Air-Leg Rock Drills & the Small-Diameter Drilling Tool Chain

In February a contractor in Kitwe sent me a photo of two pallets sitting in his yard. Six YT28 air leg rock drills. Two hundred tapered bits. Four hundred metres of Hex 22 rod. He had bought the whole package from a trader two provinces away, and now almost nothing fitted together. His bits were 11° taper. His rods were 7°. Nobody had asked him which taper he was running.

He did not need a better price. He needed a matching list.

That is the whole problem with small-diameter drilling. The individual parts are cheap. The mismatch is what costs you a month. And almost nobody sells the chain as a chain — the drill, the rod, the bit, the sleeve and the pipe all come from different suppliers who never speak to each other.

This page is that chain, written down. It covers the tools we actually ship for small and medium holes: air-leg rock drills, tapered and threaded bits, retrac bits, coupling sleeves and DTH drill pipe. If you already know your hole diameter and your rock type, jump straight to the matching chart. If not, read in order.

What “Small Diameter” Means on a Real Site

Most buyers arrive with a diameter in mind and no idea that the diameter alone does not choose the tool. Three different tool families cover what people loosely call “small holes”, and they overlap in size but nothing else.

Hole band	Typical method	Rod / steel	Bit type	Power source
30–45 mm	Handheld or air-leg percussion	Hex 22 / Hex 25 integral or taper rod	Tapered button or cross bit, 6–12° taper	Air leg + portable compressor
38–64 mm	Top hammer on a small carriage	Extension rod, R25 / R28 / R32	Threaded button bit, straight or retrac gauge	Drifter + compressor
64–127 mm	Top hammer, heavier drifter	Extension rod, T38 / T45 / T51 / GT60	Threaded button bit, retrac for bad ground	Drifter or rig-mounted hammer
76–200 mm	Down-the-hole (DTH)	DTH drill pipe, 3 m / 6 m	DTH button bit	Rig + high-pressure compressor

Notice what changes between the second and fourth row. Not just the size — the entire energy path. In rows one to three the striking energy travels through the steel: the piston hits the shank, the shank pushes the rod, the rod pushes the bit. In row four the hammer itself goes down the hole, which is why DTH pipe and top-hammer rod are not interchangeable no matter how similar their diameters look.

The three diameters people confuse

• Hole diameter — what the survey or the client specified. The only number that matters on the drawing.
• Bit diameter — the cutting head. In top-hammer work this is usually 1–2 mm larger than the nominal hole, because gauge wear eats the difference.
• Rod or pipe OD — determined by the thread system, not by the hole. A 76 mm DTH pipe can drill a 115 mm hole; a 32 mm R32 rod can drill a 64 mm hole.

When a customer tells me “I need a 45 mm bit” and nothing else, the next two questions decide the order: which taper or thread, and which shank. Everything else follows.

Air-Leg Rock Drills: YT27, YT28, YT29A and the S250

Air-leg drills are the cheapest way to put a hole in rock. They are also where most of our first-time buyers start, and where the most orders go wrong — usually on air supply, not on the drill itself.

Naming first, because it trips people up. The same machine is sold as an air leg rock drill, an air-leg drill, a pusher-leg drill, or — in Zambia, Zimbabwe and much of southern Africa — a jackleg drill. That is one tool with four labels. If two quotations use different names, you are not necessarily looking at two different machines.

Here is the honest comparison of the four models we stock. All of them are pneumatic percussion drills with a pusher leg for feed force.

Model	Cylinder bore × stroke	Working pressure	Air consumption	Blow frequency	Shank	Weight (drill only)
YT27	70 × 60 mm	0.4–0.6 MPa	≈ 40 L/s	≈ 1,800 bpm	Hex 22	≈ 24 kg
YT28	72 × 65 mm	0.5–0.63 MPa	≈ 45 L/s	≈ 1,900 bpm	Hex 22	≈ 25 kg
YT29A	75 × 70 mm	0.4–0.63 MPa	≈ 47 L/s	≈ 2,000 bpm	Hex 22 / Hex 25	≈ 26 kg
S250 pusher-leg	—	0.4–0.63 MPa	—	—	Hex 22	≈ 26 kg

The full specification sheets, spare-part drawings and current pricing sit on the individual product pages: YT27 Air-Leg Rock Drill, YT28 Air-Leg Rock Drill, YT29A Air-Leg Rock Drill and the S250 Pusher Leg Rock Drill.

Reading air consumption before you buy

Air consumption is quoted at a working pressure, and it is the number that decides whether your compressor can run the drill. A YT28 at 45 L/s is about 2.7 m³/min. Run two of them off one machine and you are asking for 5.4 m³/min of free air at 6.3 bar — before you account for hose losses, a leaky manifold and the fact that nobody ever lubricates the line properly.

