Mine got REALLY over-tightened. I hadn't used it much since it was new about 15 years ago and this was the first new disc. On mine, the disc spins on the shaft to tighten. I finally wound up putting the disc in a vise and used the weight of the grinder as leverage. It actually took two men pushing on it to break it loose. I'd tried everything and decided that if this didn't work, a $120 grinder was going in the
scrap metal pile. Also, you might see if spraying some wd40 to soak into the threads would help any.
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