Dual mount sinks are made as a replacement sink when a customer’s sink fails and they need a replacement. These sinks are slightly bigger than a standard undermount so that the sink hole can be opened up slightly in order to fit as a replacement sink.

There are two main issues we run into with these. First, they are larger sinks and the depths are too large for us to drill a faucet hole behind the sink and against a wall (we wouldn’t have this problem in an island sink for example).

Second, on a normal undermount sink, there are not faucet holes built into the back of the sink. On dual mount sinks, there are faucet holes that are exactly 1 3/8” which is the exact size of our hole saw. This makes it nearly impossible for us to hit exactly on those holes which means there’s a huge chance the sink will break when we drill the holes.

These cast iron sinks below will illustrate what I mean.

The first one is a sink made as an undermount. They make these holes at 2 1/2” which gives us ½” on each side of play.

This top mount sink has holes that are 1 3/8”, the exact size of what our hole saw is.

If the dual mount sinks had bigger holes like a sink designed as an undermount sink then we wouldn’t have this issue. The problem is that they’re designed with the same size sink holes as a top mount which make them nearly impossible to install.