This is a threaded insert with #8 machine thread inside and it fits in a 1/4-inch hole. Threaded inserts are very handy but they love to screw in crooked. My method for setting them straight is shown in this sequence of photos.
I use a long #8 screw with two nuts jammed together to drive the insert. With such a long screw I can force the insert to go in straight. I wear a glove on my left hand in case I slip with the screwdriver.
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