On my 2005 C-310 with in-mast furling (Charleston Spar) I replaced the MAIN-IN line with 3/8 Spectra, 62 ft long. At 62 ft, there is plenty extra. I used 62 ft because that is what the old one was. Both the MAIN-IN Line and MAIN-OUT line were original and seriously worn.
- Unfurl the main and release the halyard to lower the main.
- You will have two to three wraps of MAIN-IN line on the spindle
- Remove the grub screw on the bottom of the spindle and pull out the old line (you can turn the furler by hand to expose the grub screw)
- Put the new line in the hole, replace the grub screw, and turn by hand clockwise (looking down on the mast) to thread the line onto the spindle all the way to the top.
- Feed the other end through the fair leads into your clutch following the path of the old line.
- Pull the main up the mast with the halyard (have a friend help feed the luff into the track), and furl the main.
- You can actually feed the line around the spindle and thus avoid unfurling and lowering the main. It is tricky to feed through in the grooves inside the guide bars, especially the one in the center of the mast that is barely visible. I've used curved hemostats to do it. It took lots of finagling and cussing to do it. Allow 6.5 inches per wrap and feed it around the spindle two or three turns and into the hole and fasten with the grub screw. Yes, I did this with 7/16 line - it was a real bear, and then the line covering separated from the interior and refused to go into the hole at the bottom. It is easier to to with 3/8 line and 7/16 is overkill for the MAIN-IN line anyway.
I replaced the MAIN-OUT line with 7/16 spectra. The 7/16 grabs in the clutch better than 3/8 and you don't need to keep it on the winch. The worn 7/16 MAIN-OUT line would slip through the winch if the wind was higher than 14 knots. If memory serves, it was 48 ft, but might have been 46 ft. Measure the length of what is on your boat. I sewed the end of the new line to the old using 1.5 mm whipping line, butting the ends together, then wrapped with tape. The tape is just to hold things in line. Then pull on the old line and the whole thing goes through the boom quite nicely. It may hang at the turn-around on the end tip of the boom, just help it through and it works great.
BTW, Charleston Spar was purchased a couple of years ago and no longer makes masts, etc.