How to Size a Tube Brush: Diameter, Core Wire and Working Length

A tube brush is not sized to the hole. It is sized to the hole plus a little. That small difference is the whole mechanism — the filament has to be deflected against the bore wall to cut, and a brush that matches the bore exactly just slides around inside it.

Most wrong orders come from the same place: the buyer sends a hole diameter and nothing else. Diameter alone cannot be quoted, because the same bore needs a different brush depending on how deep it goes, whether it is open at both ends, and how much room the core wire leaves for filament.

This guide covers the four numbers that actually define a tube brush — bore diameter, bore depth, core wire and working length — with the sizing rules our sales engineer, Frank, uses on hydraulic valve blocks, and the one case where a lantern-style brush beats a standard tube brush outright.

1. Two Numbers Decide Everything

Frank puts it plainly: when sizing a brush for a hole, the first number is the bore diameter and the second is the bore depth. Everything else is a consequence of those two.

Diameter decides how much the filament is deflected, and therefore how much pressure it puts on the wall. Depth decides how long the brush has to be, and length is what limits rigidity — a long, thin brush deflects under load and stops cutting where it should.

Two reference points are worth keeping in mind, because they describe the ends of the range rather than a threshold. A bore of 3 mm or below is a small hole: there is very little room for filament, and the core wire has to shrink with it. A bore deeper than 300 mm is a deep hole: the brush becomes a long, flexible tool, and the sizing question shifts from "how much filament" to "how much stiffness can I keep".

In hydraulic work the common range sits in the middle. On the valve blocks described later, 6 mm to 8 mm passages are the majority, with occasional 7.5 mm bores and a few large 25 mm and 32 mm channels.

2. Brush Diameter: Bore Plus 1–2 mm

The working rule is simple: make the brush 1–2 mm larger than the bore. That is the interference that bends the filament over and generates cutting pressure at the tip.

The direction of error matters. Too small and the brush polishes instead of deburring: the filament barely touches the burr root, the burr rolls over instead of coming off, and the edge looks clean while a folded flap of metal remains. Too large and a small bore becomes a different problem — the brush will not enter, or it enters under load, heats up, and either the filament breaks or the brush grabs and stalls. Frank has seen the second failure at the machine: the brush catches at the bore mouth, the tool jumps, and the part is scrapped.

Two practical corrections apply. First, bore tolerance is real: a nominal 8 mm hole that runs 8.1 mm at the top of its tolerance needs the upper end of the brush range, not the lower. Second, an interrupted bore — a cross hole, a keyway, a port breaking into the passage — gives the filament somewhere to expand into, so the same brush behaves differently in an interrupted section than in a continuous one.

3. Core Wire Sets the Lower Limit

A tube brush is a spiral of wire with filament trapped in it. That spiral is the core wire, and its diameter decides how much radial space is left for filament — which is why it, not the bore, sets the smallest brush you can build for a given hole.

The standard heavy construction is 4.0 mm single wire, four strands twisted together, which finishes at roughly 10 mm outside diameter. Put that core in a 25 mm bore and the arithmetic is easy: 25 mm minus 10 mm leaves 15 mm, or about 7–8 mm of filament on each side. That is a stiff, aggressive brush with a short, well-supported filament — right for cast iron and a heavy burr.

A 6 mm bore has no such room. The core drops to 2.0 mm, which finishes at about 1.8–2 mm, leaving roughly 2 mm of filament per side. At that scale filament diameter matters as much as brush diameter: a coarse filament will not fit into the remaining gap at all, so a small bore forces both a thin core and a fine filament.

Core size also sets stiffness, and stiffness is what makes long and angled holes possible. A thin core bends, so the brush follows a drilled passage that is not straight — an angled port, a spiral gallery, a bore that drifts. A thick core holds its line and cuts harder but will not negotiate a bend. This is why two brushes with the same outside diameter can behave completely differently in the same bore.

4. Working Length and the 50 mm Outside the Hole

Bore depth sets the length of the filament section — the working length. Total length is working length plus the shank that stays outside the part, and that second number is where most quotes get unnecessarily long.

The shank exists to be gripped and to keep the tool clear of the workpiece. Frank's rule is 30, 50 or 100 mm outside the bore, with 50 mm the usual choice: a typical chuck grips about 30 mm, and the remaining clearance keeps the tool from striking the part. Longer than that is not better. A long overhang whips at speed, the core wire bends, and the brush stops running true — and a brush that is not running true cannot hold a consistent interference against the bore wall.

