A 1950 Cat D315

Caterpillar was on a roll before World War II and rapidly dominating the crawler market. The war sped that roll up and they were smart and agile enough to take advantage of the momentum in the post-war landscape.

Cat was a very engine-centric company and manufactured their own powerplants. When it came time to upgrade their engine lines, they went after it with a lot of energy. They must’ve spent some portion of the war developing new engine designs because, in 1947, they debuted several new engines, the D311, D315, D318, to name just three, replacing the D3400, D4400 and D4600 engines. One feature the new engines shared were cross-flow heads that allowed better breathing and they all were considerably more powerful than the engines they replaced.

This D315 is a joint father-son project by Kenny Walker II and Kenny Walker III. They bought a non-running 1950 narrow track D4, which had suffered a major engine problem in the woods. They dragged the Cat out of the woods, pulled the engine and found that one of the notorious early Cat 2-piece pistons had come apart. The engine had been freshened up sometime in the recent past, so there were a number of new parts in it already. After replacing the broken piston and rod, replacing the one intact 2-piece piston still in it, adding four new liners, rings, and freshening up everything else, the engine was ready to go. It wasn’t quite so easy with the pony engine, detailed farther on.


The D315 was destined for one of Cat’s most popular crawlers, the D4. Between the D4 and D8, those two crawlers were sucking the oxygen out of the crawler market and accounted for a lot of Cat’s sales. They still do! The D315 debuted for ‘47, replacing the D4400. The two engines were fours of similar displacement, the D4400 making 312 ci from a 3.25 x 5.50 in. bore and stroke vs the D315 350ci from a 3.50 x 5.50-in. The D315 had a crossflow head with other improvements and made 82 maximum horsepower at 2000 rpm at the flywheel (53 @ 1600 continuous) versus 55 at 1600 maximum (35 @ 1500 continuous) for the older engine.

“The D315 was destined for one of Cat’s most popular crawlers, the D4. Between the D4 and D8, those two crawlers were sucking the oxygen out of the crawler market and accounted for a lot of Cat’s sales. ”

In the D4 crawler, the D315 was rated for 54.61 horses on the belt in a ‘49 Nebraska test at 1400 rpm (44 on the drawbar), and in a ‘55 test, power was up to 62.36 ponies on the belt at 1600 (52 on the drawbar). Of course, the engine was also used in many other venues, including marine, generator, and power units. In the mid-’50s, cat built their line of generators and the D315 units could crank out 40 kilowatts continuous on a 12-hour duty cycle, with short spurts in standby duty cranking out 45 kW. The difference in power was attributable mostly to a new balance that allowed it to operate at a higher rpm. There was also a slight compression increase.

Like virtually all larger diesels of the era, the D315 started with a gasoline pony engine, in this case, a 36 cubic inch, 10 horsepower at 3000 2-cylinder. This one has an electric start on the pony with a rope start back-up, but some were rope start only. The whole process of starting a diesel with a pony probably strikes most modern diesel heads as a major PITA, but there were some benefits to it, especially in the era of marginal fuels and lubricants. First thing, the cooling system of the pony and the diesel were shared, so the small engine warmed up the large one. Second, you could engage a compression release and spin the diesel over with the pony, thus heating it some more, the oil especially, and pre-lube it well before it lit off. When the engine was ready, you lit off the diesel by disengaging the compression release and engaging the injection pump. In the case of the Walker’s D315, the pony was in bad shape and almost as much money was spent on it as with the diesel itself.

The D315 served Cat from 1947 to 1960. The D4 got a major upgrade that made it the D4C and used the D330 turbocharged engine starting in 1959 that had the same displacement as the D315 but was a new engine. Plus it was turbocharged and/or turbocharged with an aftercooler, cranking out as much as 215 maximum horsepower. A few of the last D315s were also turbocharged, making 115 horsepower at 2000 rpm. These engines only saw use in industrial, generator, or marine service and not in crawlers.

The radiator and engine are integral with the engine and it mounts in the tractor as a unit. If it was used as a power unit, the “D4” badge below “Caterpillar” would be replaced by a “D315” badge, but we have seen pictures of some power units wearing “D4.”
The D315 diesel had a big brother, the D318, a six-cylinder version of the same basic design with the same 4.5 X5.5-in. bore and stroke. It made 119 horsepower at 2000 rpm naturally aspirated and was used in the ‘47-59 D6. Near the end of production for the D315/318 family, a turbocharged version was introduced that made 175 horsepower. The D6 carried on in the lineup as a unit one step ahead of the D4 and is built to this day.
Joe Mancuso’s 1953 Cat D47U shows up how the D315 was used. In the early days, the D4 was considered a “medium” crawler but later it was the smallest in the lineup. Joe’s D4 mounts a Number 42 toolbar with subsoilers that will make the ground tremble under your feet.

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