Bending Springs: How to Bend 15mm Copper Pipe With a Tool That Costs Less Than an Elbow
The UK guide to copper pipe bending springs. Internal vs external, sizing, cord-and-knee technique, copper grades, removal tricks, and what to buy from £4-15.
You've run 15mm copper from the boiler to a kitchen sink, and the pipe needs a single 90-degree drop down the wall. You don't own a lever bender. You're not buying one for one bend. The push-fit elbow at the merchants is a few pounds. The bending spring next to it costs about the same. You buy the spring, push it inside the pipe, brace your knee against the floor, and twenty seconds later you've got a clean 90-degree bend with no fitting and no joint to leak. The whole tool fits in a coat pocket.
What it is and when you need one
A bending spring is a coiled steel spring that supports the wall of a copper pipe while you bend it by hand. It's the cheapest pipe-bending tool you can buy, and on certain jobs it's also the most practical.
There are two types. An internal spring slides inside the pipe and holds the inner wall against collapse. An external spring slides over the outside of the pipe and stops the outer wall from spreading. Internal springs are made from flattened spring steel formed into a tight coil with a small ring or eye at one end for withdrawal, and a tapered insertion end at the other. External springs are open coils with a funnel at each end so the pipe feeds through.
You need a spring when you're making a small number of bends in copper pipe and don't want to spend the price of a lever bender. Typical scenarios: a one-off radiator move, a single drop from a boiler to a tap, replumbing one section of a kitchen first-fix where the rest of the system uses push-fit. For full plumbing first-fix on an extension with thirty or forty bends, buy a lever pipe bender instead. Springs are slow, the bends are less accurate, and your knees take a beating.
The other case where a spring beats a lever bender is access. Lever bender arms are long. There are spots on a real plumbing job where the arms simply won't fit: tight against a wall, between closely spaced joists, in a cupboard. A spring fits anywhere the pipe fits. Plumbers carry one even when they have a lever bender in the van.
Get the sizing right
This is where most people buy the wrong tool. Pipe-bending springs are sized to the pipe diameter they fit, but the rule is different for internal and external springs, and almost no retailer explains it clearly.
The pipe sizes called out in UK plumbing (15mm, 22mm, 8mm, 10mm) refer to the outside diameter of the copper tube. A "15mm pipe" has a 15mm OD and roughly a 13.6mm bore. The standard is set out in BS EN 1057.
An internal 15mm spring is sized to slide inside a 15mm pipe (so the spring itself is around 13.6mm across, fitting the bore). An external 15mm spring is sized to slide over the outside of a 15mm pipe (so the spring is around 16mm across internally). They are not interchangeable. Buying an external spring when you needed an internal one means it won't fit the pipe at all.
| Pipe size (OD) | Common use | Spring type to use | Why |
|---|---|---|---|
| 8mm | Microbore central heating, gas to a single appliance | External | Pipe is too narrow inside for an internal spring to support effectively |
| 10mm | Microbore heating, gas, brake/oil lines on plant | External | Same reason as 8mm |
| 15mm | Domestic hot and cold water, central heating flow and return | Internal | Standard size; internal spring is widely stocked and works well |
| 22mm | Hot and cold mains feeds, heating mains, larger appliance feeds | Internal (but use a lever bender) | Spring exists but bending 22mm by hand is impractical for most people |
| 28mm and above | Mains supply, commercial systems | Neither (use a lever or stand bender) | Hand-bending springs do not handle this size |
External springs are sometimes sold as covering "6-10mm" or "8-16mm" depending on the manufacturer's design. Read the listing carefully before buying.
How to use one properly
Spring bending technique is unforgiving. Get the steps wrong and the most likely outcome is a spring stuck inside a kinked pipe that you can't extract without cutting the pipe in half. The good news is that the steps that prevent this are simple, take thirty seconds, and most homeowner guides bury them.
Tie a cord to the spring eye before doing anything else
This is non-negotiable. Use strong cord (paracord, sash cord, or doubled-up garden twine) at least a metre longer than the pipe. Thread it through the ring on the eye end of the spring and tie a secure knot. Without this cord attached before insertion, you have no reliable way to pull the spring back out if it sticks. Forum threads describing stuck springs almost universally end with "I didn't tie a cord on first." Do not skip this step.
Lubricate the spring
Smear petroleum jelly (Vaseline) along the full length of the spring. A light oil works in a pinch but jelly stays put better. Lubrication makes insertion easier and, more importantly, makes removal possible after the bend has gripped the spring slightly.
Insert the spring with anticlockwise rotation
The tapered end goes in first. Push the spring into the pipe while twisting it anticlockwise. The anticlockwise motion winds the spring tighter, which slightly reduces its diameter and helps it slide through any minor variations in the pipe's bore. Centre the spring across the bend point so equal lengths sit either side.
