kink in forestay?
Mike, Let's start by making sure we're referring to the same parts by the same names. On my 170's jib, there's a cringle (metal grommet) in the tack (lower front corner of the sail). A U-shaped shackle links the cringle to a thimble, a raindrop-shaped metal loop. Wrapped around the outside of the thimble is the lower end of the wire forestay. On my boat, the short shaft protruding from the top of the roller furling drum is split. The wire thimble fits down into the split and is held in place by a clevis pin inserted horizontally through the drum shaft. If I understand you correctly, the shackle on your jib is intruding downward and blocking the opening in the thimble. Since the shackle has little or no free play in it, my guess is that the wire forestay is for some reason pulling upward on the thimble. Is it possible that your forestay has developed a kink or bend, somewhere along the luff where you can't see it? The only other possiblity I can offer is that the luff of the sail has stretched, increasing its length relative to the wire forestay. I'm a newcomer to the 170 so I'm guessing, rather than speaking from experience. Maybe somebody out there with more experience can enlighten us both. I'm still experimenting with shroud length. Loose enough to allow the mast base to slide easily into the mast step, tight enough to keep the rig taut underway. As suggested in a post the archives, I installed a pair of Johnson quick-release adjusters to replace the stock adjusters on the upper stays. This relieves much of the backward pull on the mast and makes it much easier to pin the forestay to the furling drum. You may accomplish the same thing by simply unfastening the upper shrouds, but I'd leave the lower shrouds connected to give some side-to-side support to the mast. I don't know how much leverage is required to tear the mast step out of the deck, but I wouldn't want to find out. Good luck.
