GMC V6 engine
GMC Truck produced a unique 60 degree V6 engine family from 1959 through 1974, in gasoline and diesel versions. V8 and V12 derivatives of the basic design were also produced. Examples of this engine family were found in pickup trucks, Suburbans, heavier trucks and motor coaches. V6 big block engines were produced in 305, 351, 401 and 478 cubic-inch (5.0, 5.8, 6.6, and 7.8 respectively liter) displacements, with considerable parts commonality. During the latter years of production, versions with enlarged crankshaft journals were manufactured as well. GMC produced a 60° V8 with a single cam  shaft using the same general layout (bore and stroke) as the 478 V6. The 637 V8 was the largest displacement production gasoline V8 ever made for highway trucks. The largest engine derived from the series was a "Twin Six" V12, which had a unique block and crankshaft, but shared many exterior parts with the 351. Diesel versions of the 351, 478 and 637, advertised as the ToroFlow, were also manufactured. These engines had no relationship to the well-known Detroit Diesel two-stroke engines produced by General Motors during the same time period. All versions of the GMC V6 used  a six-throw crankshaft, which when combined with the 60 degree included cylinder angle, produced a smooth running engine without any need for a balance shaft. Spark plugs were located on the inboard side of the cylinder heads and were accessed from the top of the engine. This position allowed for shorter spark plug wires and kept the spark plugs away from the hot exhaust manifolds, something which was emphasized in sales literature. It was also perceived as easier to access them for maintenance. These GMC V6 engines were noted for durability, ease of maintenance and strong low RPM torque . In 1974, GMC discontinued the V6 engine; all gas-engine models were powered by Chevrolet straight-six and V8 engines while diesel engines were dropped from medium duty models and would not return until 1976. 305. The 305 had a bore and stroke. The 305 was GMC's standard pickup truck and Suburban engine from 1960 to 1974, and was one of the first V6 engines produced by an American company. The 305A was equipped with a single barrel carburetor and produced gross at 3600 rpm and gross at 1600 rpm (measured without air cleaner or accessories in an ideal  environment). GMC also made a B, C, D and E version of the 305, with the E version producing gross at 3600 rpm and gross torque at 1600 rpm. The 305A was standard in 1000–3500 series trucks in 1960-1961 and was dropped in 1962. The 305B and 305C (a 305B with a different manifold and carburetor) were used in 4000 series trucks; the 305B was dropped in 1962 while the 305C continued to 1974. The 305D was an option in the 1000–3500 series in 1961 and became standard in 1962, replacing the 305A. The 305E replaced the 305D in  the 1000–3500 series trucks in 1963 and was used until 1969. 351. The 351 was introduced in 1960 it had a bore and stroke. The 351 was available as a C series, an E series (351E) and Magnum series (351M). The E and M series featured a larger 2 barrel carburetor and an open port intake, bigger intake and exhaust ports, larger diameter valves, and larger exhaust manifolds. The 351E did not the same parts as the 305 In 1973, the 351 was replaced by the 379. The 351 was used in the 5000 series trucks (it was optional  in the 4000 series) and the 351E was used in the 1000–3500 series trucks from 1966 to 1969. The 351, 351C and 351M engines were medium duty truck engines, while the 351E was a light-duty engine(basically a 351M without the oil-driven governors) 379. The 379 had a bore and stroke. The 379 was a 351 with a 478 crankshaft. Net power was at 3600 RPM and torque at 1600. 401. The engine was further enlarged for the 401. It has a bore and stroke, and was produced from 1960 through 1972. The 401 engine produced gross  at 3200 rpm and gross torque at 1400 rpm while the Magnum version produced gross at 4000 RPM and gross torque at 1600 RPM. This engine was used in the 5500 and 6000 series as well as the H-5000; it was an option in the W-5000 and SP-5000. 432. In 1973 & 1974 there was also a version with enlarged crankshaft journals ( bore and stroke). The 432 was a 401 with a 478 crankshaft. It produced net at 3200 RPM and net torque at 2000 RPM. The 432 was a Magnum engine though it was never designated as such. 478. The  478 was one of the largest V6 engines ever built. Gross output was at 3700 rpm and at 1400 rpm. Bore and stroke was . It was introduced in 1962 for the 6500 series trucks. 702. The V12 "Twin Six" was offered in 1960 for the 7000 series trucks, and as a special order option in Canada. It was mistaken as two V6 engines welded together, but it is its own separate engine design based on a single casting. It used four separate exhaust manifolds, two separate carburetors and intake manifolds, two separate distributor caps driven by a single distributor drive , and other parts from the 351 V6. 56 major parts are interchangeable between the Twin-Six and the other GMC V6 engines to provide greater parts availability and standardization. It produced net SAE horsepower. Torque was . The 702 was in production until 1966, when it was replaced by the 637 V8.