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If you're not familiar with General Motors engines from the 1960s, 70s, and 80s, you might think that the 307 cubic-inch small block engines from Chevrolet and Oldsmobile had a lot in common. The truth is, other than their advertised displacement and use in some GM models overdue for a reboot, the two V8 engines are quite different, with no shared major components, and not easily swapped without extensive fabrication.

Based on the long-standing small block Chevrolet platform, the Chevy 307 came on the scene years before the Oldsmobile version. Introduced in 1968, the Chevy 307 featured a crankshaft delivering 3.250 inches of stroke and cylinder bores measuring 3.875 inches in diameter. Power ratings for a 307-equipped 1968 Chevy Chevelle come in at around 200 horsepower with 300 pound-feet of torque. 

The Oldsmobile 307, often referred to as a 5.0-liter, was introduced in 1980 and saw continued use by a number of GM brands through 1990. The Olds 307 holds the distinction as the last carbureted GM engine to power a passenger car. Its cylinders are bored to 3.800 inches in diameter, the smallest of any Olds-produced engine, and its crankshaft delivers 3.385 inches of stroke; the same stroke as other notable small block Oldsmobile engines like the 330, 350, and 403. For comparison, a 1984 Hurst/Olds powered by the Olds 307 V8 makes 140 horsepower with 240 lb-ft of torque.

Why would GM produce two completely different 307 cubic-inch V8s? Cheng Xin/Getty Images

Long before smartphones supported the General Motors app and General Motors' success as a maker of EVs, GM was a conglomeration of automakers. According to General Motors, GM was founded in 1908 with a partnership between Buick and Oldsmobile, but Chevrolet wasn't added to the fold until 1916. With such a complex history, it's easy to see how separate GM automobile divisions could produce similar displacement — yet very different — engines. 

Differences between the Olds and Chevy 307s go beyond model years, bore and stroke variations, and power output. They didn't even use the same engine block or cylinder head castings. Chevrolet used its venerable small-block design, often referred to as the small-block Chevy (or SBC for short), as the basis for its 307. While the SBC platform was already proven as stable, Oldsmobile decided to use its own block design for its 307. 

One way to tell the difference between Olds and Chevy 307 engine blocks is to look for casting marks. Oldsmobile engine blocks have their displacement cast into the side. In the case of the 307, the casting states the engine's 5.0-liter metric equivalent. Chevrolet did no such thing since its 307 block was part of the SBC family.

Another good visual clue to tell the Chevy and Olds 307 apart is the number of bolts in the valve covers. Chevy valve covers are held on with four bolts, while Olds uses 10 bolts to secure the valve covers on its 307.