Halyard wire recommendations:

• main halyard: about 22' of 3/32"  7 x 19 stainless steel wire  (more than 22' if your halyard exits at the foot of the mast!)
• jib halyard: about 21' of 1/8"  7 x 19 stainless steel wire  (more than 21' if your halyard exits at the foot of the mast!)

Rationale: This halyard should be 1/8" wire since it must take a much greater strain than the main halyard. It will be an equal partner with the sidestays (which you will note are also 1/8" wire!). I am assuming here, that you are replacing an old jib halyard, and that you already have a tensioning device installed.

Installation:
1. Swage a loop lined with a thimble into one end of your new halyard wire. The length of this wire will need to be around 20', depending on the luff wire length of your jib and how you will get to your tensioning device. On SHADES, our halyard comes straight down to the magic box which is about a foot below the gooseneck. If you lead your halyard out the bottom of the mast at centreboard level and then come up or aft to your tensioning device, you will, of course, need more wire.
2. Take the mast down, and thread the un-swaged end of the wire through the jib halyard entry block. On the old-style Proctor masts, this wire then needs to be encouraged to slide down the mainsail groove towards the gooseneck. On Abbott masts or the newer Proctors, where the halyards are internal and exit at the foot of the mast, the blocks at the foot will have to be moved to thread the wire through.
3. Get your jib out but do not attach its tack to the stemhead fitting. With the mast still down, shackle the thimbled loop of the halyard to the head of the jib, making sure that the other end of the halyard is prevented from going back into the mast (get a friend to hold it or tape it securely to the mast). With the loose halyard end still secured (!), put the mast back into its upright position. Now attach the tack to its proper spot. Take the loose halyard end and pull the halyard as tight as you can reasonably get it by hand. Mark the spot where the wire goes around the hook of your tensioning device. (I do this by slightly kinking the wire in that spot.)
4. Undo the tack of the jib so that you can pull more halyard out of the mast. This makes the cutting and swaging a lot easier. Cut off enough excess wire to leave about 3 - 6" of wire after the 'kink' or other such mark. Now swage a 3 - 6" loop into the 'loose' end of the halyard.
5. If you use #4 Dacron braid line (cheap!!!) as a halyard tail, you can simply tie this onto the loop with a bowline. #4 line runs nicely, even up the mainsail groove, and if its thinness makes you nervous, soak the part where rope meets wire with 5-minute epoxy glue to keep the #4 line from fraying too easily. You'll need less than 20' of this line. My apologies for not knowing more precise dimensions!
6. Now you can haul the jib down and stow your jib halyard as usual.
the bottom of the loop should be about a foot below the bottom black band (i.e. gooseneck in normal position). Mark the position of the bottom of the loop on the mast.
 

If you already have the right halyard but wish to add a tensioning device...
 

Your Highfield Lever (Holt-Allen 42601) or Magic Box (Harken 116) must be attached such that its hook in its uppermost extended position (see diagram on right) matches the position of the halyard's bottom loop when the jib is hoisted. There are numerous possibilities for attachment. To attach the tensioning device directly to the extension of a sail groove, the best set-up I've seen is the use of bolts with square nuts that have the corners turned inwards. This lets them really grip the inside of the groove ans prevents the lever or magic box from sliding along the groove under the considerable pressure that will be exerted. If you are going through the main body of the mast, beware of screws' points getting caught on line that run up and down the innards of the mast. You need to file the self-tapping screws accordingly or do as I do and use Monel rivets.

There is a picture of the Harken #116 Magic Box on line but they've blocked it from being copied - dumb, but there you have it!

Length Adjustment: As many of us have already found out the hard way, a wire halyard that is the absolutely perfect length for our current jib may suddenly be an inch or two too short or too long for the next jib. To avoid the need for new halyards or a new placement for the magic box or lever, we have added a shroud plate (see illustrations below) to the sail end of the halyard. The shackle goes into the single hole at the left end (below, left) while the swaged eye of the halyard slides in from the right end as many holes as is necessary. With our current jib, that is 6 holes in, which allowed the chain plate's sides to gape a bit and left me with the fear that the spi halyard might get caught in there. To avoid such a possibility, and to make sure we don't lose the horseshoe-shaped part of the shackle when the pin is out, we have taped up the entire sides (but not, of course, the ends!) of the shroud plate (which I believe is Ronstan #RF2331, or so it says in my local catalogue...) as you can see in the photo of our mast stored for trailering, on the right below:
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