Applying elastic with the Serger

When sewing swimwear or underwear, using an elastic foot on the serger can help save time and ensure you have the same tension in all openings.

We made a Claudia bikini top as a bralette. We used a single-layer cotton spandex jersey. Since the pattern requires an elastic the same width as the Janome Elastic Gathering Attachment, we decided to show you how nice the finish is when applying the elastic using this attachment designed for all Janome sergers.

We used the amazing Janome AT2000D Air Thread serger.

Getting the bralette ready for the elastic

Sew all the seams that you planned to use your serger for, up to the step where the elastic is applied to the openings.

Pattern modification for underwear

The Claudia bikini is designed for swimwear fabric but we decided to convert it into an unlined bralette without seaming for this tutorial. We needed to remove the seam since there would be no lining covering the seam allowance. It’s very easy to do!

On piece B, we traced a line twice the seam allowance (1.2 cm) and overlapped the side piece (C), with the piece ends at the line. Since the bralette is a single layer of fabric (which provides less support than a lined fabric), the wedge that is removed will be barely noticeable.

Sew the side and shoulder seams BEFORE you install the Janome Elastic Attachment on your serger. Once your bralette is assembled and you are ready for the elastic, it’s time to set up your elastic gathering attachment.

Setting up the elastic gathering attachment

Install the attachment according to the instructions in the box. So long as your serger has the small screw in the front as pictured below, you can use this attachment.

Place the elastic into the guide, then close it to secure. We are using 1 cm (3/8”) knitted elastic for this project.

Before you start, you will want to test on scraps of fabric and adjust to get the elastic gathering you want.

For the Claudia bikini, the elastic is approximately 95% of the opening, so make a few tests to get the gathering right.

Measuring the elastic tension/gathering (percentage)

Cut a piece of fabric approximately 18” long and mark a distance of 10”. This 10” distance will be your reference, your “100%”.

We are aiming for 90-95%, which would be 9 to 9 ½” length when the elastic is sewn onto the fabric.

On our first try, the piece with the elastic measures 7 inches. It means the elastic tension is at 70%, which is too tight for our project.

We turn the dial on the Janome Elastic Gathering attachment counter-clockwise to reduce tension on the elastic, but this time, it was a little too much. This actually lengthened our fabric, bringing our guide to 12 inches! We don’t want to make our neckline and armholes opening bigger so we need to turn the dial clockwise to increase the tension.

The third time’s the charm! We have exactly 95%, with our piece measuring 9 ½”. Our foot is set up, ready to sew!

Applying the elastic

The elastic is sewn to the wrong side of the fabric, the edge of the elastic along the edge of the fabric, the fabric by a millimeter or so to make sure the elastic does not touch the upper knife. The blade skims the fabric, not cutting the elastic of course.

No need to stretch the elastic as you sew, the attachment takes care of it! Tension will be even everywhere, so gathers are consistent. It’s magic! lol!

When applying the elastic, take your time. It’s not a race. If you race through the application, you might have to unpick the seam later on. Slowing down will save you time in the end.

What we love most about applying elastic on the serger is that the edge is nicely finished and there is no need to mark halves and quarters, like we do when applying elastic with a regular machine (where we cut the elastic to a specific length and stretch between marks to fit the fabric).


When your elastic is attached to the edge of the openings, fold it to the wrong side. The fabric should be taut over the elastic, with no “empty space” along the fold.

You are ready to topstitch. You may go with a zigzag stitch, or double needle on your regular machine or choose to go with a cover stitch for a more professional finish. We chose to use a cover stitch for a “store bought” look.

Continue with the band assembly as per the instructions. We used a contrast fabric for the band to mimic the iconic Calvin Klein bralette style from the 90’s :

We hope you enjoyed this tutorial and can’t wait to see your swim or underwear versions of the Claudia on Instagram with the hashtag #JalieClaudia


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