Bosch 8-in Spline Core Bit Extension

Why I reached for this extension

I keep a few core bit heads in the kit for anchors, boxes, and service penetrations where a full-depth core rig is overkill. To run those two-piece, shankless heads on a spline-drive rotary hammer, I’ve been using Bosch’s 8-inch spline extension. It’s a straightforward part, but it’s the kind of link that either makes the day go smoothly or turns it into a fight with chatter, stuck threads, or sloppy holes. After months of use across concrete, block, and a fair amount of limestone, I have a good feel for where this extension succeeds and where you should be mindful.

What it is and what it isn’t

This is a one-piece steel extension with a spline shank on the tool side and a threaded end for Bosch-style, two-piece shankless core bit heads. By design, it turns your head into a usable two-piece core bit for large, shallow holes. It’s not a cutting tool by itself. If you don’t already own compatible core heads, this extension won’t do anything for you.

The published length is 8 inches, which refers to its working extension. End to end, the body measures roughly a foot, including the spline shank and shoulder, and it weighs about 1.16 lb. The balance feels right on a mid-class spline rotary hammer—enough mass to carry the hammering energy without feeling top-heavy.

Bosch also makes this same pattern in other lengths (there’s an 18-inch option) and in an SDS‑Max version. If your hammer isn’t spline, stop here and get the SDS‑Max variant. This spline model will not fit SDS‑Plus or SDS‑Max tools.

Setup and compatibility

Threading the extension to Bosch’s shankless heads is quick and positive. The threads are cut cleanly, and the one-piece body has flats that let you snug things properly. My process:

• Clean the head’s threads and the extension threads.
• Add a small amount of high-temp anti-seize.
• Hand-tighten, then snug with a wrench—firm, not gorilla-tight.
• Lock the spline shank into the hammer.
• Spin it up and watch for runout before touching the surface.

On compatible heads, concentricity has been consistently good. If you see a wobble, it’s almost always debris on the threads or a bruised head, not the extension itself. A quick clean usually solves it.

Note on fit: this is built for spline drive systems. Spline hammers are less common than SDS‑Max these days, but they’re still around in industrial fleets. If you’re bouncing between job sites and loaner tools, it’s safer to standardize on SDS‑Max and buy the SDS‑Max extension.

In use: concrete, masonry, and limestone

With a 3- to 5-inch head, the extension transmits hammer energy well. The one-piece design makes a difference; there’s no mid-body joint to loosen or flex, and the hammer’s blows feel crisp. In concrete, I get predictable bite and steady progress. In limestone, which can be grabby, the extension’s stiffness keeps the head from “walking” under percussion.

On hollow CMU, I shift to a lighter touch and let the bit score before committing. The extension doesn’t add meaningful whip, so the entry hole stays cleaner than with some two-piece systems that have looser thread tolerances.

Most of the shallow work I do—anchor holes for rail bases, conduit knockouts, and tap-ins on retrofit—falls within 3 inches of depth. That’s where this 8-inch length is ideal: you get enough standoff for clearance and dust evacuation without making the assembly unwieldy. If you routinely core through thicker walls or need to reach past insulation, the 18-inch model offers more flexibility.

Accuracy and vibration

Runout is the make-or-break metric for extensions. With this one, runout at the head rim has been minimal across several heads, provided the threads are clean and I take the time to seat things square. That translates to:

• Rounder holes and less chipping at the perimeter.
• Reduced load on the hammer because the cutting edge isn’t rattling.
• Fewer stuck heads in cured, hard aggregate.

Vibration levels are very manageable. You’ll still feel the percussion—this isn’t a magic wand—but compared to a worn or two-piece jointed extension, the one-piece body clearly reduces shake. That helps with precision and fatigue over a long day.

Durability and maintenance

The steel body has held up well. The spline shank shows only minor polish from the chuck, no rounding or burrs. The thread faces on the head side are still crisp, and the shoulder hasn’t mushroomed despite plenty of wrench tightening. I’d expect long service life if you keep the threads clean and avoid cross-starting the head.

A few maintenance tips I’ve adopted:

• A dab of anti-seize on the threads every session prevents galling, especially on hot, dusty runs.
• Don’t over-torque; firm and square beats brute force.
• If you feel grit while threading, stop and clean—forcing it is how you bruise the lead thread.
• Wipe the spline shank before every insert; concrete dust acts like lapping compound in the chuck.

The finish is just raw steel—no coating to speak of—which is fine. I oil it lightly before tossing it back in the case to stave off surface rust.

Limitations

• Spline-only. If you don’t own a spline hammer, this is the wrong part. The SDS‑Max variant is the better long-term investment for most modern crews.
• Thread dependency. The extension is only as true as the head you attach. A dinged head will still wobble.
• Heat and binding. Any two-piece threaded system can seize after prolonged dry coring in hard concrete. Anti-seize and patience solve it, but it’s a reality to plan for.
• Depth. The 8-inch length is optimized for shallow to moderate reach. For deep penetrations or thick walls, consider the 18-inch version or a dedicated core rig.

Who it’s for

• Trades still running spline-drive rotary hammers who need to core large, shallow holes.
• Facility maintenance teams working on anchors, boxes, and service penetrations in concrete, masonry, or limestone.
• Installers who prefer the flexibility of two-piece, shankless heads and want a stout, low-runout extension to drive them.

If you’re a general contractor outfitting from scratch, I’d gently steer you toward SDS‑Max for broader compatibility and easier tool sharing across subs and future projects.

Alternatives and length options

Bosch offers an SDS‑Max shank version of this same extension, and it mirrors the strengths here: one-piece construction, clean threads, and solid energy transfer. If you’re moving off spline or mixing tool platforms, that’s the logical path.

On length, the 8-inch extension is the sweet spot for day-to-day shallow coring. The 18-inch version makes sense if you regularly need extra reach—past insulation, through block and furring, or when clearance around the hammer is tight. Just remember that longer assemblies magnify any runout present in the head.

Value and practicality

There’s not much glamour in a core bit extension, and that’s the point. This one disappears into the workflow: it threads on cleanly, stays tight, runs true, and shrugs off jobsite abuse. Over time, those small wins save you minutes on every hole and prevent the kind of binding or wobble that can burn hours. In a pro kit, that’s real value.

The pack quantity is one—exactly what you’d expect—and the 1.16 lb weight rides comfortably in a drill case without being precious. I appreciate that I can match it across different head diameters without worrying about a proprietary interface.

Recommendation

I recommend the Bosch 8-inch spline extension to anyone working with a spline-drive rotary hammer who uses two-piece, shankless core bit heads for large, shallow holes. The one-piece steel design keeps runout low, transfers impact energy efficiently, and holds up under daily use. It’s easy to set up, easy to keep tight, and easy to trust on concrete, masonry, and limestone. The main caveat is platform: make sure you actually need spline. If your fleet is SDS‑Max, get the SDS‑Max version instead. Within its intended ecosystem, this extension does exactly what it should, and that reliability is what I want from a part that sits between my hammer and a spinning, expensive core head.