The practical rule we give customers: size the compressor for the number of drills running simultaneously, add 20% for line losses, and check the compressor’s rating is quoted as free air delivery (FAD) rather than displacement. Piston displacement numbers always look better and mean less.

For a single air leg working shallow holes up to about 4 m, a towable machine in the 3–4 m³/min class is comfortable. If you are running a small fleet of legs plus an occasional DTH hammer, you are better off stepping up to a proper screw machine — the C650 Air Compressor and the bigger C850 Air Compressor both sit in that bracket. Piston compressors are cheaper up front but they do not like continuous duty; if the drill runs all day, the compressor should be rotary screw.

What the air leg actually does

Beginners often ask why they need the leg at all. The drill weighs 25 kg and its own percussion will not pull it into the rock. The pneumatic leg supplies feed force — the operator adjusts its pressure so the bit stays pressed against the face without being hammered into it. Feed too light and you grind the buttons. Feed too heavy and you stall the rotation. It is a two-second judgement, made a hundred times a shift.

That is why air-leg work gets handed to the driller who has done it longest, not the one with the newest machine.

Tapered Bits vs Threaded Bits: The Decision That Actually Matters

Once you accept that you are drilling small holes with percussion, there is one real fork in the road, and it is not the diameter.

	Tapered system	Threaded system
Connection	Friction fit; bit is pressed onto a tapered rod end (6°, 7°, 11°, 12°)	Screw connection: R25, R28, R32, T38, T45, T51, GT60
Typical hole	30–45 mm	38–127 mm
Rod type	Integral, taper rod, Hex 22/25	Extension rod with coupling sleeve
Depth ceiling	Shallow — 2–4 m per setup is realistic	Deep; rods extend indefinitely
Energy transfer	Loses energy at the taper joint, especially as it wears	Keeps energy through the thread; tolerates modern high-frequency drifters
Cost	Low per piece	Higher per piece, far longer life
Failure mode	Bit walks off the taper and drops down the hole	Thread wear, then galling, then a very bad day
Best for	Handheld and air-leg work, secondary breaking, shallow anchor holes	Production drilling, benches, tunnels, anything deeper than a few metres

Why tapered bits fall off

Almost always one of three causes. The taper angle does not match — an 11° bit on a 7° rod will never seat, which is exactly what happened to the pallets in Kitwe. The rod end is worn or burred, so the bit cannot reach full engagement. Or the operator is drilling in a direction the taper was never designed for: gravity does part of the retaining work in down-hole handheld drilling, and a tapered bit asked to hold sideways has very little to hold with.

Check the angle stamped on both parts before they go down the hole. It takes ten seconds. A bit at the bottom of a 3 m hole costs you the rest of the day.

Our Tapered Drill Bits are stocked across the common taper angles, and the matching steel is on the Tapered Rods page. If you are drilling with a handheld machine and want an integrated steel-and-bit arrangement, the Integral Drill Rods page covers the sizes we hold.

Where threaded bits earn their price

Thread bits — usually listed as threaded bits — are three to five times the price of tapered bits and worth it the moment your hole gets deeper than a rod length. The thread geometry keeps the joint closed under the rapid, high-energy blows of a hydraulic drifter; a taper joint simply cannot. Our Thread Drill Bits run R25 through GT60 with ballistic, spherical or cross inserts, in straight, drop-centre and retrac gauge styles.

If you are already running a top-hammer rig and want the thread standards explained properly, the drill rod sizes and thread types guide on this site walks through the whole table.

Retrac Bits: The Bit You Buy for Bad Ground

Retrac is short for retraction, and the design is easy to describe. Suppliers list the same product as retrac button bits, retractable-skirt bits or anti-stick bits; the specification that matters is the skirt angle and button layout, not the label. the gauge row of buttons sits on a stepped, flared skirt instead of a straight one. That flare does two things. When you withdraw the string it bores a slightly larger path on the way out, so the bit is far less likely to jam. And because the skirt keeps the hole gauge open behind the bit, the string tracks straighter.

You buy retrac bits for one reason — ground that closes. Fractured rock, softened zones, mixed faces where the hole wants to squeeze back onto the steel. In competent, uniform rock a straight or drop-centre bit will drill faster and cost less.

Specification	Range
Bit diameter	51 – 127 mm
Thread profile	R25 / R28 / R32 / T38 / T45 / T51
Insert type	Ballistic or spherical tungsten carbide buttons
Body	Alloy steel, heat treated
Gauge design	Retrac (step gauge) skirt
Application	Fractured rock, tunnelling, quarry

Full specs and available sizes are on the Retrac Drill Bits page. One note from the field: a retrac bit in soft, uniform ground is wasted money. Buy it when the ground tells you to.

