Introduction: In-Sink Espresso Knock Box

About: Grad student and jack of all trades

As any barista who uses a manual machine knows, getting the used grounds out of a portafilter without a knock box is not only annoying, but can likely either burn your fingers, cool down your portafilter, or damage your portafilter. But no home barista wants to make room for one of those big clunky knock boxes you see in cafes or on Amazon. I saw one person's tutorial for an in-sink version, but I wasn't sold on the metal-on-plastic. This tutorial is based on that concept, but the materials all come from Home Depot (instead of the bar coming from a specialty coffee retailer), and I added fasteners to absorb the shock of use to try to both dampen the sound and reduce wear. It's small enough that it fits on top of my Gaggia Classic, under the cabinets (4in. tall, and 7in. at the widest part), and it only requires a drill and a file for assembly. My materials list came out to under $20, including enough rubber pipe insulation to change out the bar wrap close to 20 times.

PLEASE NOTE: The increaser that I listed was chosen because I measured my sink, and it came to 3-1/4". Measure your sink! Also, you probably shouldn't do this project if you don't have a garbage disposal. I'm not sure how coffee grinds do in plumbing, but I doubt they do any good.

Step 1: Materials

You will need:

  • a plastic "increaser" (these are technically supposed to be for ductwork)
  • 1/2-inch diameter rod (preferably either stainless steel or galvanized; remember, this will get wet a lot), cut to at least 2 inches longer than the largest diameter of your increaser
  • 2 1/2-inch diameter neoprene grommets
  • 4 1/2-inch hex nuts (again, try to get either stainless steel or galvanized)
  • 1/2-inch rubber pipe insulation
  • 2 1/2-inch gray rubber pipe protectors (optional, but you can worry about sharp edges from the rod less)


  • Drill with 5/8-inch bit
  • Dremel (optional)
  • Rat-tail file (if not using Dremel)

Step 2: Holes

First, figure out where you want to place your holes. I like where I placed mine, which was slightly off-center, because it gives a little extra room for the portafilter. However, realize that we will be holding the bar in place with hex nuts that will bend the plastic perpendicular to the rod, so you do not want the holes at too much of an angle. I wanted the rod to be as close to the top as possible, so I measured the grommets and added about an eighth of an inch just to be careful (I really didn't want to break the increaser when drilling), and that was my distance between the edge of the holes and the top.

Mark and drill.

The holes are going to be just a smidge tight for the grommets, so this is when I used the Dremel to both clean up the rough edges and enlarge the holes. If you have a 41/64-inch drill bit, by all means, use that (that's what my calipers read the diameter of the grommets inside the lip to be), but for the rest of us, I think this works just fine.

Fit the grommets in now.

Step 3: Rod

Thread the rod through with nuts on either side of the increaser, as shown. Tighten until snug.

Step 4: Pipe Insulation

The nice thing about using pipe insulation for the cushion of your knock bar instead of buying an assembly from an espresso supply store is that, for roughly the same price, you get enough of the stuff for 18+ replacements. Even if it doesn't last as long (which I'm not even sure is the case--mine seems to be holding up pretty nicely), it's a steal.

This step is simple. Eyeball how much you need and chop it, using whatever tool strikes your fancy. Wrap it around your rod.

Step 5: Coffee

Brew some espresso and give that portafilter a nice whack over your sink.