Mark and align the bend
Mark the centre point of the bend on the outside of the pipe with a pencil. Some plumbers use the three-point method: a centre mark, a mark one pipe radius back to show the start of the bend, and a mark half a radius forward to show the end. For a 15mm pipe expect the bend radius to be around four times the OD (roughly 60mm), which is gentler than a lever bender produces.
Bend over your knee with a rocking motion
Sit down. Rest the pipe across the front of your thigh (top of the thigh works better than the kneecap and is kinder on your joints). Pull the two ends down towards you. Critically: do not pull from one fixed point. Move the pipe sideways across your knee as you bend, walking it back and forth. This distributes the bending stress along the bend rather than concentrating it at one spot, and the difference between a kinked pipe and a smooth radius is almost entirely down to this rocking motion.
Slightly over-bend, then ease back
Pull the bend a few degrees past your target angle (5-10 degrees of overshoot is enough), then ease it back to the angle you actually want. This releases the spring's grip on the pipe wall. Without the over-bend-and-ease step the spring is much harder to extract because copper springs back slightly and clamps onto the spring.
Withdraw the spring with cord and anticlockwise twist
Pull on the cord while twisting the spring anticlockwise. The twist tightens the coil, reducing its diameter and breaking the friction grip on the pipe wall. If the spring resists, slip a screwdriver shaft or thin metal bar through the eye ring and use it to twist harder while pulling. The bar through the ring is the standard recovery move when a spring is stubborn.
Warning
Repeated knee bending of copper pipe over many years is a recognised cause of plumber's knee and chronic joint problems. For occasional DIY use this isn't a concern. If you find yourself doing more than a handful of bends, it's a sign you should be using a lever bender, not a sign you should push through.
When the spring gets stuck
If the cord-and-twist withdrawal fails, work through the steps in this order before resorting to cutting the pipe.
First, push a screwdriver shaft or short metal bar through the ring on the spring eye. Use it as a handle to twist the spring anticlockwise hard while pulling. This tightens the coil and reduces its diameter more than your fingers can. It rescues most stuck springs.
Second, over-bend the pipe further. If there's room to push the bend a bit tighter, do that, then ease it back out. The release sometimes loosens a spring that wouldn't budge.
Third, gently tap the pipe along the bend with a piece of timber. Mild vibration can free a friction-stuck spring. Do not hammer the pipe directly with the spring still inside, because that crushes the pipe onto the spring and makes everything worse.
Last resort: cut the pipe and unthread the spring through the cut sections. The pipe is scrap at this point. Better to lose a length of pipe than the spring.
Check your copper before you bend
Three grades of copper pipe are sold in the UK under BS EN 1057. Not all of them can be bent.
R220 (soft, fully annealed) comes in coils. Used for long underground runs and microbore central heating. Bends easily by hand, with or without a spring. The softest of the three.
R250 (half-hard) comes in 3-metre or 6-metre straight lengths. This is the standard pipe at Screwfix, Toolstation, and plumbing merchants for domestic hot and cold water and central heating. Bends with a spring or a lever bender.
R290 (hard, thin-wall) is sold for commercial systems where wall thickness matters less and rigidity matters more. Cannot be bent by hand. The thin walls collapse or crack before the bend forms. If you try to bend R290 with a spring you will destroy the pipe.
Check the printed grade stamp on the pipe before bending. If you can't find a stamp, treat the pipe as suspect until you've confirmed what it is.
If your half-hard R250 keeps creasing on the inside of the bend even with good technique, the pipe has work-hardened from previous handling. The fix is to anneal the bend zone. Heat the pipe with a butane or propane blowtorch until the section glows a dull red, then let it air cool. The copper turns black where it was heated; clean it back with wire wool before bending. Annealed copper bends like soft butter compared with cold half-hard. This is a trade trick that no published homeowner guide spells out, and it rescues bends that would otherwise be junk.
The 22mm question, and why it's a question
Every published guide that mentions 22mm bending springs warns about the difficulty. DIY Doctor calls 22mm spring bending "a herculean task which none of us would attempt." Forum threads describe it as "very tough on knees and pecs." Multiple sources converge on the same point: it is technically possible, it is not practical for a homeowner.
The problem is force. A 22mm copper pipe is roughly twice as stiff as a 15mm pipe under bending load. The lever arms of a bender solve this with mechanical advantage. Using your knee, you don't have that mechanical advantage. You're applying raw strength to a stiff length of metal, and the smallest mistake (the spring drifting off centre, the pipe rocking unevenly under your knee) puts the bend on the wrong axis or kinks the wall.
The honest answer for 22mm: don't try to bend it with a spring. Pick one of three alternatives.
Hire a lever bender. Tool hire shops rent 15mm + 22mm lever benders for around £150 per day. One day is enough to do all the 22mm bends on most extension first-fix plumbing.
