Burr alignment is the process of making sure both burrs in your grinder sit perfectly parallel to each other. When they’re off, even a little, you get uneven particle distribution, channeling in the puck, and shots that taste different pull to pull, no matter how carefully you dial in grind size. If you just swapped burrs and suddenly can’t get consistent extraction, alignment is the first thing to check.
What burr misalignment actually does
Think of it this way: if one side of the burr gap is 200 microns and the other side is 400, you’re grinding two different particle sizes simultaneously. The fine side over-extracts, the coarse side under-extracts, and what ends up in the portafilter is a blend of both. You’ll notice it as a wide spread of fines and boulders, shots that run unevenly (one side of the basket flows first), or a grind that produces good shots at one setting in the morning and tastes off by afternoon when the shop heats up.
Thermal expansion makes misalignment worse. A burr carrier that’s slightly warped or a spindle bearing that’s worn a few hundredths of a millimeter will show up more aggressively once the grinder reaches operating temp.
How to check alignment before you start shimming
You need a dry-erase (whiteboard) marker, patience, and good lighting. Don’t use a permanent marker — it won’t wipe cleanly and gives you a false read. This is called the marker test, and it’s been the standard diagnostic in the specialty coffee world for years.
- Remove the upper burr carrier. With the burrs installed, coat the flat face of the upper burr lightly with dry-erase ink.
- Reassemble the grinder and bring the burrs together gently, just enough to touch. Don’t force them.
- Back off slightly, disassemble, and look at the upper burr face. If the ink is worn evenly all the way around, you’re aligned. If only one arc shows wear, that’s your high point.
On commercial grinders (Mazzer, Mahlkonig, Victoria Arduino, and similar), the upper burr carrier typically sits on three or more mounting screws. The high side is where you see ink removed; the low side is where you need to add a shim.
The shimming process
Shims for grinder alignment are thin metal washers, usually brass or stainless steel, cut to fit under the burr carrier mounting tabs. Purpose-made sets are available from coffee equipment suppliers. Aluminum foil works for iterative testing (standard household foil runs around 16 microns per layer, which gives you a useful rough increment), but foil doesn’t hold reliably once you torque the screws down. Use metal shims for anything you intend to leave in place.
The process is iterative:
- Identify the high point from your marker test.
- Add a shim at the low point(s), opposite the wear mark.
- Reassemble, run the marker test again.
- Repeat until the ink wears evenly.
On a commercial grinder like a Mazzer Major or Mahlkonig EK43, getting within a few microns of parallel takes multiple rounds of testing. It’s slow work. Don’t rush it.
One practical note: check the burr carrier itself for flatness before you start. A warped carrier (common on older grinders or after improper cleaning) can’t be shimmed into alignment. It needs to be replaced or machined flat.
What’s DIY-safe and what isn’t
The marker test and shimming with metal shims is something a careful technician or experienced cafe owner can do. You need good lighting, clean hands, and time. No special tools beyond what the grinder’s service manual calls for to remove the burr carrier.
What gets harder:
- If the spindle bearing is worn, alignment will shift every time the grinder runs. You can shim perfectly and still get inconsistent results. Bearing replacement requires pressing the old bearing out and seating a new one correctly, which needs a press or at minimum a proper bearing driver. Doing it wrong damages the housing.
- Some grinders have alignment adjustment at the motor mount or through the lower burr carrier, not the upper. On a Victoria Arduino Mythos or a Nuova Simonelli MDX, the geometry is different and the service procedure in the manual matters more than YouTube tutorials.
- If the grinder took a physical hit or was dropped, the frame itself may be out of true. Shimming the burr carrier doesn’t fix a bent chassis.
Grind consistency checks after alignment
Once you think you’re dialed in, grind a dose at your normal setting and look at the spread. A good aligned grinder should produce a relatively tight particle distribution. The practical way to check: weigh three doses back to back from the same setting. If they vary more than 0.3 to 0.5 grams without any user input changes, something’s still off, whether alignment, a worn burr, or a dosing mechanism issue.
Pull a few shots and watch how the basket fills. A well-aligned grinder with fresh burrs should produce a puck that extracts evenly, with no visible channeling or one side of the basket flowing significantly before the other.
When to call a technician
If you’ve done two or three rounds of shimming and the marker test still shows one dominant wear point, or if shots are inconsistent even with a fresh set of aligned burrs, it’s usually a mechanical problem downstream of the burrs. Bearing wear, a bent spindle, or a damaged carrier are the likely culprits, and they’re not DIY repairs on commercial equipment.
On commercial grinders running significant weekly volume, burr carriers and bearings wear faster than most people expect. A full grinder service, which includes bearing inspection, carrier flatness check, and alignment, makes sense every 12 to 18 months depending on volume.
If you’re in the Bay Area and the grinder still isn’t behaving after you’ve worked through the alignment process, we’re happy to take a look. More at bayareacoffeeservice.com.