The bore is the hole in the centre of a circular saw blade where it mounts on the arbor (the spinning shaft). It must match your saw’s arbor size exactly â too large and the blade wobbles dangerously, too small and it won’t fit at all. Common bore sizes are 16mm, 20mm, 25mm, and 30mm. This guide shows you which bore size you need and what to do if your blade and saw don’t match.
What Is Bore Size?
Imagine the circular blade as a wheel on a car. The bore is the hole in the centre where the axle (arbor) passes through. The hole must be sized precisely:
- Too small: blade won’t slide onto the arbor
- Too large: blade rattles and spins dangerously
- Just right: blade is secure and safe
Your saw’s manual will specify the arbor size. For example, “This saw accepts blades with a 20mm bore.” Match that exactly.
Common Bore Sizes & Which Saws Use Them
| Bore Size | Common Saws | Availability in UK |
|---|---|---|
| 16mm | Some older DeWalt, Ryobi, budget brands | Moderate â not all retailers stock it |
| 20mm | Makita DHS680Z, DeWalt DCS391, Bosch GKS 18V-57, many others | Excellent â most common metric bore |
| 25mm | Rare â some table saws and bench models | Poor â harder to find |
| 30mm | Makita DHS710Z, DeWalt larger models, Milwaukee BLCS66, table saws | Good â available from major suppliers |
| 15.87mm (5/8″) | Milwaukee M18 CCS55 only | Limited â imperial spec, not standard metric |
20mm is by far the most common. If you own a typical cordless handheld circular saw (Makita, DeWalt, Bosch), it almost certainly takes a 20mm bore blade.
Brand-Specific Bore Sizes (Quick Reference)
Makita: DHS680Z = 20mm, DHS710Z = 30mm
DeWalt: DCS391 = 20mm, DCS570 = 16mm, larger models = 30mm
Milwaukee: M18 CCS55 = 15.87mm (non-standard!), M18 BLCS66 = 30mm
Bosch: GKS 18V-57 = 20mm, GKS 18V-68 GC = 30mm
Ryobi: RWSL1801M = varies (check manual)
For more detail on each brand, see the dedicated brand-specific pages on this site.
What If Your Blade and Saw Don’t Match?
If the bore is too large, you have one safe option: use a bore reduction ring.
Using a Bore Reduction Ring
A reduction ring (also called a bush or arbor adapter) steps a larger bore down to fit a smaller arbor. For example:
- Your saw arbor: 20mm
- Your blade bore: 30mm
- Solution: 30mm-to-20mm reduction ring
The ring fits over the arbor, then the blade slides over it. It’s secure and safe if properly sized.
Buy bore reduction rings on Amazon UK
Common reductions available:
- 30mm to 20mm
- 30mm to 16mm
- 25mm to 20mm
- 20mm to 16mm
- 20mm to 15.87mm (for Milwaukee M18 CCS55)
Important Safety Note
Always ensure the reduction ring is properly seated and secure. If there’s any play (the blade can wobble), don’t use it. A loose blade is a safety hazard. Check that the ring is made for your exact bore sizes â not all rings are universal.
What If the Blade Bore Is Too Small?
There’s no safe adapter that makes a smaller bore larger. If a blade won’t fit over the arbor, you need a different blade. Don’t force it or try to modify it.
The Milwaukee 15.87mm Challenge
Milwaukee’s M18 CCS55 uses a 15.87mm bore (which is imperial 5/8″). Most UK market blades come in metric sizes: 16mm or 20mm. Neither is the same as 15.87mm.
Your options:
- Buy Milwaukee-branded blades â they fit directly (but cost more)
- Use a 20mm blade with a 20-to-15.87mm reduction ring â opens up cheaper options
- Contact Milwaukee support to confirm the exact bore size
How to Measure Your Saw’s Arbor Size
If you can’t find your saw’s manual:
- Use digital callipers to measure the arbor shaft diameter
- Check the spec label on the saw body
- Contact the manufacturer with your model number
Frequently Asked Questions
What’s the most common circular saw blade bore size?
20mm. If you own a typical cordless handheld circular saw, it almost certainly uses 20mm bore.
Can I use a 30mm bore blade on a 20mm saw with a reduction ring?
Yes. A 30mm-to-20mm reduction ring is common and reliable. The blade will fit safely.
What if the reduction ring is loose?
Don’t use it. A loose blade is a safety hazard. Replace the ring with a better-fitting one or buy a blade with the correct bore.
Why does Milwaukee use 15.87mm when everyone else uses 20mm?
Milwaukee is a US-owned brand and uses imperial (inch) specs. 15.87mm is exactly 5/8 inch. It’s a legacy design choice that makes cross-brand compatibility awkward for UK users.
Is it safe to force a blade onto an arbor that’s slightly too small?
No. Never force a blade. If it won’t fit, buy a blade with the correct bore.