New to all this: I have a JD 550G: Does anyone have an idea of how much it would be to change the pins and bushings?
On that size tractor , I think most people just install new rail system. If your pads are still good use them back. Rails are about 2000$ a side and pad bolts would be about 600$ and New sprockets about 250 per side. I have been told by more than one person that turning or even installing new pins and bushings on a smaller tractor , the repair won't hold up and by the time you factor in labor to remove and reinstall its more cost effective to put new ones on. They also say the pins and bushings won't hold oil so they grease them , which they eventually get noisy and wear accelerates. Depending on how bad they are , I'd run them out then replace.
Is the rail system another name for the Chain?
Yes