I have a model year 2000 K3 and ended up disposing of the 315 mm blades I naively purchased when I bought the saw. I don't specifically remember exactly how far below the table top a 315 mm blade could go on this saw. But the bigger problem I encountered was that swapping between the 300 mm blades I normally use and a 315 mm blade entails resetting the riving knife, at least if you desire the riving knife to be positioned a hair below the top edge of a 300 mm blade (so as to stay on for non through cuts). And on the K3, resetting the position of the riving knife is a very fiddly and unfun procedure, as discussed a number of times previously here. I also have at least a vague recollection that the interplay between the Hammer riving knife and the mechanism may not have allowed for sufficient range to position the riving knife below the top of a 315 mm blade. I could be mistaken on this last point, however.
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As a separate matter, even with a 300 mm blade, I always remove the zero clearance insert to change blades. Maybe it's possible not to do so--I'm not sure I have ever tried. But certainly, it makes things easier, given the need to maneuver the blade in a confined space and line up the bore holes.
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David