We get lots of emails like this:
"My son just bought the wagon vise parts for his workbench,
which I am building. I have a most basic question... The top will be 3 inches thick. Your specs call out for 4 inch top..... and if thinner, to place the templates so that the top is the registered surface..... So what happens with a 3 " top?"
The Benchcrafted Tail Vise can be mounted in any thickness top. The important thing to keep in mind is the vertical placement of the vise, specifically the hand wheel. The top of the handwheel needs to fall below the top of the bench, and far enough below to allow for some future flattenings.
Our templates provide about 1/4", but you can make it whatever you like. The lower you place the vise in the bench, the thicker the end cap will need to be to seat the flange adequately.
For benches less than 4" thick you'll need to shim the under side of the top to move the vise vertically down. If your top is a uniform thickness your shims will be the same thickness. For example, if your top is 3" thick, your shims need to be 3/4" thick. This places the vise in the same vertical placement as if your top was 4" thick. See the drawing above.
If you have a thicker apron around your bench, as in the example below, where the apron is 4"+ and the rest of the top is less, you'll only need to make one shim for the inner guide rail--the outer rail is mortised into the bottom edge of the apron. The advantage of this arrangement is that there is virtually no cavity to excavate.