Basket Headspace and Overfilled Pucks

What headspace does

When the pump starts, water fills the space above the puck before it begins to pass through. A small, consistent headspace lets water distribute across the surface before it finds the bed. A large gap can let the puck shift, and the top layer may become uneven. A puck that touches the screen has nowhere to expand; as it wets and swells, it can press into the screen, deform, and create a path for water.

You are looking for a middle ground: the puck fills the basket properly, and there is a small gap at the top.

Signs the basket is overfilled

• The shower screen leaves a clear imprint or a ring in the puck.
• Locking the portafilter feels tight or scrapes.
• The puck sticks to the screen when you remove the portafilter.
• Flow is erratic, often slow then fast, or spurts.
• Grounds remain on the screen after the shot.
• The puck is difficult to knock out cleanly.

Any one of these is worth testing. A screen imprint alone is not always fatal, but it is a strong hint that the dose is at or beyond the basket's practical limit.

Signs the basket is underfilled

• The puck is shallow and soupy.
• The surface is uneven after the shot.
• Flow is very fast regardless of grind.
• The puck falls apart on removal.
• The shot tastes thin even at a fine grind.

Underfilling is less dramatic than overfilling but makes the bed hard to control.

How to find your dose

You do not need a specification sheet. You need a repeatable test.

1. Start with the basket's nominal dose if it has one, or a dose you know is close.
2. Prep the puck normally and lock in. Note whether it feels tight.
3. Pull a shot and inspect the puck. Look for a screen imprint and how it knocks out.
4. Adjust dose in small steps, keeping grind and yield ratio sensible. If the puck touches the screen, reduce dose. If the puck is soupy and shallow, increase it.
5. Re-check the shot. A dose change alters flow, so you may need a small grind adjustment afterward. Change only one thing per shot.

A common practical range for a double basket is somewhere around 16 to 20 g, but baskets vary widely. Treat any number as a starting point, not a standard.

Dose versus basket rating

Basket labels are approximate. A basket sold as an "18 g" basket may work best at 17 g with one coffee and 19 g with another, because bean density and roast level change the volume for a given weight. Darker roasts are less dense, so the same weight takes more space. That is one reason a dose that fits one coffee can overfill with another.

Record the dose and the coffee together. If you switch beans and the puck suddenly touches the screen, the dose may need to come down even though nothing else changed.

Why overfilling causes channeling

An overfilled puck is compressed against the screen. When water enters, the top surface cannot expand evenly. The path of least resistance may open at the edge or through a weak point, and the shot channels. Grinding coarser can reduce the symptom by lowering the puck's expansion, but it does not fix the fit. The durable fix is to match dose to basket.

The coin test and its limits

A common check is to place a coin on the puck, lock in, remove, and see whether the coin is pressed into the coffee. It is a rough indicator of clearance. It does not tell you whether the puck will swell into the screen during the shot, and coin thickness varies. Use it as one signal among several, not as a measurement.

Headspace and yield

Changing dose changes the ratio if you hold yield constant. If you reduce dose from 20 g to 18 g and keep 40 g out, you have moved from 1:2 to about 1:2.2. That is a different recipe. Decide whether you are changing dose to fix fit or to change the recipe; do not confuse the two. Record both.

A fit-check log

Two or three rows usually reveal the practical ceiling for that basket and coffee.

Bottom line

Headspace is a fit problem, not a grind problem. If the puck touches the screen, reduce the dose; if it is shallow and soupy, increase it. Test with your own basket and coffee, record the dose alongside the bean, and re-dial grind after any dose change.