The GMC straight-6 engine was a series of gasoline-powered straight-six engines introduced in the 1939 model year by the GMC Trucks division of General Motors.
Prior to the introduction of this new engine design GMC trucks had been powered by straight-six engines designed by the Buick, Pontiac and Oldsmobile divisions of GM.
This OHV (overhead valve) engine was produced through 1953.
With a cylinder bore of 3.5625 inches (90.49 mm), this is the smallest low-deck engine.
The 236-cubic-inch (3.9 L) 236 was introduced in 1941 and only in limited production through World War II.
It was used in military 21⁄2-ton trucks with the Hydramatic transmission; however, the engine was a sealed engine for snorkel/submersion use, had an electric fuel pump, and other features such as a deep sump oil pan.
From 1952 to 1959, GMC manufactured the civilian 302 engine, which was not sealed, had a mechanical fuel pump, and used a standard oil pan.
This engine was popular with hotrod enthusiasts because it delivered tremendous power for an inline six engine at the time (although now obsolete), is built with a heavy cast block, and can take quite a bit of abuse.
[2] The 308 was discontinued in 1954 and replaced by the 324 cubic inch Oldsmobile V8.
[1]: 24 The 361-cubic-inch (5.9 L) 360 was released in 1950, replacing the physically larger 361 "Group 3" engine.
The 425.6 cubic inches (7.0 L) with a bore and stroke of 4.25 by 5 inches (108 mm × 127 mm) GMC inline six appeared in 1940s 4x4 Cab Over Engine (COE) trucks made in Pontiac, MI.
It also appeared in large GMC trucks in the 1950s ending in 1955.
It was the largest in the GMC straight-6 line until the 477 debuted late in the 1940 model year.
The 477-cubic-inch (7.8 L) 477 was released late in the 1940 model year, replacing the 451.