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 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, a 432-cubic-inch (7.1 L) version with enlarged crankshaft journals was manufactured as well.
GMC produced a 637-cubic-inch (10.4 L) 60° V8 with a single balance 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 702-cubic-inch (11.5 L) "Twin Six" V12, which was basically two 351 V6s placed end-to-end with a common block and crankshaft.
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 reduced the likelihood of water being splashed on to the spark plugs and causing the engine to misfire or stall. GMC V6 engines were noted for durability, ease of maintenance and strong low RPM torque.
The 304.6-cubic-inch (5.0 L) 305 had a 4.25 in (108 mm) bore and 3.58 in stroke (90.9 mm). The 305 V6 was GMC's standard pickup truck and Suburban engine from 1960 to 1969, and was one of the first V6 engines produced by an American company. The 305A was equipped with a single barrel carburetor and produced 150 gross horse power at 3600 rpm and 260 gross torque at 1600 rpm. GMC also made a B,C,D and E version of the 305 v6. The E version producing 165 gross HP at 3600 rpm and 280 gross torque at 1600 rpm.
The 351-cubic-inch (5.8 L) 351 had a 4.56 in (116 mm) bore and 3.58 in stroke (90.9 mm). The 351 was available as an E series (351E) and Magnum series (351M). Magnum being a larger 2 barrel carburetor and an open port intake, bigger intake and exhaust ports, larger diameter valves, and larger exhaust manifolds. The 351E shared the same parts as the 305 with the exception of a larger bore. In 1973, the 351 was replaced by a 379.
The 378.6-cubic-inch (6.2 L) 379 had a 4.562 in (115.9 mm) bore and 3.86 in stroke (98 mm). The 379 was a 351 with a 401 crank shaft. Basically a stroked 351. Net power was 170 hp at 3600 RPM and 266 Ft. pounds torque at 1600.
 The engine was further enlarged for the 400.7-cubic-inch (6.6 L) 401. It has a 4.87 in (124 mm) bore and 3.58 in stroke (90.9 mm), and was produced from 1960 through 1972. The 401 being a Magnum engine produced 205 gross horse power at 3200 rpm and 377 gross torque at 1400 rpm. This engine was used in the 4000 series and larger trucks.
In 1973 & 1974 there was also a 432.2-cubic-inch (7.1 L) version with enlarged crankshaft journals (bore 4.875", stroke 3.86"). The 432 was a 401 v6 with a 478 crank shaft. Basically a stroked 401. The 432 produced 190 net horse power at 3200 RPM and 336 net torque at 2000 RPM.
The 477.7-cubic-inch (7,828 cc) 478 was one of the largest V6 engines ever built. Gross output was 254 hp (189 kW) at 3700 rpm and 442 lb·ft (599 N·m) at 1400 rpm. Bore was 5.125 in (130.2 mm) and stroke was 3.86 in (98 mm). It was introduced in 1962 as a Magnum engine in the 6500 series trucks.
The 702-cubic-inch (11.5 L) V12 "Twin Six" was offered in United States GMC commercial 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, two separate distributor caps driven by a single distributor drive, and other parts from the 351. 56 major parts are interchangeable between the Twin-Six and the other GMC V6 engines to provide greater parts availability and standardization. It produced 250 net SAE horsepower (190 kW). Torque was 585 lb·ft (793 N·m).
- gmc truck parts & illustration manual 1955-1964,1965-7
- "GMC Twin-Six V12 Myths". 6066 GMC Trucks. June 1, 2007. Retrieved 2008-10-05.