When buying a new Circular Saw Blade, it's easy to get caught up in the details of tooth count, coatings, and diameter. However, there is one critical specification that is often overlooked until it is too late: the bore size. This single measurement determines whether a blade will even fit on your saw, and getting it wrong is not just an inconvenience—it is a serious safety risk.
The bore is the heart of the blade-to-saw connection. A perfect fit ensures stability, accuracy, and safety. A poor fit can lead to dangerous vibrations, ruined cuts, and damage to both your blade and your power tool. In this complete guide, we will explain everything you need to know about bore sizes, arbor compatibility, and how to ensure you always buy the right blade for your machine.
What is Bore Size and Why Does It Matter?
The bore size is the diameter of the hole in the exact centre of a Circular Saw Blade. This hole is designed to fit precisely onto your saw's arbor (also known as the spindle)—the threaded shaft that the blade mounts onto.
Think of it like the wheel of a car. The wheel must fit the hub perfectly. If the central hole is too small, it won't go on. If it's too large, the wheel will be off-centre and wobble uncontrollably. The same principle applies to your saw. The bore and arbor must match perfectly to ensure the blade spins true, without any lateral movement or vibration. This concentricity is fundamental for a clean, accurate, and safe cut. An incorrect fit compromises the blade's stability, turning a precision cutting tool into an unpredictable and dangerous object.
Common UK Arbor & Bore Sizes
While there are many different saw manufacturers, the industry has thankfully standardised most arbor and bore sizes. In the UK market, you will almost always encounter one of these three primary measurements:
20mm Bore
This is the standard bore size for the vast majority of handheld circular saws and track saws (plunge saws) sold in the UK and Europe. If you have a cordless or corded circular saw from brands like Makita, DeWalt, Bosch, or Festool with a blade diameter between 120mm and 190mm, it almost certainly has a 20mm arbor.
30mm Bore
This is the go-to standard for larger, stationary machines. Most table saws and mitre saws (chop saws) use a 30mm arbor. This larger spindle provides the extra rigidity needed to support bigger, heavier blades (210mm to 305mm in diameter) that are common on these benchtop tools.
16mm Bore
This size is less common today but is often found on older or smaller DIY-grade circular saws. If you have an older power tool, it's always worth double-checking for a 16mm arbor before buying a new blade.
What about Imperial Sizes like 5/8"?
The 5/8-inch bore (approximately 15.88mm) is the standard in the United States. While most blades sold in the UK are metric, you may encounter 5/8" bores on American-made saws or blades. It is crucial not to mix these up with 16mm bores, as the slight difference is enough to cause an unsafe fit.
Using Reduction Rings Safely
What happens if you find the perfect Circular Saw Blade for your project, but it has a 30mm bore and your handheld saw has a 20mm arbor? This is where reduction rings (or 'reducer bushes') come in.
A reduction ring is a thin, precision-engineered metal washer that fits snugly into a larger bore to reduce its diameter. For example, a 30mm-to-20mm ring allows a table saw blade to be fitted onto a handheld saw.
However, they must be used correctly:
- Use High-Quality Rings: Flimsy or poorly made rings can deform under pressure, throwing the blade off-balance. Always use a solid, precision-machined reducer.
- Ensure a Snug Fit: The ring should snap into the bore with a firm push. If it is loose or rattles, do not use it.
- Check Blade Compatibility: Just because a blade can be adapted to fit doesn't mean it should be used. A heavy, standard-kerf table saw blade might put too much strain on the motor of a small cordless saw.
Reduction rings are a safe and effective solution when used with care, offering you greater flexibility in your choice of Circular Saw Blades.
The Risks of a Mismatched Bore Size
Attempting to use a blade with the wrong bore size is one of the most dangerous mistakes you can make with a power saw.
If the Bore is Too Small
This is straightforward—it simply will not fit onto the arbor. Never attempt to enlarge the bore yourself, as this will destroy the blade's balance and structural integrity.
If the Bore is Too Large (Without a Reducer)
This is where the real danger lies. A blade with an oversized bore will not be centred on the arbor. When you turn the saw on, the blade will spin eccentrically, causing:
- Violent Vibration: The saw will shake uncontrollably, making it impossible to guide accurately. This vibration can also damage the saw's motor bearings over time.
- Catastrophic Failure: In a worst-case scenario, the extreme off-balance forces can cause the blade to warp, fracture, or even shatter during operation, sending sharp fragments flying.
- Awful Cut Quality: Even if the blade doesn't fail, the wobble will result in a wide, rough, and burnt cut, ruining your workpiece.
There is no situation where using a blade with a bore that is too large is acceptable. Always use the correct size or a proper reduction ring.
How to Check Your Saw’s Arbor Before Buying
Before you add any Circular Saw Blade to your basket, you need to confirm your saw's arbor size. Here’s how:
- Read the Manual: The quickest and most reliable method is to consult your saw's user manual. The specifications section will list the required blade diameter and bore size.
- Inspect the Old Blade: Remove the existing blade from your saw. The bore size is almost always stamped or laser-etched onto the blade's body, along with the diameter and tooth count.
- Check the Saw's Data Plate: Look for a sticker or plate on the motor housing of your saw. This usually contains key information, including the model number and blade specifications.
- Measure It: If all else fails, you can measure the saw's arbor directly with a set of digital calipers. This will give you a precise reading.
Product Recommendations Based on Bore Type
At Key Blades & Fixings, we stock a wide range of professional blades to fit all common UK saws.
For 20mm Bore Saws (Handheld & Track Saws)
Our selection of blades in the 120mm to 190mm diameter range is designed for professionals using handheld tools.
- Recommendation: Our 160mm "Cougar" Track Saw Blades come with a 20mm bore as standard. They are perfect for delivering precise, clean cuts in sheet materials like plywood and MDF.
For 30mm Bore Saws (Table & Mitre Saws)
Our larger blades, typically 210mm and above, are built for the power and stability of benchtop machines.
- Recommendation: Explore our fine-finish 254mm Blades with a 30mm bore. With high tooth counts and advanced tooth geometry, they are ideal for cabinetry and joinery work where accuracy is paramount.
Getting the bore size right is the first and most important step to a successful cut. By taking a moment to confirm your saw’s arbor size, you ensure a safe connection, protect your tools from damage, and guarantee the blade performs exactly as the manufacturer intended.