Sometimes copper tubing swells, especially during freezing temperatures if they have water in them. A cheap set of digital calipers are your friend here if they go to 3 decimal places. You can compare to the new size. If it's just tight but you think it could be made to fit then the tubing has probably swelled. That's not the end of the road as there are many many ways around the problem. Easiest is most likely a rubberflex coupling for lower pressures and I would expect those to work fine for cooling runs (engine radiator and heaters). A tool can be found or made to swage the tubing back to the correct size. The fitting can be expanded to fit the tubing. Or sometimes a compression or other (PEX) type fitting can be used. My choice would be to drive the copper fitting on after cleaning both pieces and then braze them using a fluxless brazing rod for copper plumbing. A propane torch with MAP gas should get hot enough if you don't have an acetylene plumber's torch.
Jim
