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;