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.
The new engine family featured a valve-in-head design, pioneered by Buick and also used by the Chevrolet division's contemporary "Stovebolt Six" engine. Many displacements were produced using three block sizes: "Group 1" (small), "Group 2" (mid-size) and "Group 3" (large).
The straight-6 engine was replaced by the GMC V6 engine in 1960, remaining in use only in certain light-duty models of the P-series step van until 1962.
Group 1
The Group 1 engines were the smallest in displacement and outer dimensions, and differed most significantly from the larger Group 2 and Group 3 engines by having only four main bearings (whereas the Group 2 and 3 engines had seven) and a different firing order (1-5-3-6-2-4, whereas the others are 1-4-2-6-3-5).
228
GMC replaced the Pontiac 223 with their own 228 in 1939. This OHV (overhead valve) engine was produced through 1953. With a cylinder bore of , this is the smallest low-deck engine. All four low-deck engines have a stroke of and used 7 inch long connecting rods.
236
The 236 was introduced in 1941 and only in limited production through World War II. This is a low-deck engine, with stroke and bore. Production ended in 1946.
248
The 248 was released in 1939 alongside the 228. The 248 was similar to the 236. The bore was . Stroke is 3.8125. Power in 1955 listed as 100 HP @ 3100 rpm; torque 202 lb ft @ 1000 rpm. The 248 was discontinued in 1955.
256
The 256 was similar to the 236 and 248. It was also an OHV/pushrod engine, and was built for just two years, 1940 and 1941. This is the largest low deck engine. Bore was 3.78125 and stroke was 3.8125. Power listed as 91 Net HP @ 3000 rpm; torque 201 Net lb ft @ 1000 rpm.
270
The last GMC-only straight six was the 270. It was produced from 1941 through 1963, and was an OHV/pushrod engine. This is a raised-deck engine. The bore was 3.78125" (same as 256) and stroke was . Power listed in 1963 as 133 HP @ 3600 rpm; torque 244 lb ft @ 1300 rpm.
302
The GMC inline six was produced from 1952 to 1960, when it was replaced by the V6. It has a square bore/stroke ratio of . This is the largest raised-deck engine. It was originally designed for the GMC military M135 and M211. It was used in military 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. Power listed in 1959-160 HP @ 3600 rpm; torque 268 lb ft @ 1600 rpm. 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.
Group 2
278
The 278 was released in 1939 alongside the 308, sharing the same stroke but with a bore. It produced a maximum 110 hp @ 3000 rpm and 230 lb•ft @ 1000 rpm. The 278 was discontinued in 1946.
308
The 308 was released in 1939 alongside the 278, sharing the same stroke but with a bore. It produced a maximum 122 hp @ 3200 rpm and 241 lb•ft @ 1000 rpm. The 308 was discontinued in 1949 and replaced by the 318.
318
The 318 was released in 1950, replacing the 308. It shared the same stroke as its predecessors but with a slightly larger bore. It produced a maximum 136 hp @ 3400 rpm and 245 lb•ft @ 1400 rpm. The 308 was discontinued in 1954 and replaced by the 324 cubic inch Oldsmobile V8.
360
The 360 was released in 1950, replacing the physically larger 361 "Group 3" engine. The 360 and 361 shared the exact same bore and stroke, but the 360 used the mid-size "Group 2" block. It produced a maximum 127 hp @ 3000 rpm and 289 lb•ft @ 1300 rpm. The 360 was discontinued in 1955 and replaced by the 324 cubic inch Oldsmobile V8.
Group 3
361
The 361 was released in 1939 alongside the 426 and 451. The 361 was discontinued in 1949 and replaced by the 360 "Group 2" engine. The 360 and 361 shared the exact same bore and stroke, but the 361 used the larger "Group 3" block. It produced a maximum 136 hp @ 3000 rpm and 273 lb•ft @ 1000 rpm.