When shopping at Screwfix or Toolstation, you will notice that small angle grinders typically come in two standard sizes: 115mm (4.5-inch) and 125mm (5-inch). If you have recently upgraded from a 115mm model to a more powerful 125mm machine, you might find yourself with a drawer full of leftover 115mm slitting and grinding discs.
The short answer is yes, you can absolutely use 115mm discs on a 125mm angle grinder.
Because both sizes share the identical European standard 22.23mm (7/8 inch) centre bore, a 115mm disc will slot perfectly onto a 125mm grinder’s backing flange. However, while it is physically possible and entirely safe if done correctly, there are a few practical trade-offs and safety golden rules you need to know.
Why Putting a Smaller Disc on a Larger Grinder is Safe
In the power tool world, putting a smaller accessory on a machine rated for larger accessories is usually fine.
- The RPM Safety Margin: A standard 125mm angle grinder usually spins at a maximum speed of around 11,000 to 12,000 RPM. Meanwhile, smaller 115mm discs are structurally engineered to handle higher rotational speeds—typically up to 13,300 RPM. Because the 125mm grinder rotates slower than the maximum speed limit printed on the 115mm disc, there is zero risk of over-speeding the disc and causing it to shatter.
- The Bore Size Matches: Both 115mm and 125mm discs feature the exact same 22.23mm centre mounting hole. The disc will sit perfectly flush against the inner flange without any dangerous wobble or imbalance.
The Golden Rule: Never Do the Reverse!
While putting a smaller disc on a larger grinder is perfectly safe, never, under any circumstances, try to fit a larger 125mm disc onto a smaller 115mm grinder.
To make a 125mm disc fit on a 115mm machine, you would have to remove the protective safety guard.
Doing this is incredibly dangerous. A 115mm grinder spins too fast for a 125mm disc, which can cause the disc to explode under centrifugal force. Without a guard in place, the resulting flying shards will cause catastrophic injury.
The Trade-offs of Using a 115mm Disc on a 125mm Grinder
While it is safe to downsize your discs, you should be aware of a few structural limitations before starting your project:
- Reduced Depth of Cut: A 125mm disc gives you roughly 5mm to 10mm more cutting depth than a 115mm disc. When you drop down to a 115mm wheel, the bulky metal gear housing of your 125mm grinder will hit your workpiece much sooner, limiting how deep you can slice into metal or stone.
- Faster Wear Rate: Because a 115mm disc has less surface area and fewer abrasive particles along its circumference, it will wear down to the nub significantly faster than a proper 125mm disc would under the same workload.
Comparison at a Glance
| Factor | 115mm Disc on 125mm Grinder | 125mm Disc on 115mm Grinder |
| Bore Hole Alignment | Perfect Match (22.23mm) | Perfect Match (22.23mm) |
| RPM Speed Compatibility | Safe (Grinder is slower than disc limit) | ❌ Dangerous (Grinder spins too fast for disc) |
| Safety Guard Clearance | Perfect (Ample room inside the guard) | ❌ Will Not Fit (Requires removing the guard) |
| Verdict | 100% Permissible | 🚫 Strictly Forbidden |
How to Set Up Your Grinder Safely
1.Isolate the Power Source:
Unplug your corded grinder from the mains or pull the 18V lithium-ion battery pack completely out of the tool casing.
2.Keep the 125mm Guard Attached:
Leave the original 125mm safety guard fixed firmly to the grinder housing. Do not adjust it closer or remove it; the extra clearance is perfectly fine and provides excellent spark protection.
3.Mount the Disc Concentrically:
Drop the 115mm disc onto the inner backing flange. Double-check that the raised metal ring on the flange slots directly inside the disc’s 22.23mm centre hole so it sits completely flat.
4.Tighten the Locking Nut:
Press the spindle lock button on top of the gearbox and thread the outer locking nut on. If using a thin slitting disc, ensure the raised collar of the nut faces outward. Tighten firmly using your official pin spanner wrench.