This tutorial describes how to fix an elastic in a casing, such as Fuzzi Bunz, Bumgenius, Blueberries, etc. I get the most repair orders from FB and BG, I'll be working on a Blueberry soon, but it looks as though it follows the same instructions as a FB. The following pics were taken during the repair of a BG one size pocket.
First, turn the diaper inside out and find the points where the elastic attaches. You'll be able to see a small bit of elastic sticking out of the top stitching most of the time, sometimes you'll only see where the top stitching gets close to the edge again, at the end of the casing, you can tell where this is because you'll be able to see the seam allowance.
Using a seam ripper, gently undo the stitches hiding the end of the elastic. You'll want to unstitch all the way to the end of the casing. Sometimes you'll find the manufacturer was in a hurry to sew and caught the elastic in the top stitching for a half inch or so, just unstitch until the elastic is free from the casing but still attached at the end.
Using the seam ripper or scissors, carefully cut the elastic close to the stitching at the end. Leaving the end of the elastic helps you see where to attach the new elastic to. You can also unstitch the end and mark it, but I find it is much easier to accidentally unstitch some of the serging that way...
Go to the other end of the elastic and unstitch the casing in the same way. Pull the broken elastic out, and snip it off, either leaving the stitched end for reference, or marking where to attach the new elastic. Repeat this on both legs and the back, as needed. I recommend replacing both leg elastics at the same time.
For Fuzzi Bunz leg elastic (except for some older ones), it is stitched to the seam allowance the whole way down the casing. You have a choice of just threading new elastic through and leaving the old in place, or unstitching the entire leg casing, removing the elastic, and applying new.
Measure the new elastic and pin one end in place, on the seam allowance, using the markings you made or by pinning it over the old elastic end that you left attached. I suggest using Stretchrite braided elastic, but be sure you've got the kind that says it's good for swimsuits. It is made to use in temperatures up to 200 degrees and will withstand chlorine. Stretchrite makes elastic that looks exactly the same but does not withstand high temps, making it unsuitable for diapers, so read the back of the package!
You can use Lastin (I recommend 3/8") but you will have to unstitch the entire casing and attach the Lastin to the seam allowance with a 3 step zig zag.
BG and FB use 1/4 inch elastic. I'll try to remember to post exact lengths as I find my notes on them... BG one size uses 5" in the back and 6" in the legs. You can estimate how much to use from your broken elastic you removed too. Adjust according to your preference of course!
Stitch across the elastic a few times to sew the end in place tightly. Be careful not to stitch past the seam allowance. Then, using a safety pin, knitting needle, or your preferred method, insert the elastic through the casing.
You can see the new casing here, it looks as good as new besides some overlapping top stitching.
Ta-Da! Ready for lots more use!