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My flush-fitting MastGates are made for sail slug diameters of 3/8" and 9mm or smaller.
The MastGate insert has side barb rails that make it snap-lock in place and can be removed with a large tweezer or needle-nose plyer.
You will have to cut two 1.5mm deep side grooves into the aluminum sidewalls of the feed slot with a Dremel type tool. See my video on how easy it is done.
The groove cut Video: https://youtu.be/kjJ8F6KNM1Q
To make your life easier, consider using 3/8" or 9mm round sail slugs with shackles. This combination reduces friction during reefing and hauling up the sail. Easier for you to replace slugs in the future. I add a small dab of Starbrite lube to the sail slug's necks for a quantum leap in friction reduction.
Request the 3D-D1 install document from: info@MastGates.com