Bosch 3-1/2 In. Starlock Oscillating Multi-Tool Diamond Grit Grout Blade

I reached for the Bosch diamond grout blade during a bathroom regrout and kept it out for a plaster repair a week later. In both cases, it did exactly what I wanted a segmented diamond blade to do: cut precisely, resist wear, and stay controllable in tight spots where an oscillating multi-tool shines.

Build and interface

This Starlock grout blade is a 3.5-inch segmented wheel with a diamond-grit edge and a pointed segment that actually matters in the real world. The tip helps start cuts cleanly and reach into inside corners without overcutting adjacent tile. The diamond coating is consistent across the rim, with a uniform grit that doesn’t shed prematurely when you lean on it a bit. It’s a shallow cutter by design—about a quarter-inch of immersion depth—so you’re less likely to blow past grout and into substrate.

The Starlock interface is the other headline feature. It locks onto my Starlock and Starlock Plus bodies with zero play, and torque transfer feels immediate—no jangly chatter, no slop. On an older OIS body with the proper adapter, it’s solid, though you’ll get a touch more vibration than on a true Starlock mount. Either way, the quick-change is tool-free and reliable, and the blade sits true.

Swiss-made blades typically have clean edges and tighter tolerances, and this one fits that pattern. The body is flat, the segment joint is even, and the diamond band is properly bonded. It feels like a premium accessory in the hand before you even power up.

Performance on grout

On cementitious grout—floor and wall—the blade chews at a predictable pace. With the oscillating tool at medium to high speed and light, steady pressure, I was able to clear a standard 1/8-inch joint to full depth in one or two passes. The 3.5-inch width helps you cover lineal footage quickly: long runs go faster than with small-diameter blades, but you still keep the control you expect from an oscillating tool. That pointed segment makes a difference at corners, where you can steer the tip into the joint and lift out cleanly, instead of oscillating past your mark.

Dust management is a big factor with grout. Diamond grit on cementitious material produces very fine silica dust. I paired the tool with a compact vac and a helper nozzle close to the cut, and also ran painter’s tape along the tile edges to prevent scuffing from accidental contact. With suction nearby, visibility stays usable and the blade doesn’t overheat. If you’re solo, a light water mist can keep dust down, but be cautious with electrical tools and GFCI-protect everything.

On epoxy grout (and old, hardened cement grout), the blade is slower but still effective. The diamonds don’t glaze quickly as long as you don’t push too hard. If you do notice a shiny “burnished” band and the cut rate drops, a few passes through a dressing block or even across a scrap of abrasive concrete will refresh the edge.

That 0.25-inch immersion depth is a feature, not a flaw, for wall work. It acts as a guard against accidental substrate damage. On thicker mud-set floors or deep joints, you may need to widen the kerf or make a second pass at a slight angle to reach the full depth. For most modern thinset installations, a quarter-inch gets you where you need to be without drama.

Vibration is well controlled on a Starlock head. Cut feel is more like a steady nibble than a harsh buzz, which leaves your hands fresher after an hour of joint work. Noise is typical oscillating-tool loud; ear protection recommended.

Precision work on plaster and soft tile

This blade isn’t just for grout. I used it to score and dissect a plaster wall to open a switch box, where depth control and a clean edge mattered. The shallow immersion depth, semi-circular footprint, and diamond edge make it excellent for this. It’s not fast—plaster rarely is—but the cut is precise, the kerf is tidy, and I didn’t chase into the lath or worry about nicking unknowns in the cavity. With a marked perimeter and a gentle touch, you can creep up to the line and stop exactly where you intend.

For soft ceramic tile (think wall tile and trim), the blade can undercut a shoe, nibble a notch, or widen a grout line without exploding the glaze. It’s not a tile-cutting wheel—don’t expect it to slice porcelain or stone cleanly—but on softer tile and glaze corrections it’s a controlled option that’s less prone to chipping than coarse carbide.

Durability and value

Diamond grit earns its keep in longevity. Compared with carbide-tooth grout blades, the diamond band outlasts by a comfortable margin, especially on tough or old grout. After several rooms’ worth of joints and a few linear feet of plaster scoring, my blade still cuts evenly, with only light wear on the leading third of the rim. The segment hasn’t shed grit in chunks, which is what separates good diamond blades from cheap ones.

A note on technique: pressure is not productivity here. Let the diamonds do the work. Too much force overheats the bond and shortens the blade’s life. Keep the speed moderate, maintain a steady feed, and dress the rim if it slows. Done right, the blade pays for itself over multiple projects.

Limitations

• Depth: The 0.25-inch immersion depth is intentionally conservative. It’s ideal for most wall work and many floors, but deep mortar beds may require multiple angles or a different accessory.
• Material scope: It’s built for grout and soft ceramic. It will scratch or nibble porcelain, but you’ll burn time and glaze the rim. For porcelain/stone cutting, use a dedicated diamond tile blade on a different platform, or a specialized oscillating accessory rated for hard tile.
• Width: The 3.5-inch segment is great for straight runs. In very tight spaces, you might wish for a smaller diameter blade to maneuver around obstructions.
• Compatibility: It shines on Starlock and works with OIS via adapter, but if your tool can’t accept Starlock-style blades, you’ll lose some of the interface’s stiffness and quick-change convenience.

Tips for best results

• Mark joints and tape tile edges to avoid scuffs. The blade’s body can touch glaze when you widen a kerf.
• Run medium to high oscillation speed; apply light pressure. Listen for a consistent “sizzle,” not a bogged hum.
• Use dust collection or local extraction. Silica dust is no joke; a HEPA vac and a proper respirator are smart.
• Dress the rim if cutting slows. A few strokes across a dressing stone or abrasive concrete renews the diamonds.
• Let the pointed segment start and finish each joint. It minimizes accidental overcuts at corners and fixtures.
• For plaster, score lightly on the first pass to set the line, then deepen gradually. The blade will track cleanly.

The bottom line

As a grout removal and precision scoring accessory, the Bosch diamond grout blade is a dependable, controlled performer. The Starlock interface keeps the cut smooth and the blade rigid, the diamond edge holds up across real projects, and the pointed segment earns its keep in corners and delicate starts. It’s not a universal cutter—and it’s not meant to be—but within its lane (grout, soft ceramic, and careful plaster work), it does the job with minimal fuss.

Recommendation: I recommend this blade to anyone with a Starlock-compatible oscillating tool who needs reliable grout removal and controlled, shallow cuts in plaster or soft tile. The combination of durability, cut quality, and interface stiffness makes it a better long-term value than carbide alternatives. If your work regularly involves porcelain, stone, or very deep joints, pair it with a more specialized accessory for those tasks. Otherwise, this becomes one of those blades you keep at the front of the kit because it quietly does what it’s supposed to do, project after project.