From another forum (reprinted with permission):
I am making a fleece jacket that uses a separating zipper. Unfortunately, the zipper size recommended is too short, probably because I was using thick fleece and it enlarged when I was cutting the pattern out. So I had to buy a larger size that is too large. The zipper has nylon teeth. I know how to shorten a regular zipper, but I need to shorten this from the top because it is separating. Any ideas would be really appreciated. Thanks.
My first piece of advice is to recut the jacket front to eliminate the growth, and correct the back accordingly too. If the piece has grown like that, you may find other surprises mid process when you’re least capable of dealing with it.
Secondly, be sure to fuse that zipper inset area, keeping the length stable. There are two reasons for this but the first is that people will mistakenly fail to fuse zipper inset areas (or anything) on heavier weight goods reasoning the goods are already heavy enough but you have to look at this comparatively. The goods are heavier than what you may be used to working with but look at that area in relation to the rest of the goods; those areas haven’t been sliced down the middle and forced to perform the work of entry and exiting from the garment. The closure must withstand performance commensurate with the weight of the rest of the garment. As a rule of thumb, fuse all outside edges as these get the most wear and tear.
The second reason to fuse that area on this particular fabric -fleece- is that when you’re sewing in that zipper, it is common for the fleece to grow lengthwise in process. Fusible will keep the area length stable. I like to use that nylon tricot fusible for this job. If you’re concerned the knit fusible will also grow, cut it crossgrain. Experiment with a strip of fused fleece, machine baste in the zipper tape to test it. I’ve done it and have had excellent results.
With regard to sewing in the zipper itself, I just noticed I don’t have a separating zipper tutorial on site. Hmmm. [Note to self: use photos from the vintage blouse step by step tutorial (2005) for this. Someone could also email me to remind me if I haven’t done this within the next week or so.
Back to shortening the zipper -nylon, metal or otherwise, this is really easy! I will show you using a separating zipper with metal teeth. First mark off the proper length (below).
Using a pair of pliers, pull the teeth off of the tape. Just grip one individual tooth at a time and give it a good tug. While my zipper has metal teeth, if anything, this is easier to do on plastic zippers. Sometimes the plastic teeth break but this just means you haven’t grabbed the base of the tooth, reposition your pliers and tug.
Below you can see I’ve cleared away all of the excess teeth on both sides leaving about 3/4″ clearance. I’d recommend at least 5/8″ of clearance for heavier goods. This gives an allowance of 1/4″ for outside edge seam allowance, 1/4″ for top stitching and a remaining 1/8″ for turn of cloth (essential on heavy goods).
Once you’ve cleaned the edges of excess teeth, then you can cut the zipper tape (below).
And that’s all there is to it.
Process review: lapped zipper
Lapped zipper template
Lapped zipper construction
Lapped zipper specs
Centered zipper template
Centered zipper construction
Invisible zipper tutorial pt.1
Invisible zipper tutorial pt.2
Shorten a separating zipper
Zippered welt pocket tutorial
Deconstructing a zippered pouch
Zippered welt pockets