Retrac, drop-centre and straight gauge — choosing in one minute

• Straight gauge — hard, competent rock. Fastest penetration, cheapest bit, least deviation control.
• Drop-centre — the middle of the face is relieved. Good all-rounder for blocky rock and general bench work.
• Retrac — fractured, squeezing or unconsolidated ground, or any hole where you have ever lost a bit.

A driller who has pulled a stuck string out of a collapsed hole once tends to keep retrac bits on the shelf afterwards. That instinct is usually right.

Coupling Sleeves: The Cheapest Part That Stops the Job

Nobody photographs a coupling sleeve. It is a short threaded cylinder that joins two rods into one string, and it is the part that quietly decides how much torque actually reaches the bit.

In our Coupling Sleeves range the sizes run to rod outside diameters of 50–168 mm, in standard DTH and API profiles, machined from high-strength alloy steel and quenched and tempered. The bore is full-flush so the air and water pass through without creating a pressure step.

Item	Specification
Rod OD range	50 – 168 mm
Thread profile	Standard DTH / API / custom
Material	High-strength alloy steel, quenched and tempered
Connection	Female–female threaded coupling
Typical duty	DTH drill string, rotary drilling, water well

How a coupling sleeve fails

Three patterns, in order of how often I see them.

1. Thread flank wear. The joint loosens between rods, torque stops transferring cleanly, and the shoulders start hammering each other. You will hear it before you see it.
2. Galling. Thread metal cold-welds and tears on make-up. Usually caused by dry threads, contaminated thread grease, or re-using a joint that was already damaged.
3. Bore erosion. High-velocity air plus abrasive cuttings eats the internal step. Air pressure at the hammer drops and penetration falls off — while every other component looks fine.

A sleeve is a wear item, not a permanent part. If you are tracking cost per metre properly, log sleeves alongside bits. When a string starts needing more torque to break out, or you are turning the joint more than a quarter-turn to make up, replace the sleeve instead of re-torquing it harder.

DTH Drill Pipe: Friction Welded or Not

DTH pipe carries the hammer, transfers rotation and feeds air down to it. Everything about the design serves those three jobs, and the manufacturing method is what separates a pipe that lasts from one that leaks.

Friction welding joins the tool joint to the pipe body by rotating one against the other under pressure until the interface reaches forging temperature. There is no filler metal. The result is a continuous, coaxial joint with strength close to the parent metal — which matters because the thread is where DTH pipe almost always fails first. After welding, the joint zone is heat treated (quenched and tempered) to even out the microstructure. Skip that step and the weld area becomes the weak link rather than the strong one.

Our DTH Drill Pipe uses a one-piece steel tube body — no longitudinal weld seam — with CNC-machined threads, available in the standard 3 m and 6 m lengths and compatible with the DHD, QL and SD hammer families.

Specification	Range
Outer diameter	73 – 219 mm
Wall thickness	8 – 18 mm
Standard length	3 m / 6 m (custom available)
Thread type	Standard DTH thread — DHD, QL, SD series
Steel grade	S135 / G105 alloy steel
Surface finish	Phosphate treated or paint coated

The wall thickness question

Thicker wall means longer life and less vibration, and it also means more weight on the mast and a higher freight bill. For water well and general blast-hole work in the 76–114 mm range, 8–10 mm wall is the common working choice. If your holes are deep, your rock is abrasive, or your string is long, go heavier and accept the weight.

Be careful with quoted “cost per metre” comparisons between suppliers. A cheaper pipe with 1.5 mm less wall will look good on the invoice and then cost you a full string in six months. Ask for the wall thickness in writing.

What to check when the shipment arrives

• Thread form and pitch, checked with a gauge — not by eye.
• Shoulder faces square and undamaged; a dented shoulder will never seal.
• Straightness across the full length, rolled on a flat surface.
• Weld zone: no undercut, no porosity, no visible colour banding from an uneven heat treatment.
• Bore clear of scale and welding spatter.

If you are matching pipe to a hammer you already own, tell us the hammer model and thread rather than the pipe part number. Hammer threads have more naming variants than any other part of the chain.

Matching Chart: Hole to Tool

This is the table to print and keep in the store. Read it from your hole diameter leftwards.

