HERE’S THE ACCURATE WAY TO MEASURE FOR LENGTH: Drive a finishing nail into the dock or a wood deck of a patio, or any horizontal surface that will hold the nail and with enough length to stretch the wires out.
Don’t let the kids step on it! A safe way might be to put a chair over the nail.
The wire will probably have an eye or a fork on one end. Hang it over the nail and stretch it out, and hang a tape measure over the same nail, and measure to the end of the stud, which is the measurement I need to duplicate the wire. But think about this first. Rarely, even on a new boat, are the turnbuckles right in the center of the adjustment range. Now is the time to correct this, if you care. If a turnbuckle is close to be bottomed out when set up on the boat, now is the time to correct it; same thing if it’s uncomfortably close to the ends of the studs. Add or subtract whichever is appropriate, remembering that the wire can draw (elongate) a little, but it’s never going to shrink unless you tie a knot in it. (Just kidding- DON’T tie a knot in it) IF YOU SEND THE RIGGING, INCLUDING THE TURNBUCKLES, FOR ME TO DEAL WITH, I’LL AUTOMATICALLY MAKE THESE ADJUSTMENTS FOR YOU SO BE SURE TO KEEP THE TURNBUCKLES IN THE SAME PLACE AS THEY WERE ON THE BOAT! If I get confused because it looks like the turnbuckles appear to be unscrewed I’ll email you. If this confuses you, email me!
Another thing, regarding forestays, and also headstays on boats where the wire is not highly loaded (and therefore not likely to stretch, or draw), we can clean things up a little by leaving out the adjuster or turnbuckle, adjusting the length of the wire as needed, and putting (usually) a toggle on the bottom of the wire (depending how it’s anchored to the chainplate) instead of the stud for a turnbuckle or an eye for a stay adjuster.
The replacement of wires is the time to think about replacing some of the terminal hardware, like changing over the Nicopress terminals on some kinds of boats with eyes or forks. Swedged terminals as opposed to Nicopress terminals are stronger and cleaner, and not much more expensive. Many boats, like some Macgregor/Ventures, Potters, and the Catalina 22 have Nicopressed fittings instead of swedged fittings, usually just as a cost-cutting factor but sometimes to allow freedom of movement in all directions. So think about each application before making changes. Or ask me.
Below are common 1/8″ terminals; first is a marine eye, then an aircraft fork, a turnbuckle stud, and a Nicopress eye. Note that the marine eye is drilled for a 1/4″ pin, but an aircraft fork uses a 3/16″ pin.

Jerry Montgomery
All Rights Reserved 2021
Email Jerry Montgomery: jmbn1@outlook.com