See the attached link, it illustrates the components and basic knowledge on how to measure same. The important thing with a track type tractor undercarriage is to be able to determine by measuring what condition each of the components are in. You take the specifications of "new" compare against what you measure and determine a percentage of wear of each component, NEVER take someones word as to this number. Now there are obvious signs such as the track tension adjuster, at the maximum allowable adjustment, meaning there is no more, and if the tracks are loose, there is considerable wear on all or some parts. There are ways to assess things and do things like turn the pins and bushings to a fresh unworn surface, that is if the links, rollers, sprockets and idlers are in reasonable shape. As an undercarriage wears these kinds of things used to be typical to do, turning pins bushings, building up links to bring em back to spec etc.
I'm no expert on D2's but you should measure up the undercarriage, links (rail height) and looking at the pins/bushings, ( those are what ride in the sprocket )to see how oblong or if they are worn through to the pin inside etc. etc. A good visual inspection can tip you off about severe wear, which then can be confirmed by actual measuring. One handy little tool is a track gauge, simple tool that you can place against each component and literally see what and how much is worn, I have one for my D7 Cat.
Overall the undercarriage can be expensive to rebuild, more than a tractor might be worth, but you don't really know until you measure. You might determine that the pins/bushings are worn, but the rails are ok, all other parts within reason ( meaning not wore to destruction or close to that) so to get enough tension back so they stay on the track frames, you have new pins/bushings pressed in, or you find a set of tracks in better shape. Now, even a tractor with a worn set of tracks may still have plenty of life for part time, hobby or occasional use, hopefully this gives you an idea about what you are dealing with.
If the tractor runs, has no serious leaks, signs of abuse or repairs like any of it's major component's housings welded or similar, next thing you need to get a look at is the final drives, you can pry against the sprocket to see if it is loose, that means a bearing or something is gone awry on the dead axle, you can take an appropriate magnet and remove the filler plug, dip the fluid and look for metal chunks or just fine metallic particles etc. will tip you off those will need some work. These are major repair jobs, requires specialize tools, heavy duty puller etc. Now if the tractor runs good, has nice tracks, no major leaks and or what have you, in my opinion they are worth repairing, better to know up front before you buy any crawler, as to things like this. Again just some thoughts.
Caterpillar designed and built tractors in those days to last and to be rebuilt, 40's-50's and some later were considered bullet proof designs, meaning with proper care, operation, they hold up for thousands of hours without overhauls, they were designed to perform.
Starting engines on these are 2 cyl, horizontally opposed, kind of high rpm little engines, you want to make sure it runs correctly, carburetors and magnetos, ( fuel & spark ) are usually the most common problems, always check the crankcase oil for thinning from gasoline leaking into it, if so change it immediately, do not run it, gas thinned oil will cause a catastrophic failure. Also you do not want to run just the starting engine by itself, I believe on these too, you have to spin the diesel to get coolant flowing, it will overheat, these are all things anyone new to these should know.
D2 undercarriage components may be hard to source other than from salvage yards, as new components may not be available, but you can convert them to much later D3 undercarriage, but there are some modifications or things that have to be done to make this work. A fellow from Canada provided a great photo by photo progress of one he did this too, on the ACMOC (antique caterpillar machinery owners club forum) Both ACMOC and ACME have excellent forums and resources via its members for all of these older tractors. A gent named Bill Glenn from PA also has some great posts on his D2, in short he's had a lot of fun with it, shows how durable they are, don't think he took it down to complete dis-assembly either, like what the fellow from canada did, and it has provided excellent service, a good example of their reputation for reliabilty. Again just some thoughts, and some things you may want to know.
Undercarriage