Hole Ø	Drill	Steel	Bit	Joining part
32–40 mm	Handheld or air leg (YT27 / S250)	Hex 22 integral rod	Tapered bit, 7° or 11°	None — integral
34–45 mm	Air leg (YT28 / YT29A)	Hex 22 / Hex 25 taper rod	Tapered button or cross bit, matching taper angle	None — taper fit
38–51 mm	Small top hammer	R25 / R28 extension rod	Threaded button bit	R25 / R28 coupling sleeve
45–76 mm	Top hammer drifter	R32 / T38 extension rod	Threaded button or retrac bit	R32 / T38 coupling sleeve
64–102 mm	Heavy top hammer	T45 / T51 / GT60 rod	Threaded button bit, retrac for bad ground	T45 / T51 / GT60 sleeve
76–152 mm	DTH rig	DTH drill pipe, 3 m / 6 m	DTH button bit	Pipe thread (DHD / QL / SD)
90–200 mm	Integrated DTH crawler	Ø76 / Ø89 DTH pipe	DTH button bit, 3″–5″ hammer	Pipe thread + shank adapter

Two things the table cannot tell you: the rock, and the rig you already own. Send both and we will narrow it further.

Air Supply: The Part of the Order Nobody Plans

We see more under-sized compressor orders than under-sized drill orders. The arithmetic is not difficult, but it has to be done before the invoice, not after.

1. Add up the consumption of everything running at once. Two YT28 legs = roughly 5.4 m³/min.
2. Add line losses: 15–25% depending on hose length and how many joints you have. Round up.
3. Check the compressor figure is free air delivery, not displacement.
4. Confirm the working pressure suits the tool. Air legs run around 0.5–0.63 MPa; DTH hammers want much higher pressure and a machine rated for it.

If you are running DTH work rather than air legs, the sizing logic is completely different and lives in our DTH hammer air requirements and compressor sizing guide. For the tools themselves, the full range sits under drilling tools, and complete rig packages under drilling rigs.

Spares Worth Keeping on the Shelf

Downtime on small-diameter work is rarely caused by a broken machine. It is caused by waiting two weeks for a part that costs less than the courier charge.

Item	Recommended holding	Why
Bits (taper or thread)	2–3 per drill in use	Gauge wear is continuous; rotating bits evens the load
Coupling sleeves	1 per 3 rods	First component to wear in a threaded string
Air-leg leg seals and springs	1 set per drill	Cheap, fails silently, kills feed force
Shank adapters / shanks	1 spare per drill	Splines wear faster than anything else on the drill
Hose couplings and O-rings	Box quantity	Line leaks are the most common cause of “the drill got weaker”
Drill pipe joints	1 spare per 5 pipes	Thread damage ends a pipe’s life, not the tube

Consumable kits and mixed spares groups are listed on the Kits and Consumables page. If you are running DTH, keep hammer spares separate from drill spares — the DTH Hammer and DTH Drill Bits pages cover the common items.

Ordering Checklist: What to Send Us

Customers who send these six lines get a correct quotation the first time. Customers who send “please quote for rock drill” get a conversation instead.

1. Hole diameter in mm, and hole depth.
2. Rock type — granite, limestone, fractured, mixed. Anything you know.
3. Method already in use or intended: handheld, air leg, top hammer, DTH.
4. Thread or taper you are already running, if any. This is the line that prevents the Kitwe pallets.
5. Air supply — compressor model, FAD in m³/min and working pressure.
6. Quantities and whether you need the complete chain (drill, steel, bits, sleeves, pipe) or a single part.

Attach a photo of the shank, the taper end and the thread if you are unsure. It is faster than describing it.

FAQ

Can I run a YT28 on a small piston compressor?

Short answer: not continuously. The drill wants roughly 2.7 m³/min of free air at 6.3 bar for one unit. A piston machine rated near that will do it in bursts but will not enjoy all-day duty. For production work, use rotary screw.

My tapered bit keeps coming off. What is wrong?

Nine times out of ten, mismatched taper angle. Check the stamped angle on both the bit and the rod, then check the rod end for wear or burrs. If both are correct and the hole is overhead or horizontal, you are asking the taper to hold in a direction it was not designed for — switch to a threaded system.

How do I know when to replace a coupling sleeve?

Three signals: the joint needs noticeably more breakout torque than it used to, the shoulders show shiny contact marks, or you can make up the joint past the normal turn count. Any one of these means replace. Thread grease is not a repair.

Is friction welded pipe really better than a machined joint?

For DTH work, yes — provided the weld zone is properly heat treated afterwards. A friction weld gives a coaxial, high-strength joint without filler metal, and the thread is where DTH pipe fails. Ask any supplier for the heat treatment specification. If they cannot answer, that is your answer.

Retrac or standard bit for a quarry?

If the rock is competent and the face is stable, standard straight or drop-centre bits drill faster and cheaper. Choose retrac when the ground fractures, closes or has ever swallowed a bit. You are buying withdrawal reliability, not speed.

Do you supply tools for rigs we did not buy from you?

Yes. Give us the hammer or drifter model and the thread, and we will confirm compatibility before quoting. See the full tool range on the drilling tools page.