Featured here is the all time classic Cat D6. The D6 first appeared in the later 1930's, but got a revised engine in the late 1940's. While the D6 is still around, later models all have a letter suffice such as the D6C, etc. This model is the real thing. It's soul mate was the D4 in that the engine was identical to that of the D-4 except that it 'gasp' had a 6 cylinder engine instead of a 4 cylinder one. Compared to the older Cat engines it was also a high speed engine in that it turned up 1200 RPM instead of 900. Earlier models in particular had a horizontally opposed 2 cylinder gas engine sitting on top of the bellhousing that was the 'starting engine' Sometimes it was rope start, and some models had a 6 volt electric start for the starting engine. To start the main engine, you first started the pony engine, either by pulling the rope or with an electric starter, and then engaged a pinion into the main engine flywheel and set a clutch to spin up the main engine. After the main engine had been spinning for a while you opened the throttle of the main engine and if all went as planned, the main engine would start and you could shut down the starting engine. The starting engine besides cranking the main engine did a number of other things to facilitate starting. It shared a water jacket with the main engine and circulated its water directly into the back of the main engine head. Likewise the fuel filter system was 'water warmed' so the warm water would also circulate into a water jacket around the fuel filters. Finally the exhaust from the starting engine was piped inside the intake manifold with the ultimate exhaust to the outside out of the front of the intake manifold. This allowed the exhaust heat to be recovered for warming the intake air to the diesel engine. These features all allowed the Cat engine to function in pretty severe weather, but alas, as the years went by fuels for diesel engines got better, and did not wax up as badly as they did in the early days, and many places did not have the severe weather (pretty much anything below freezing was severe for these early diesel engines), and so Cat also offered on later models direct electric start. The starting engine was replaced by a couple of large batteries, and a classic electric starting motor was installed where the pinion had been. This provided the convenience of push button starting, something we now sort of take for granted.
The D6 shown here is clearly a late model one. The presence of a strong canopy and a hydraulic blade give it away. The owner tells me it was converted from a pony engine start to direct electric start. Once again, this is a classic mid sized dozer.