Quick Answer
In North America a 4" angle grinder has a 3/8"-24 UNF spindle, and every grinder from 4-1/2" through 9" has a 5/8"-11 UNC spindle; in Europe and most metric markets the equivalent is M14 × 2 (KB: Spindle Thread and Arbor Standards). The open-arbor wheels they carry use a different number: a 7/8" plain bore, exactly 22.23 mm, not 22 mm (oSa marking rules under EN 12413; KB: Spindle Thread and Arbor Standards). And 5/8-11 and M14 are never interchangeable.
Spindle thread and wheel bore are two different numbers
Most fitment confusion comes from mixing up two interfaces. A disc attaches one of two ways: it either screws on (a threaded hub with a 5/8-11 or M14 internal thread) or it drops over the spindle on a plain centre bore and is clamped between flanges by the lock nut (KB: Arbor and Hub Mounting).
That is why a "5/8 grinder" does not take a 5/8" bore wheel. The 5/8-11 figure is the spindle thread. The matching open-arbor wheel needs a 7/8" bore so it clears the threads and seats on the inner flange's raised pilot, a shoulder about 0.040" tall (KB: Arbor Hole). The bore only centres the wheel; the flanges clamp it and transmit the torque through friction on the wheel faces (KB: Arbor Hole).
Thread chart by grinder size
Thread choice tracks the grinder's wheel diameter, not the brand: the 4" class uses the fine 3/8"-24 die-grinder thread, and everything from 4-1/2" up shares one standard, which is why a 4" and a 4-1/2" grinder take incompatible accessories (KB: Spindle Thread and Arbor Standards).
| Grinder size | North American thread | Metric-market thread | Typical no-load RPM |
|---|---|---|---|
| 4" (100 mm) | 3/8"-24 UNF | M10 × 1.25 | ~11,000 |
| 4-1/2" (115 mm) | 5/8"-11 UNC | M14 × 2 | 10,000–12,000 |
| 5" (125 mm) | 5/8"-11 UNC | M14 × 2 | up to ~10,000 |
| 6" (150 mm) | 5/8"-11 UNC | M14 × 2 | 9,000–10,000 |
| 7" (180 mm) | 5/8"-11 UNC | M14 × 2 | 6,500–8,500 |
| 9" (230 mm) | 5/8"-11 UNC | M14 × 2 | ~6,000–6,600 |
(RPM figures: KB: Angle Grinder.) The 3/8"-24 thread also runs most pneumatic die-style grinders; M10 × 1.25 shows up on some smaller imports (KB: Spindle Thread and Arbor Standards). One caution: a threaded-hub disc threads onto both a 4-1/2" and a 9" grinder, but the small tool spins nearly twice as fast; the disc's marked max RPM must still clear the tool's no-load speed. See grinding wheel max RPM and ANSI B7.1.
Bore size chart
Wheel dimensions are marked D × T × H (diameter × thickness × hole) under ISO 525 and EN 12413 (KB: Arbor and Hub Mounting). H is the bore:
| Bore (inch) | Bore (mm) | Where you meet it |
|---|---|---|
| 5/8" | 15.88 mm | Threaded-hub discs (the 5/8-11 internal thread that screws onto the spindle) |
| 7/8" | 22.23 mm | The open-arbor standard on both sides of the Atlantic: Type 27/41 wheels, flap discs, fibre-disc backing pads; the EU hand-grinder standard for 115–230 mm discs |
| — | 20 mm | 12"–14" chop-saw and masonry cut-off wheels |
| 1" | 25.4 mm | Bench-grinder wheels and large cut-off blades |
| 1" / 1.26" | 25.4 / 32 mm | 300–400 mm large-diameter bonded wheels |
(KB: Spindle Thread and Arbor Standards.) Watch the 5/8" trap: some imported cut-off wheels carry a 5/8" plain bore meant for chop-saw arbors. Those will not seat on a 5/8-11 angle-grinder spindle; an angle-grinder wheel needs the 7/8" bore (KB: Arbor Hole). Unsure which wheel format you need? Start with how to choose a cut-off wheel.
Why 7/8" means exactly 22.23 mm
The bore is a controlled dimension, not a nominal one. oSa, the independent European abrasive-safety organisation, spells it out in its EN 12413 marking requirements: 7/8" is 22.23 mm, not 22.2 mm and not 22 mm (oSa marking requirements; KB: Spindle Thread and Arbor Standards). Under EN 12413:2019 the permitted limit deviations and run-out for that bore are governed by ISO 13942 (EN 12413:2019; KB: Spindle Thread and Arbor Standards). The upshot: the hole is a deliberate slight-clearance fit on the flange seat, never an interference press.
5/8-11 vs M14: close, but never compatible
M14 is 14.0 mm in diameter, about 0.551". A 5/8" thread is 15.875 mm. The pitches differ too: 11 threads per inch versus a 2 mm metric pitch (KB: Spindle Thread and Arbor Standards). Close enough to start by force, wrong enough to ruin the thread when you do, and a cross-threaded wheel can be left insecurely seated: a burst hazard, not just a fitment annoyance.
To bridge the regional split, use a proper machined thread adapter, a short steel collar (for example M14 × 2 internal, 5/8"-11 external). Every added joint adds stack height, run-out and a wear point, so treat adapters as a workaround, not a substitute for the correct mount (KB: Spindle Thread and Arbor Standards; Arbor Hole). And an adapter converts spindle threads only; it cannot change a plain bore size (KB: Arbor Hole).