Use compression or push-fit elbows. A 22mm compression elbow costs a few pounds and seats in a couple of minutes. For a small number of bends this is cheaper than buying a lever bender. The trade-off is more potential leak points.
Borrow. Most builders' merchants will sell you a budget lever bender for not much more than a day's hire if you might use it again. The Magnusson 15+22mm combo at Screwfix sits at the bottom of this range.
What to buy
The whole product category sits in a narrow price band. There is no high-end professional bending spring market because anyone bending pipe at volume already owns a lever bender.
Internal springs (15mm and 22mm)
Bending spring (15mm or 22mm, internal)
£4 – £7
The two brands that dominate the UK retail market are Rothenberger (German manufacturer, widely stocked) and Monument Tools (UK manufacturer based in London).
Rothenberger 15mm internal sells at the cheapest end of the range from Screwfix (product 39016, 500mm length, 1-year guarantee, around 3.7 stars from nearly eighty reviews). The Toolstation equivalent is a few pence more. The Rothenberger 22mm internal sits a pound or two above the 15mm at Toolstation.
Monument's 830V is the 15mm internal model: square-section polished spring steel, 559mm long, with a pegboard hook on the eye end. Pricing varies more widely across retailers (FFX, Trading Depot, and specialist tool shops at the upper end of the range). Monument's springs are noticeably stiffer and feel better made than Rothenberger's at twice the price, but for occasional DIY use the Rothenberger is fine.
If you only ever expect to bend one or two pipes, buy the Rothenberger. If you want a tool that will be in the toolbox for the next twenty years, the Monument is worth the extra fiver.
External springs (8mm and 10mm microbore)
External springs are stocked less widely than internal ones. Most homeowners never buy one because microbore work is a niche of central heating that's largely been superseded by 15mm runs. When you do need one, expect to pay a similar few pounds as the internal version from specialist plumbing suppliers, eBay, or via Toolstation/Screwfix when in stock. Faithfull and Monument both make ranges. The Rothenberger external 15mm (product 25185) covers 8-16mm OD pipe.
Skip the 22mm internal spring
The 22mm spring exists, costs only a couple of pounds more than the 15mm, and gets used by almost nobody. Buy a lever bender, hire one, or use elbow fittings. The 22mm spring is a tool that solves the wrong problem.
Alternatives
Lever pipe bender. The professional choice. Bends 15mm and 22mm in seconds with a degree-marked former that gives you angle accuracy. If you're doing more than a handful of bends, this is the right tool. Springs are an old technology that sits in the toolbox for tight access, not a primary bending tool.
Compression elbow fittings. Each fitting adds a small amount to material costs and one more potential leak point. For a small project with three or four direction changes, elbows are the simpler answer. For a full first-fix, the cost adds up.
Push-fit elbows (Speedfit, Hep2O). Even faster than compression. If you're plumbing the entire extension in plastic push-fit pipe rather than copper, you don't need a bender at all. Push-fit systems are designed around fittings.
Pre-formed copper bends. Plumbing merchants sell ready-made 90-degree and 45-degree copper bends for both 15mm and 22mm. They're more expensive per direction change than bending your own, but the bend is perfect every time. Useful for exposed pipework where appearance matters.
Sand fill. An old-school alternative when nothing else is available. Plug one end of the pipe, fill it with dry sand, plug the other end, and bend over your knee. The sand prevents collapse the same way a spring does. Slower and messier than a spring, mentioned here for completeness rather than recommendation.
Where you'll need this
- First fix plumbing - making occasional 15mm bends in tight spaces during the first-fix run
- Kitchen plumbing provisions - bending pipework into appliance positions when a lever bender won't fit
Bending springs turn up at first-fix plumbing stage on any extension or renovation project where copper pipe is being run. They're a backup tool to the lever bender on professional jobs, and the primary tool on small DIY jobs where the cost of the lever bender isn't justified.
Common mistakes
Inserting the spring without a cord attached. This is the single most common cause of stuck springs. The eye on the spring is small, fiddly, and impossible to grip with pliers once it's deep inside a bent pipe. Tie the cord on at the workbench before the spring goes anywhere near the pipe.
Skipping the lubrication. Dry insertion grips the pipe wall on withdrawal. Petroleum jelly costs almost nothing and saves the spring.
Pulling from one fixed knee point. A static pull concentrates all the bending force at one spot on the pipe, which produces a kink rather than a curve. Walk the pipe back and forth across your knee as you bend.
Trying to bend hard-tempered R290 pipe. Check the grade stamp. If it isn't R220 or R250, don't bend it.
Bending 22mm with a spring on principle. The spring exists but bending 22mm by hand is impractical. Hire a lever bender or use elbow fittings.
Hammering a kinked pipe with the spring still inside. Crushes the pipe onto the spring and turns a recoverable problem into a scrap-pipe problem. If the bend has gone wrong, withdraw the spring first using the screwdriver-through-ring trick, then assess the pipe.