Prior to 1936, the GMC name represented the General Motors Corporation. It was reserved for only their line of large commercial trucks. They had their own Big Truck dealerships and very rarely mixed with other GM-made cars and trucks.
The Great Depression of the 1930’s changed the automotive market forever! Sales of cars as well as large trucks had dropped to very low levels. The fraction of original dealers that had survived were also trying to sell more used cars, doing repairs on all vehicles and, even farm equipment.
Over half of automotive stand-alone names ceased production and never reopened. The remaining GMC dealers were crying to General Motors to do something to save them from bankruptcy
This is what GM did to save its Big Truck line! It is why we still have the GMC small trucks today.
General Motors knew getting more Big Truck sales was not likely during hard times. That was already the problem! However, offering a less expensive small truck with more than just GMC logos might just turn the tide.
There was no time to wait! Without a doubt, many night hours for management and designers were spent to create a much lower-cost pickup to appeal to individuals.
It could not be a Chevrolet that moved over to a GMC dealership. This would be an insult to Chevrolet dealers, and it would not increase GM’s overall purchases. The sales volume would then be the same, but from two divisions. The Chevrolet Division would never get over having this done to them!
Because time was so important, it was decided to use the Chevy pickup cab and bed, but not the name or their six-cylinder, low oil pressure engine. Other differences were a redesigned grille (a major focal point), different color options, changed chrome hubcaps to a GMC logo, front and rear bumper, etc.
All was a go except for one BIG problem. GMC had no smaller engine! They were “Big Truck” people, and their existing engines would not fit in a “Chevrolet” pickup chassis, which they were going to partially use!
What now? The decision was made to start (in 1936) with an existing engine from the Oldsmobile. This was a flat head (the values were in the block close to the 6 pistons). With almost no moving parts in the cast-iron head, it was about 2.5” high.
This was repeated in 1937. However, the 1938 engine of choice was changed to the Pontiac. (It appears this was less costly.) There was no secret, as the large Pontiac Indianhead logo was still on the side of the engine block.
Finally, after a three-year wait for a totally new, smaller GMC engine became available in 1939 for their new pickup truck. It was a more traditional inline six-cylinder with overhead valves in the head. This was very successful and continued through 1959. As the trucks got larger, the piston diameter was increased to get more horsepower.