from 1966 to 1978, is a member of the FT truck engine series --- closely related to the more common FE-Series. The FT-Series consists of the 330, 361 and 391 cubic inch engines and have differences related to their truck usage. FE and FT engines are first identified in the same manner, beginning with locating and decoding the engine casting number, then by noting differences unique to FT-series engines. As Ford casting numbers do not identify displacement, measuring the engine's stroke may ultimately be necessary.

Step 1

Count the number of bolts on one of the valve covers. The covers are bolted to the cylinder heads, and according to's V-8 identification page, there are five bolts on all FE/FT-Series engine valve covers, differentiating them from other Ford engine families.

Step 2

Locate the engine block casting number. The American Automobile Casting Numbers Club locates it on the passenger side of the engine block. The known 391 code example, D3T1-6015-1, identifies the engine block part number, year of manufacture and original vehicle application.

Step 3

Decode the casting number. According to MustangTek, the example D3T1-6015-1 decodes as: "D" for the 1970s decade, with the 1960s represented by "C." The decade year is next; "3" designating 1973. The next position, "T," for truck application. The remaining positions are general part number designations and engineering codes not necessary for general identification.

Step 4

Check for FT engine block characteristics. Differences between FT and FE blocks include: four engine mount attachment holes --- the FE-series have three. Also, the cylinder head casting number under the valve covers will likely match the block casting, as FT-series engines used different cylinder heads than the FE-series.

Step 5

Measure the engine's stroke. According to's FE engine block casting number page, crank the engine until number one cylinder, the forward-most cylinder on the passenger side, is at top dead center. Remove the spark plug and insert a wooden chopstick into the cylinder until you hit the top of the piston. Make a mark on the chopstick, using the valve cover lip as a reference. Remove the spark plug from the number four cylinder, the last one on the passenger side and make another mark on the dowel. Measure the difference with a machinists' ruler. According to the website Flxible, the stroke on the 391 cubic inch equals 3.97 inches.

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