I drilled the holes for my fender style 6 inline headstock and bought some Grover tuning machines to fit it. My headstock is slightly under 1/2 inch thickness and these Grover tuners will not tighten down all the way to that thickness. I measured them with calipers and they are above a 1/2 inch when screwed all the way down. Is this normal for tuning machines? I figured I could add an extra washer what would be other options.