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Someone asked the other day about my bionicle minifigure heads, they can't normally fit on a torso, but you can hollow out the peg holder so that it does
I think I took a 3/8ths drill bit to these years ago and it fit perfectly. They make for a decent alien head imo, and the modification is incredibly minimal.
I had them from the Toa Terrain Crawler and they are fairly cheap parts anyway.