though it was never designated as such. 478 The 478 had a bore and stroke. It produced net at 3200 rpm and net at 1400 rpm. It was one of the largest V6 engines ever built. It was introduced in 1962 for the 6500 series trucks. 702 The V12 "Twin Six" had a bore and stroke. It produced gross at 2400 rpm and gross at 1600 rpm in 1965. It 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 horsepower. Torque was . The 702 was in production until 1966, when it was replaced by the 637 V8. See also GMC straight-6 engine GMC V8 engine List of GM engines 1960-1966 GMC Truck Club References V6 V6 engines