Jamie, my main bench is 4" thick birch. The holes started out as 3/4", but they've been reamed out a bit since the bench was new, so they are probably closer to 7/8 by now. The Grammercys grab just fine in all the holes in that bench (excepting a 1" hole that was done as a test, that ones a gamble and doesn't always grab tight), and in my 2" thick Continental style bench (which was my first woodworking project years ago). I've even used them in the square dog holes on the Conti bench, they work fine.
I started out with the cheap cast iron holdfasts from Woodcraft, they do not work at all in either bench.
I look forward to trying the Williamsburg holdfast and will post about it once it arrives.
Last edited by Zach Dillinger; 01-15-2013 at 11:45 AM.
Your endgrain is like your bellybutton. Yes, I know you have it. No, I don't want to see it.
Ask me why I use hand tools, and I'll tell you