D4 7U & 6U built from 1947-1959, 7U is wide gauge at 60" center of track to center of track, 6u is 44" narrow gauge.
The 2 links I attached should help you understand how to measure undercarriage components. The IH U/C guide is lengthy, but on page 28, (26 in .pdf file) there is an illustration for measuring each component. The 2nd link is a simple form to fill out, but they left one important component out, the track link, and what is referred to as link or rail height, its the bottom portion of the track link that rides on the track rollers, idler and top carrier rollers. The thickness is what needs to be measured and compared to new dimensions. The IH guide shows it. That same component, can be observed and suspected of being worn out if you look at the track chain pin/bushing, its what rides in the root of the sprocket and is where power is transferred to the track chain, if the ends of the pins/bushings are contacting the bottom roller flanges, its sign that link height (rail height) is worn significantly, to be verified by measuring. Another one is the amount of sag in the track, and no more adjustment left to tension the track, you can measure to see whats going on there too, goal is to see if the adjuster is all the way out. To measure the stretch or chain wear, track must be taut and you measure across 4-5 pins, its described in the links attached. Once its measured up, it may reveal options that can restore track life, increase tension again, giving some hours and some value to the tractor. Problem with loose and worn out tracks is that they are prone to come off, wear accelerates and value of the tractor is reduced, with older ones, to scrap value.
The reason its important to consider, is its often the case that rebuilding, replacing or repairing an undercarriage will cost more than what the tractor is worth, more so with older, obsolete models.
Good news is, this being an ag tractor, likely used for drawbar work, is likely to not been subject to harsh conditions or hard work like one with a dozer kit installed. It will likely have smaller idlers on the track frames to facilitate easier turning, being used for drawbar work. PTO is not rare on one of these, possible it was used for stationary work too, so that could mean very little track or other component wear.
Final drives and the internals say within the bevel gear compartment are another costly area to repair, you can remove the filler plug on the final drive compartment, dip a magnet in their and see if you have metal fines or bits, signifying there maybe a bearing or other imminent failure due to excess wear. You can pry with a bar against the sprocket to see if its loose or has any play on the tapered dead axle, which means something is loose, may need to be tightened or its been run like that too long and will require final drive work. These are worst case scenarios, easily checked out, but for dozers its not a bad practice, given the abuse and lack of maintenance which is never a good thing.
These can go a few different ways, real nice find, lots of service life left on the undercarriage, to its so worn out, the tractor is scrap value. Or, you may have nice tracks but it needs significant repair to say the final drives or drive train. One needing engine work, but with nice tracks could pan out to be worthy of repair, or say if you built up the track links, (rails) and had the pins and bushings turned to the new or unused side, which could restore adjustment left, the money spent to do this could give you X amount of service life and hours of work once done, increasing the value of the tractor.
Thats typically how it works with track type tractors, you don't want to blindly assess one of these, then represent that to a buyer, whom could be well informed or experienced and determine the above on their own, which may conflict with what a seller states, often times these kinds of tractors are "eyeballed" as far as undercarriage wear, and the percentage worn or say track life left is roughly and inaccurately stated, suggested, this is why measuring is required, you can't eyeball much on these and be accurate doing so.
IH Undercarriage Guide
West Track
