The sprue plate screw on my old and heavily used 3118 4-cavity mold will not stay in. It doesn't come unscrewed, it just plain slides out. Rust is the presumed cause. The threads on the screw seem to be reasonably good, so I assume it is the ones in the mold body. I tried filing a recess for the set screw, but that did not hold it. I haven't tried loctite, as I figured the heat would defeat it. Has anybody tried anything that worked?