Working length has a floor as well. Below roughly 30 mm of filament the brush wears out quickly, because the working section is consumed from the front and there is very little of it. The exception is a genuinely shallow bore: where depth is only 25 mm or 10 mm, a short filament section is correct and extra length would never enter the hole.

5. Through Holes and Blind Holes Are Sized Differently

This is the point that changes the quote most often, and it is easy to miss on a drawing.

In a through hole, the brush passes completely through: every millimetre of filament contacts the wall at some point in the stroke, so working length equal to bore depth is fully used.

In a blind hole, the brush enters from one end and stops. Only the front section ever touches the wall; the rear of the filament section stays in the mouth of the hole or outside it, doing nothing. Frank has quoted customers who asked for 300–400 mm of filament to clean a 300–400 mm deep blind bore — the rear two-thirds of that brush will never see the burr. It adds cost, adds overhang, and makes the brush more likely to deflect.

The correct figure for a blind hole is the reachable depth plus a small margin, not the full bore depth. Where the burr sits matters too: in a blind hole produced by drilling, the burr is usually at the bottom and at any cross port, not along the full wall.

6. Case: 6–8 mm Passages in Hydraulic Valve Blocks

The clearest example of these rules in production is a heavy equipment manufacturer that machines hydraulic valve bodies and valve blocks in cast and ductile iron. Their parts range from small components to several-tonne housings, and the internal passages are drilled in every direction: straight bores, angled ports, spiral galleries, and cross holes that intersect a main gallery.

The burr sits where the drilling operation leaves it — at the intersection of two passages, at the bottom of a blind bore, and along the break-out edge of a cross hole. None of it is reachable from outside, so the brush is the only tool that gets there.

Sizing follows the pattern above. Most passages are 6–8 mm, so the specification library is built around those two diameters, with 7.5 mm and the occasional 25 mm and 32 mm channel as separate items. Each bore size has its own brush; one brush is not stretched across several diameters. For the material, the choice is silicon carbide filament in almost every position — cast iron is hard and the burr is substantial, so the abrasive has to cut rather than polish. Diamond was trialled and set aside on cost: the passages are within what silicon carbide can move.

The result is a specification list rather than a single part number — a brush per bore diameter and depth, each one matched to the passage it cleans. That is what a silicon carbide spiral tube brush looks like when it is specified properly rather than bought off a size chart.

The customer is identified by industry only.

7. When a Lantern Brush Wins, and When It Loses

A lantern-style brush is a different geometry: the filament is expanded in a barrel around the tip, so it concentrates pressure at one spot instead of along a length of bore. That makes it very effective at a cross hole intersection — it cleans the breakout edge thoroughly, and it reaches the intersection from either passage.

The limit is time. A lantern brush works one location at a time. Where a part has one or two intersections, that is an advantage: precise, clean, and fast enough. Where a single gallery carries seven or eight cross holes, each intersection needs its own stroke, and the cycle time multiplies. A standard tube brush passes through once and contacts every intersection on the way, plus the rest of the wall — less precise at each individual edge, but far quicker overall.

The choice therefore follows the part, not the brush. Few intersections and a critical edge: a brass wire lantern tube brush. Many intersections and a cycle time to hit: a standard run of tube brushes. For the burr mechanics behind that trade-off, see our guide to cross hole deburring.

8. What to Send With a Sizing Request

Five lines of information are enough to quote a tube brush correctly, and they remove almost all of the back-and-forth:

• Bore diameter, with tolerance if the drawing has one.
• Bore depth, and whether the hole is through or blind.
• Passage shape: straight, angled, spiral, or intersecting with other holes.
• Workpiece material and where the burr sits — breakout edge, bore bottom, or full wall.
• How it will be driven: hand drill, drill press, CNC spindle or dedicated machine, with the speed you intend to run.

With those five, diameter, core wire, working length and shank length all follow from the rules above — and the brush that arrives will be the one the bore was asking for.

FAQ

Get the Brush That Fits the Bore

Longguang manufactures abrasive filament and wire tube brushes for internal deburring, cross hole cleaning and bore finishing, in silicon carbide, aluminum oxide, diamond and ceramic filament, with diameter, core wire, working length and shank matched to your drawing. Send the five sizing lines above and we will specify the brush for each passage.

Useful starting points: our tube brushes collection, the diamond spiral tube brush for bores beyond silicon carbide, and our contact page to send your drawing.

Note: Longguang is a manufacturer and exporter of industrial brushes. We do not provide on-site deburring services or local installation.

Henry Zhu
International Sales Manager

Phone / WhatsApp / WeChat: +86 13818514531

Email: info@shlgbrush.com

Email: zhulei@shlgbrush.com

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