Bore clearance: the hole is oversize on purpose
ANSI B7.1, the North American abrasive-wheel safety code, requires that wheels "fit freely on the spindle" with proper clearance under all grinding conditions (ANSI B7.1 §6.3; KB: Spindle Thread and Arbor Standards). The spindle is held to nominal +0/−0.002", while the wheel hole is deliberately oversize, at or above the minimums in the standard's Table 22; for portable and off-hand wheels that runs from about +0.001" to +0.006" depending on diameter and duty (ANSI B7.1 Table 22; KB: Spindle Thread and Arbor Standards). The clearance exists so heat-driven spindle expansion and clamping pressure cannot crack the wheel; a 7/8" bore disc is intentionally a few thousandths over 22.23 mm. Never force a wheel onto a spindle, and never run an oversize bore loose: an eccentric wheel vibrates and risks burst (KB: Spindle Thread and Arbor Standards).
Reducing bushings: the rules
A wheel with too big a hole may only be adapted down with a proper reducing bushing, and ANSI B7.1 §6.5 is strict about what that means (KB: Spindle Thread and Arbor Standards):
- A purpose-made bushing only. Flat shim stock is never an acceptable substitute.
- It must fit freely on the spindle, just as the wheel must.
- It must not exceed the width of the wheel and must never contact the flanges: the flanges drive the wheel, the bushing only centres it.
- Loose bushings (not mechanically held) must not be used on wheels under 1/8" thick, which rules them out for thin cut-off wheels.
- On portable and hand-held tools, separate bushings are out unless supplied or recommended by the wheel maker.
- Never use a bushing to mount a larger wheel than the machine was designed for.
Threaded-hub wheels: the extra rules
Threaded-hub versions of flap discs, depressed-centre wheels and cup wheels skip the loose flange nut, but ANSI B7.1 §6.10 adds conditions: the internal thread is a Class 2B fit, the thread must tighten in the wheel's running direction, the spindle must not bottom in a blind hole, no spacers between wheel and back flange, and threaded-hole mounting should not be used above 6" diameter (ANSI B7.1 §6.10; KB: Spindle Thread and Arbor Standards). For open-arbor wheels, both flanges must be flat, clean, matched in diameter and not less than one-third the wheel diameter, with a compressible blotter between each flange and the wheel face (wheels with moulded plastic or rubber hubs, like threaded-hub flap discs, are the exception) (OSHA 1910.215; ANSI B7.1; KB: Arbor and Hub Mounting).
Mounts that skip the 5/8-11 thread entirely
Flap wheels and mounted points ride a 1/4" to 1/2" round shank in a die-grinder collet (KB: Arbor and Hub Mounting). Small 2" and 3" surface-conditioning and sanding discs use the Type R / TR quick-change twist-on system on a 1/4-20 holder; that family is covered in fibre disc backing pads and Type R quick-change. Bench and pedestal grinder wheels run a plain unthreaded bore, commonly 1/2", 5/8", 3/4" or 1", clamped between two matched flanges (KB: Arbor and Hub Mounting). And one proprietary tool-free system clicks discs onto an X-shaped 22.23 mm interface with no nut or spanner; those discs keep the round 22.23 mm bore, so they also fit a threaded grinder, but a standard wheel will not click into the tool-free tool, which has no matching recess (KB: Spindle Thread and Arbor Standards; Arbor Hole).
Five checks before you buy
- Grinder size → thread: 4" means 3/8"-24; 4-1/2" through 9" means 5/8"-11 (or M14 on a metric-market tool).
- Mount family: open-arbor (7/8" bore plus flanges), threaded hub (5/8-11 or M14), or tool-free click-in. Not substitutes for each other.
- Bore in mm: for an angle-grinder wheel it should read 22.23 mm, exactly.
- Marked max RPM at or above your grinder's no-load speed, always.
- Region check on threaded-hub product: a 5/8-11 hub will not start on an M14 spindle without an adapter.
Every format above is stocked in Whitby: grinding wheels and cut-off wheels in the standard 7/8" bore, and flap discs in both open-arbor and 5/8-11 threaded-hub versions, with the bore or thread stated on every listing.
FAQ
What size thread does a 4-1/2 inch angle grinder have?
5/8"-11 UNC in North America, M14 × 2 in metric markets. The same thread runs across 4-1/2", 5", 6", 7" and 9" grinders (KB: Spindle Thread and Arbor Standards).
Are 5/8-11 and M14 threads interchangeable?
No. M14 is 14.0 mm in diameter with a 2 mm pitch; 5/8" is 15.875 mm with 11 threads per inch. Forcing one onto the other cross-threads the spindle and can leave the wheel insecurely seated; bridge them only with a proper thread adapter (KB: Spindle Thread and Arbor Standards).
Why does my 5/8-11 grinder need a 7/8 inch bore wheel?
Because 5/8-11 is the spindle thread, not the wheel hole. An open-arbor wheel needs the 7/8" bore to clear the threads and seat on the inner flange's raised pilot; the flanges then clamp and drive the wheel (KB: Arbor Hole).
Is a 22 mm bore the same as 7/8 inch?
No. The exact conversion is 22.23 mm, and oSa's EN 12413 marking rules state it that precisely: not 22.2 mm, not 22 mm. Its permitted deviations are set by ISO 13942 (oSa; EN 12413:2019; KB: Spindle Thread and Arbor Standards).
Can I use a reducing bushing on an angle grinder wheel?
Only if the bushing is supplied or recommended by the wheel maker; ANSI B7.1 §6.5 bars separate bushings on portable and hand-held tools otherwise. Any bushing must fit freely, stay narrower than the wheel, never touch the flanges, and never be shim stock (ANSI B7.1; KB: Spindle Thread and Arbor Standards).
What thread do 4 inch angle grinders use?
3/8"-24 UNF in North America, M10 × 1.25 in metric markets. It is the same fine thread as most pneumatic die-style grinders, and it is why 4" and 4-1/2" accessories do not interchange (KB: Spindle Thread and Arbor Standards).