Oxy-acetylene welding nozzle numbers: No. 1-25 explained

UK and European oxy-acetylene welding nozzles are numbered by gas-flow capacity, from No. 1 (smallest) to No. 25 (largest). The number is stamped on the nozzle body and printed on every product page - match the stamp, not the memory.

The number tracks the orifice and the acetylene flow, and the flow tracks the metal you can weld. The launch range covers No. 1-10 for welding duty, with the high numbers (No. 16 and up) reserved for heavy heating and pre-heat work. Acetylene values are shown on the tables; propane equivalents differ.

Thin sheet and fine work: No. 1-3. General plate: No. 4-6 - with No. 5 the workshop default. Heavy plate: No. 7-10. From No. 16 up, you are in heating and pre-heat territory, not welding. Read the flame as you work: a too-small nozzle drowns the puddle; a too-large one blasts it apart.

The direction is what matters: the number climbs as the section thickens, and the step between numbers is a step in heat, not a step in size you can see with the eye. Work down when the puddle is sluggish and up when the flame is blowing the metal away. The number gets you close; the flame does the fine adjustment. If you are unsure where your torch sits, the No. 1-25 series tables list each number and its duty.

Not every oxy-acetylene torch mixes its gases the same way, and the difference changes how a number feels. An equal-pressure torch feeds acetylene and oxygen at similar pressures into the mixing chamber; the gases meet and mix before they reach the nozzle. A low-pressure or injector torch uses the oxygen stream to draw the acetylene in, which suits rigs running acetylene at very low pressure.

For selection, the practical point is this: the nozzle you need depends on the torch design as well as the number. A nozzle set built for an equal-pressure head is not automatically right for an injector head, even at the same number. Confirm the series against your torch before you order - the oxy-acetylene welding nozzle collection is grouped by seat so you are not mixing families.

The No. series looks like one ladder, but in practice it splits into three jobs. Welding sits at the low end: a small, hot, well-shaped flame that melts the base metal and feeds a rod. Pre-heat sits in the middle-to-upper range: a softer, wider flame that brings a casting or a heavy plate up to temperature before welding or brazing. General heating sits at the top: a big, soft flame for straightening, loosening and warming large sections.

The same metal can need all three in one afternoon. You might pre-heat a heavy lug with a higher number, weld it with a mid number, then straighten the result with a heating nozzle. That is why a set covers more ground than a single - and why knowing which of the three jobs you are doing is the first half of picking the number. For the brazing side, the brazing nozzle guide walks the same logic, and the brazing nozzle collection lists the No. series sized for that duty.

A set covers a shop full of jobs - the No. 1-10 set is the one most workshops buy first. When you know your staple thickness, buy the single: No. 5 is the UK reorder king for general plate, and No. 16 is the heating and pre-heat specialist.

Two nozzles stamped with the same number are not the same nozzle. The number codes a capacity band, not an exact orifice, and each maker sits slightly differently within the band. One No. 5 throws a slightly hotter, tighter cone; another runs a touch softer. Both are legitimate No. 5 nozzles.

That is why the flame, not the stamp, is the final check. Light the torch, set the flame neutral, and read what it does on the metal. If a familiar number behaves differently on a new make, trust the flame and adjust - and if you are swapping makers, re-prove your regulator settings rather than carrying the old numbers over. The compatibility chart sorts the seat before you buy.

The No. series is a sizing code, not a universal thread. Your torch head must accept the British No. series seat. Check the compatibility chart, or send us your torch model and we will confirm the fit before you order.

A nozzle number only does its job when the flame is set right, and the flame is set with the regulators and the torch valves, not the stamp. Light the torch, then balance oxygen against acetylene until the inner cone is a clean, sharp blue-white with no feather. A feathering, yellow inner cone means too little oxygen; a soft, hissing flame that blows the puddle apart means too much.

The number and the flame work together. A too-small nozzle needs a hard, lean flame just to hold the puddle, and that invites oxidation. A too-large nozzle makes you run a soft, lazy flame that overheats the metal around the joint. When the number is right, the flame falls into neutral without a fight.

That fall into neutral is the feeling to learn, because it is the same check on every job. It also catches a dirty nozzle early: if the cone will not come clean and sharp, give the orifice a pass with a tip cleaner set, and if it will not recover, the nozzle is worn rather than dirty.

For a learner, the quickest way to feel the difference is to weld the same scrap with two numbers either side of the recommended one. The wrong number fights you at the torch before it shows in the joint - too hot, too soft, too hard to hold a puddle. The right number feels like the metal is doing what you ask, and that is the whole point of the series.

Set regulator pressures from the nozzle rating, light with a flint rather than a lighter, and never run acetylene above its working pressure. Keep nozzles clean with a tip cleaner set and store the set in its case.