I noticed my rails are pretty worn and 3 pins are worn badly. What might cause 3 pins to wear more ? They are next to each other. Sprockets are thin at the ends. I understand they will need to be pressed off. Is this something that requires special tools and needs to be jobed out? Can all these parts still be had? Will any other rails and pin work? Is the pitch measured as a distance between the pins? I would like to thank everybody for all the information on this site. It has helped solve many problems and avoid others.

You say the pins are worn? You can't really see the pins, you will see the bushings. The bushings wear where the sprocket engages them and become out of round before wearing completely through into the pin bore.

Both the pins and bushings wear internally to effectively increase the pitch of the track links. This eventually leads to increased sprocket tooth wear as the bushings ride closer to the sprocket tips even if the track tension is kept adjusted. When the idler gets to the front of the track frame/the tensioner adjustment is maxed out, you will probably be at the wear limit on the pitch. The recommended "turn at" % wear on both the outer dimension and the pitch is to maximize the total life of the pins and bushings. With newer, sealed and lubricated track (SALT), internal wear is vastly decreased and the most important thing to watch becomes the external bushing wear. A properly equipped shop can disassemble the track and turn the bushings and pins and reassemble the track to essentially double the life of the assembly.

The rail links wear on the surface that runs against the track rollers and becomes shorter. The pin bosses will get close to the track roller flanges and if the rollers are worn as well, they will hit and create severe wear in a hurry and result in loosening of pins in the links. At the 100% wear dimension the hard surfacing on the links is gone and rapid wear will follow.

Look at these links for dimensions and measuring instructions.
www.crawlerheaven.com/tracklinks.htm

Your track links should have some numbers on them so you can compare the measurements with the % worn for the correct part number link. Over time, most links became taller and so the % worn dimensions might not be correct if you have updated rails on the tractor. I found a set of rails on a D2 I bought that are almost new measurment in height, but are obviously worn a lot. It doesn't make sense unless that side has a D3 track which had a taller link than the D2 to begin with.

For the 3 worn bushings, someone put in 3 that were not of the same quality as the others, were already worn some, forgot to turn those 3 when turning the rest, etc.

If the seals on the final drive gear case are not leaking along the sides of the sprocket, it can be cheaper to re-rim the sprocket in place. Most folks don't have the equipment to pull and press the sprockets, but a reasonably competent welder can replace the rim. The old sprocket rim is cut off with a torch and the new ring or bolt on segment adapter ring is welded to the spokes.

Good luck!

It's not unusual to have uneven wear on some pins and bushings, especially if you are at or beyond the normal service wear limits.
Yes, sprockets will need to be pressed/pulled and there are special tools for that. I'd recommend you get a field mechanic to help you out there although some have made improvised tooling.
Pitch is measured from pin to pin (using same measurement location) and is originally 6.75 in. on the D6.
If your flush with cash the preferred upgrade is to use the D5 chain and sprocket rings with bolt on segments and upgraded dual-cone final drive seals.
These chains are a 6.91 pitch and require the matching sprocket.

All the undercarriage parts are still available either from Cat or after market.
If the sprocket hubs are in good condition you can get weld on replacement rings to use with the original pitch chain.

The three badly worn pins could be either a result of a cracked bushing that has had a chunk fall out of it .. or the three bushings are so badly worn, the ends have fractured, and are no longer being supported by the link counterbore .. or you may have run the undercarriage to the point where the wear has gone so deep, it has worn through the deep hardening, and is into the softer underlying metal.
The deep hardening on Cat undercarriage on the older tractors is only about 1/4" deep on the links, and it's less on pins and bushings.

Measuring internal wear involves placing a pin in the top teeth of the sprocket at about 10 o'clock, backing up until the track chain is tight .. locking the brakes .. then measuring the distance over 4 links, or 5 pins, to come up with a figure that is 4 x the pitch.
On the D6, 100% worn measurement comes in at 27.51", when the standard internal and external .12" wear allowance is used.

If delayed Pin and Bushing turning is employed .. as is possible, if the tractor is operating in dry sandy soil, with light loading that involves no shock loading .. then, .19" internal and external wear allowance can be used, and the internal wear measurement can be taken to 27.79" before it is classed as 100% worn.

If you go over 100% worn, on internal wear measurement, you are shortening your undercarriage life, raising your undercarriage $ cost per hour .. and eliminating any chance you can turn your pins and bushings. Once you're well over 100% worn, link counterbore wear, and too-thin bushings, then means that any money spent on a P&B turn is effectively wasted.