Yeah, when I first head about that, I scratched my head also.
We are all familiar with the Fairbanks Morse 38D opposed piston engine. The engine has its roots back to 1933 with a 6-cylinder design. It used a very boxy, cast iron block, with a small 5″ bore and 6″ stroke, hence the 38A5 model designation. The odd thing about this engine, is that it used a chain driven upper crankshaft!
Check out the original patent for this engine, filed by Heinrich Schneider and Percy Brooks on behalf of Fairbanks Morse in September of 1933.
By 1938, a new welded frame was introduced, and gave the OP the appearance we all know of today, however, that upper crankshaft was still chain driven.
The engine, with a 5″ bore and 6″ stroke was first used in a doodlebug railcar for the Milwaukee Road. At the same time, a larger 8″ bore engine was introduced. The engines were a success, and would catch the eye of the US Navy.
I was recently able to purchase a manual, which best I can tell is from 1937, for the Fairbanks Morse 38C5 engine, the slightly newer version of that prototype mentioned above. While the manual is extremely primitive, it does illustrate that chain driven upper crankshaft assembly. A second chain was used for the camshafts and timing.
According to C.H, Wendel in his 100 Years of FM Engine Technology book, the chain would be replaced in 1937 with a vertical drive shaft operating off of bevel gears on each crankshaft, which is still used to this day, however that second timing chain is still in place. The engine would go through a multitude of improvements, in which the engine reached its two main production sizes, the smaller 5 1/4″ x 7 1/4″ introduced in 1939 and the 8 1/8″ x 10″ in 1938. The larger of the two still being produced, with the smaller being discontinued in the early 1970’s. A full post on the smaller engine is planned.
While Diesel engines were in their infancy with design work changing daily, I still look a this and go “What were they thinking!?”
Amazingly enough, a pair of those original FM 38A5 engines still exist, from the USS Enterprise (CV-6). The engines were saved by the Rock River Thresheree, north of Beloit, WI.