I use a half inch diameter grommet, which is just about 1/8th inch thick. It's a tight fit, but the thin edges allow it to slip in place. I used a 3/8 ths dowel to press it in place. That worked in my 1617. When I put the bit in, I basically just drop it in and press down lightly. When you lock the bit in, it will position itself just right and the next bit will do the same. Particularly nice if you're using a door or trim set with matched-height shanks.