I have long wished my ID eVent rain jacket had pit zips, and I finally decided to install some myself. First, I practiced on my cheapie Driducks rain jacket. It worked just fine, so I put them in my ID jacket.
It turns out it takes more courage than technical skill. The skills required are straightforward sewing. The courage is cutting into a $240 jacket! Here are the steps I used and pictures of the ID eVent jacket during surgery.
Step 1: I scotch-taped a standard polyester 16" zipper, with the zipper tab face down, onto the inside of the jacket, making sure the zipper teeth were aligned on top of the jacket seam. It doesn't matter that it's a separating zipper, because the installation takes care of that, as described below. I put the mid-point of the zipper at the intersection of the side seams and the sleeve seam. I also pulled the zipper tab down a little over an inch.
Step 2: Using a zipper foot on the sewing machine, I stitched the zipper to the jacket, reinforcing the seams across the top and bottom of the zipper. Note that I stitched about an inch below the metal finishes on the zipper at each end, making the zipper shorter by about 2 inches than its original length. The needle has no trouble stitching across the scotch tape or across the zipper. This takes care of the fact that the zipper originally separated at the top. It's now stitched together at both ends. Peel off the tape after stitching. Here are 2 pictures of the stitching.
Here's a picture of the stitching from the right side.
Step 3: Cutting! First, trim the excess zipper off at both ends.
Then, turn the jacket to the right side, take a deep breath, and using small, sharp scissors, start cutting along the seam. I began and ended the cutting about an inch inside the stitching across the ends of the zipper. Unzip the zipper! Pit zips!
The jacket, formerly 9 oz, now weighs 9.4 oz with the trimmed zippers.