Answer: The absolute bare minimum opening is 2". Anything less than that and you'll struggle to get the seal inside the rail cavity. The lugs (ears) on the seal don't fold enough to be able to "pop" the seal in very well.
D.S. Brown Strip Seal Tool
D.S. Brown offers customers an installation tool that provides a huge benefit to customers when installing strip seal glands in our Steelflex® Strip Seal Expansion Joint System and Steelflex® Modular Expansion Joint System.
This tool is available for purchase on D.S. Brown's online store.