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 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). The 305E was equipped with a two barrel carburetor and produced gross at 4000 rpm and gross at 1600 rpm in 1969. The 305 was GMC's standard pickup truck and Suburban engine from 1960 to 1974. The