Carbon brush size is expressed as three dimensions in a specific order: thickness × width × length. Getting this sequence right matters because mixing up even two of the numbers will give you a brush that won’t fit the holder. A digital caliper is the best tool for the job, and the whole process takes about two minutes once you know what you’re measuring.
The Three Dimensions and Their Order
Every carbon brush is a small rectangular block with a lead wire or spring attached. The three measurements always follow the same convention:
- Thickness: The narrowest face of the brush, which slides between the walls of the brush holder. This is the dimension that determines how snugly the brush fits side to side.
- Width: The wider face of the brush, perpendicular to thickness. Together with thickness, these two measurements define the cross-section that contacts the commutator or slip ring.
- Length: The distance from the contact face (the end that touches the commutator) to the top of the brush. This is the dimension that shrinks as the brush wears down.
If your brush has a beveled edge on the contact face, measure the length along the longer side. Some brushes have a colored cushion pad on top (sometimes called a “Red Top”). If yours does, include that pad in your length measurement.
How to Take Accurate Measurements
Use a digital caliper for this. Calipers are faster and more precise than a tape measure or ruler, and carbon brushes are small enough that even a millimeter of error can cause problems. Open the outside jaws, place them around the dimension you’re measuring, and gently close until they make firm contact. Stop as soon as you feel resistance. Forcing the jaws can compress the carbon slightly and give you a false reading.
There are a few things to watch for. Make sure the caliper jaws sit parallel to the surface you’re measuring. If the brush has chipped edges, uneven wear, or a glazed contact face, position the jaws on a clean, unworn section. A brush with pitting or edge chipping will give you an undersized reading if you measure across the damaged area, and that reading won’t reflect the size you need for a replacement.
Write down all three numbers in the correct order: thickness first, then width, then length. Most manufacturers list sizes in millimeters, though some power tool brands use inches. Note which unit your caliper is set to before you start.
Why the Fit Tolerance Matters
A carbon brush needs a small gap between itself and the holder to slide freely as it wears. Too tight and it sticks in the holder, losing contact with the commutator. Too loose and it rocks, causing arcing and uneven wear. Industry standards from Mersen specify that for standard graphite brushes, the clearance between the brush and holder should fall between roughly 0.04 mm and 0.33 mm per side, depending on the brush’s nominal size. Metal-graphite brushes run slightly looser, with clearances ranging from about 0.07 mm to 0.57 mm.
You don’t need to calculate these tolerances yourself when ordering a replacement. But they explain why eyeballing the size or rounding your measurements isn’t good enough. A brush that’s 0.5 mm too wide can jam in the holder. One that’s 0.5 mm too narrow can rattle and arc.
Checking Spring Pressure
If you’re measuring brushes because you suspect a problem, the spring is worth checking too. The spring pushes the brush against the commutator, and incorrect pressure causes most brush-related failures.
Constant-force springs (the coiled ribbon type) should be measured at the position they’d sit with a new brush installed, not pulled to full extension. A common mistake is stretching the spring all the way out and reading the force there. Instead, extend it to where it would rest with a new brush, then let it retract until only about one coil diameter of spring remains extended. Take your force reading at that point.
Helical coil springs (the traditional wound type) change force depending on how compressed they are, so they need to be checked differently. Compress the spring to the position it would occupy with a new brush, then slowly release it to its free length. Record the force at the compressed position. This tells you the maximum pressure the brush will experience when new.
When to Replace Based on Length
The length measurement is the one that changes over time. As the brush wears against the commutator, it gets shorter. The general rule is to replace brushes before they wear down to the lead wire, rivet, or spring connector embedded in the carbon. Running a brush to the wire risks metal-on-metal contact with the commutator, which can score or permanently damage the contact surface.
Many manufacturers embed the wire or rivet at roughly one-quarter to one-third of the original brush length, giving you a visible reference point. Once you can see the wire or rivet approaching the contact face, it’s time. Some power tools have a wear line molded into the brush for exactly this purpose. If you’re measuring a used brush to determine remaining life, compare your length reading against the original specification. If you don’t have the original spec, check for a part number stamped on the brush or printed on the spring clip, and look it up from the tool manufacturer.
Signs That Affect Your Measurement
Before measuring, inspect the brush face for abnormal wear patterns. A healthy brush has a smooth, slightly polished contact surface that conforms to the curve of the commutator. Several conditions can distort your measurements or signal a deeper issue:
- Chipped edges: Chunks missing from the corners or sides. Measure away from the damage, but note that chipping often means excessive vibration or a poor holder fit.
- Glazed surface: A shiny, glassy film on the contact face. This doesn’t change dimensions but indicates the brush isn’t making good electrical contact.
- Uneven wear: One brush shorter than the other, or one side of a single brush worn more than the other. Measure both brushes in a pair separately. Uneven wear suggests misalignment or unequal spring pressure.
- Pitting: Small craters on the contact face, usually from electrical arcing. This can make the length measurement inconsistent depending on where you place the caliper.
If you see any of these patterns, replacing the brushes is the right call regardless of remaining length. And when you order replacements, use the original dimensions from the manufacturer’s spec sheet rather than measuring the worn brush, since the thickness and width of a damaged brush may no longer reflect the correct size.