Product review (4 stars): Turned one and a half 56" high shelves into one 84" shelf.
These fit in my 56" ShelfTech System shelves (metal shelving sold by at least one of the big box stores) and allowed me to add an extra 28 inches of height. The threaded inserts did not screw in all the way (hence four instead of five stars) but went in far enough to make a secure connection. The extra ~3/16" of visible threading was not a problem for a shelf in a storage area, but could be if you have aesthetic considerations. The included threaded rods are 3/8" in diameter and worked very well. These are great problem solvers- I wish they were in stock at my local hardware store!