Chook
The hole may need to be slightly wider to hold on your deep benchtop. Try one hole first though. The holdfast needs to be able to angle in the hole. As the hole deepens there is less of an angle possible.
Regards
Paul
Printable View
My bench is a bit over 100mm thick, following the Benchcrafted Roubo style. To get my Gramercy holdfasts to grip, I back drilled the 3/4" holes from underneath, at 1", so that my bench effectively "became" a 2" or 50mm top. This solved the problem. If this is something you're interested in doing, it is certainly a PITA to enlarge a hole of that size with a forstner bit, for example. In the past I've stuck a bit of dowel (as long as it fits tight), or failing that, a whittled piece of square stock into the hole and centred my forstner bit on that. Seems to work. Good luck.
Oh, and on the Gramercy holdfasts, rough sandpaper run radially around the stock, so you get horizontal lines, works wonders for grip. Cheers, Lucky.