The engine had it's roots as a Diesel engine developed by General Motors during WWII and fitted to Submarine Chasers and after the the war, enlarged and fitted to submarines. By
mounting the engines with the crankshaft vertical and the generator beneath, four engines
could be fitted and the submarine made shorter because of the compact installation. However problems with oil leaking into generators and restricted room for maintenance coupled with a possible conflict with the Navy's choice of oil which created bearing problems, resulted in the submarines being lengthened, to be repowered with three Fairbanks-Morse OP engines.

The engine is a spark-ignited, loop-scavenged, two-cycle gas engine, rated 3500 hp at 600 rpm. It is built with engine-driven scavenging air compressor and attached engine accessories in addition to generator applications, it can be direct connected to standard speed increasers or reduction gears, and will drive either pumps or centrifugal compressors.

The engine is arranged in four rows of four cylinders each, with 90° spacing between the banks. The connecting rods are of the slipper type. The full-floating, cast-iron pistons are oil-cooled by spray nozzles and also through drilled connecting rods. The barrel-shaped crankcase is built up entirely of steel forgings which are welded together. The crankshaft is counterweighted to give perfect engine balance.

Interesting crankshaft setup as the  connecting rods use a similar principle to the blade rod on the EMD V series 567 Diesel engines (see pic below). As this is a two stroke engine, any inertia forces of the piston assembly away from the crank are more than counter-acted by the cylinder pressures, so there is no real need for lower big end bearings. The rod ends being retained by rings which encircle the crank journal and within which, shoulders on the sides of the rod ends are held captive, if you get my drift...

The main bearings are designed so they can be removed without disturbing the crankshaft. The main bearing carrier is made in three cast-steel sections, which are bolted together at the split lines and are held in position in the crankcase by retractable dowels.

Air is supplied to the engine through four intake manifolds by a centrifugal type air compressor mounted at the front end of the engine. There are nine air inlet ports, and five exhaust ports in each cylinder.

The cylinders are mounted on the crankcase by means of clamping rings made in two halves for easy assembly and removal in service. Water pumps, lubricaing oil pumps, governors and timers are driven from the rear end of the engine.
A 1-1/2" diameter gas inlet valve is located in the top of the cylinder. 
One water-cooled spark-plug is centrally mounted in the cylinder.

Bore &  Stroke in.	Displacement	Rated  B.H.P.	Output R.P.M.	Engine Weight	Width	Length O/A.
12.5 x 14.5	28,470 cu. in.	3,500	600	86,500 lbs	10' 6"	16' 5"