A brief history of GM’s Inline Six

by Levi Watson

From 1901 to 1928, all GM vehicles were powered by valve-in-block four-cylinder engines. These engines were under-powered and inefficient. GM was looking for a suitable replacement and in 1929, the General introduced the world’s first mass-produced inline six-cylinder engine. From there, GM would go on to create a long line of inline sixes, cementing a place for the “straight six” in hot rod history.

In 1929, GM brought their brand new “stovebolt” inline six-cylinder engine to market. The new engine used an ‘Over Head Valve’ (OHV) layout, conceived by inventor and founder of Buick Motors, David Dunbar Buick. The term “stovebolt” referred to the use of ¼”x 20 slotted bolts on various engine components. They were similar to bolts used on cast iron wood burning stoves, which were popular at the time, giving a good reference to the time period.

The overhead valve layout was relatively new in design and was not being used by the big manufacturers of the day. It represented an entirely new way of packaging and designing engines. With this new technology applied to an inline six, engineers realized they could make engines more powerful and durable, while keeping them economic and affordable at the same time. The original Stovebolt sixes used forged rods and cranks, making the engines quite durable. This durability, combined with the new power (50 hp in 1929), allowed Chevrolet and GMC to upgrade its pickup trucks from 1-ton to 1 ½-ton duty ratings for the 1929 model year. This was a huge move for GM in the sense of market share. These new engines also provided the customer with an incredibly efficient 25+mpg. Car buyers took notice, and by 1931 GM had 40% of the US market, up from just 14% in 1921.

In 1937, GM released a new inline six with slight improvements in both durability and performance. This generation of six-cylinder engine is commonly known as the “Blue Flame” engine. “Blue Flame” was an advertising term used by GM in the ’30s and ’40s to describe the efficiency of GM engines. According to the advertising guys, a blue flame (rather than yellow) meant perfect combustion was being achieved. The name was actually only used on specific engines from ’53-’62, not the entire generation. This engine was used in multiple different applications, and was produced in various displacements. The most popular I-6 from this era was the 235 ci. This engine was used in the majority of GM cars and trucks from its introduction in 1941 to 1955, when the Chevrolet small block V8 was introduced. 235s offered great economy and reliability, as well as good performance for the time.

1953-Chevrolet-16In 1953, GM took a standard 235 ci engine, added higher compression pistons, a higher lift cam, and solid lifters, creating a special, more powerful version of the 235. This version was painted blue and mated to a Powerglide automatic trans, crowned the moniker ‘Blue Flame’. The most famous example of the Blue Flame had triple side-draft Carter carbs and a dual exhaust manifold, increasing power even more. This was the first engine used in the brand new Chevrolet Corvette. All ‘Vettes came with Blue Flame engines until 1955, when the small block was born.

When the small block V8 arrived, they were all the rage. The straight six was relegated to being the “lowly” base engine from there on. By 1962 however, the popularity of the small block lead to some necessary revisions of the straight six motors. The third generation of the engine received several performance and durability upgrades. The crankshaft now had seven main bearings, up from four in the previous version. This gave the crank more support, allowing the engine to rev higher and smoother than before. There were also changes to the combustion chamber, and the newly introduced stud-mount rockers received a higher ratio than the older shaft mounted rocker design (1.75:1 versus 1.477:1). These helped the new I-6 make more power than before, and increased durability at the same time. One important change was directly based on the ever-popular small block V8. Starting with the new engine, all inline six-cylinder engines receive revised bellhousings, making transmissions and starter motors interchangeable between the I-6 and V8 engines.

This third generation inline six was the base engine for the entire GM muscle car era, and came in some of Chevrolet’s most popular models. Its introduction coincided with the introduction of the brand new Chevy II/Nova platform in 1962. Five years later, it was the base engine for Chevy’s own pony car, the Camaro. Between 1967 and 1969, Chevy produced 146,025 Camaros with six-cylinder engines. It also came in Chevelles and El Caminos, as well as several Oldsmobile and Pontiac models. By the mid-seventies however, another Buick creation, the more compact V6, was beginning to gain popularity. In 1973, Chevy dropped the straight six from full-sized cars for the first time since 1928.

The I-6 stuck around in trucks and vans until GM completely phased them out in the North American market. General Motors Brazil kept using the engine as the workhorse of the GM platform in South America, but the legacy of this engine will always be an important part of America’s hot rod and muscle car history.