This Instructable illustrates how to add a 12in zipper to a sweater, useful for regulating your body's temperature when cycling or hiking. A similar zipper sweater from Ibex for example retails for $75-105. I made this one from a thrift store sweater and zipper for less than $10.
Merino wool mock turtleneck sweater
14 inch garment zipper
matching polyester thread
Look for a slim fit for cycling use. A mock turtleneck isn't necessary, but matches a traditional wool long sleeve jersey. Even though we're adding a 12 inch zipper, the extra length of 14in is needed for flipping inside-out.
Step 1: Mark for Zipper Placement
Step 2: Pin the Zipper
Again, with the sweater laid flat and right side out, lay the zipper over the chalk line right side up as well. Following the first illustration, unzip the zipper part-way and flip the two sides open like a book. The teeth of both ends should be opposite each other and should face down towards the sweater.
Begin by aligning the zipper stops with the collar and insert pins horizontally along the tape to secure. In making this Instructable I neglected to leave a void at the termination of the zipper to account for the extra width of the exposed zipper teeth. This caused a slight gathering of the wool. As illustrated with the red shading, pin the zipper sides at least 4mm apart at the 12 inch mark to account for the zipper width. A trick to inserting pins without also pinning down the back layer of the sweater is to stick a thin book or quilting ruler in between the layers to guide the pin back up.
Step 3: Baste and Sew
Unzip the zipper past the 12 inch mark and then proceed to baste stitch down each side of the zipper to the 12 inch mark. Don't be tempted to run the stitching across the bottom and back up the other zipper side. We need for the zipper to freely move for a later step.
Fold the top extension tabs of tape over the collar and baste stitch them down as well using a horizontal stitch, perpendicular to the zipper. The reason for this is that if you were to stitch vertically, the feed dogs on your sewing machine will tend to pull the tape back out, leaving you with a sloppy finish (my second mistake.)
With the zipper basted in place and all the pins removed, switch to your zipper foot and stitch along the zipper coils. Run the stitches about 1.5-2mm away from the coils. This helps to prevent the wool later getting stuck in the zipper pull.
Step 4: Cut Seam and Fold Zipper Over
Cut along the tape edges 11 1/2 inches, leaving 1/2 inch to the 12 inch mark.
Now, fold the zipper right side out and flip the end of the zipper inside the sweater. You will need to be careful on this step not to unravel the end of the cut. Tuck the last 1/2 inch inside the sweater to hide the raw edge.
Reattach your regular presser foot to the machine and determine the edge distance you'll need from the zipper teeth to stitch through the edge of the zipper tape on the inside of the sweater. This is illustrated below. Stitch in a continuous step, from one collar side, through the zipper termination, and up the other collar side. It would be good to pass over the zipper termination in a Z-pattern: forward-reverse-forward, to help secure the hemmed edge and provide a strong zipper stop.
Turn the sweater inside out and apply another Z-stitch to the loose zipper end, about 3/4 inch from the slit termination. This prevents the zipper from separating. Cut off the excess zipper and you're done!