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COIL ZIP 3 - The zip coil stitching has worn and the zip is coming apart

The polyester coils that run along each side of a coil-type zip are held on the edge of the zip flange tape by a fine chain-stitch. On the coil side of the zip this stitching sits up on the surface of the coils where, unless protected it can be easily abraded. On the tape side of the zip it embeds into the tape weave as is well protected there.

The reason outdoor gear designers go to some trouble to cover zips with flaps is not simply to deflect rain or achieve a particular 'look' but to protect the zip from wear and tear. A coil zip dragged against a rock will quickly have its chain-stitching worn through and then the coil will detach.

These days you often see coils zips inserted into gear so that the tape side is outwards, sometimes with a coating or lamination to repel water. This type of 'reverse' insertion overcomes the external coil stitching abrasion problem but, because of the way the slider is pulled upwards when it is being operated, it is possible for the slider itself to abrade the coil stitching, now on the underside. This problem can occur more so around curves. So called 'waterproof' coated, or laminated, zips are a little more prone to this problem because slider clearances are reduced by the laminated film thickness and operation along such a film can require more force. As you can see, nothing is simple!

If the coils of your zip come unstitched as a result of abrasion it is possible to re-stitch them, either by hand in the field or with a sewing machine. The main problem is getting the coil back into just the right position on the edge of the tape. The sewing machine must be fitted with fine needle and fine synthetic thread, set at a stitch length that matches the pitch of the zip coil and then the work guided very carefully while hand winding the machine's balance wheel. With the right thread and some practice it is possible to make a good repair.

If coils come adrift in the field, because chain stitch quickly unravels, you should do something to prevent more and more coil detaching. A smear of glue (like 5-minute epoxy) or a careful 'melt-back' of the thread ends with a match or lighter works ok. Be sure to keep the glue away from the meshing, inside part of the zip coils.
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