Stainless Steel Hex Nuts and Threaded Fittings for Gas Sensors

A sensor hex nut is the threaded hex body that carries a gas sensor element into its boss: it seals against the exhaust or the pipe, provides the wrench flats for installation, and holds the bore that locates the element. Balford turns these from stainless bar to drawing, in the thread and hex sizes gas sensors actually use - M12 to M27 threads on hex sizes S14 to S30 - For the same assemblies we also supply the small PTFE insulators, bought out to drawing from qualified suppliers rather than made in our own plant.

Families we turn

Sensor family	Thread	Hex across flats	Material	Typical height
Oxygen sensor (automotive)	M18	S22	430F / 430 stainless	24.8 - 29.5 mm
Oxygen sensor (motorcycle)	M12	S14 / S17	430F stainless	16.5 - 23.9 mm
Nitrogen oxide (NOx) sensor	M20 / M22 / M27	S22 / S24 / S30	304 / 430 stainless	13.5 - 36.3 mm
Exhaust gas temperature sensor	M14	S17	304 stainless	13 - 18 mm
Other sensor fittings	to drawing	to drawing	PTFE (bought out)	11 - 20.5 mm

Outer diameters run from about 10.5 mm to 25 mm and bores from about 1.8 mm to 16.5 mm across the range. Every item is a non-standard part: the drawing decides the thread, the bore, the sealing face and the hex.

How they are made: CNC turning from bar

These are turned parts, not drawn or stamped. Round bar is machined on CNC lathes in one or two setups so the thread, the hex flats, the bore and the sealing face stay concentric to one another - which is the whole point of the part, because a hex nut that is not concentric to its bore will not seat the sensor squarely.

• Turning and threading on CNC lathes, with the bore and the thread produced from the same datum
• Bar material is bought in; turning, drilling, threading and inspection are in-house
• Stamped blanks are also machined in-house where a part is better started from a pressing
• Typical lead time for a new turned part: 7 to 15 calendar days from a confirmed drawing
• Bought out, not made here: the PTFE insulators and bushes on this page, together with bulk heat treatment, slow-wire EDM, passivation, electropolishing, plating, phosphating and liquid painting. Powder coating is split: large sheet-metal parts run in our own manual booth, small parts are coated by a partner on an automatic line. Hot forging, cold extrusion, cold heading, casting and powder metallurgy are bought as near-net pre-forms and finished by machining in-house.

For the machining side in more detail, see CNC machining services and CNC turning.

Materials

• 430F - free-machining ferritic stainless. The usual choice for oxygen sensor hex nuts: it machines cleanly, takes a thread well and resists exhaust corrosion without the nickel cost of 304.
• 430 - the same ferritic family without the free-machining addition, where a part is formed rather than heavily turned.
• 304 - austenitic stainless, used on NOx and exhaust gas temperature fittings where higher-temperature corrosion resistance matters more than machinability.
• PTFE - not a material we machine. The PTFE insulators and bushes used in these assemblies are bought out to drawing from qualified suppliers and supplied alongside our turned stainless parts. They are electrically insulating, low friction and unaffected by exhaust condensate; the drawing and the material grade are confirmed with the supplier before the lot is accepted.

Where these parts are used

• NOx and nitrogen oxide sensors
• Oxygen and lambda sensors
• Motorcycle oxygen sensors
• Exhaust gas temperature sensors
• Temperature sensor housings and deep drawn sensor housings - the drawn parts that sit alongside these fittings
• Automotive sensor and exhaust systems

Technical background: sensor housing tolerance guide, concentricity guide, NOx housing material selection.

Sensor hex nuts by thread size

Thread and hex size travel together, and the pair is normally fixed by the sensor the fitting has to take. The combinations below are the ones we have actually run; anything else is quoted to drawing.

Thread	Hex across flats	Sensor family	Material we run
M12	S14 / S17	Motorcycle oxygen sensor	430F stainless
M14	S17	Exhaust gas temperature sensor	304 stainless
M18	S22	Automotive oxygen sensor	430F / 430 stainless
M20	S22	NOx sensor	304 / 430 stainless
M22	S24	NOx sensor	304 / 430 stainless
M27	S30	NOx sensor	304 / 430 stainless

The thread, the bore and the hex flats are produced from the same datum, which is why the hex cannot drift relative to the bore. A fitting that is not concentric to its bore will not seat the sensor squarely.

By material: which grade for which fitting

Grade	Magnetic	Why it is chosen	Typical part
430F free-machining ferritic	Yes	Best machinability of the three, takes a clean thread, resists exhaust corrosion without the nickel cost of 304	Oxygen sensor hex nuts, motorcycle fittings
430 ferritic	Yes	The same family without the free-machining addition, where a part is formed rather than heavily turned	Formed fittings and brackets
304 austenitic	Very slightly	Corrosion resistance for NOx and exhaust gas temperature fittings, and the choice where the assembly cannot tolerate a magnetic part	NOx and EGT fittings
PTFE (bought out)	No	Insulator and bush, not a structural part	Sensor insulators

Magnetic behaviour is a real selection criterion on a sensor assembly, not a curiosity: see which materials are magnetic and why 304 and 430 behave differently in service.

By surface finish

Finish	What it does	When it is quoted
As machined	Turned finish straight from the insert	Default for most fittings; the drawing may still call a maximum Ra
Passivated	Removes free iron and restores the passive layer on stainless	304 and 430 parts for exhaust or humid service
Zinc or nickel plated	Corrosion protection, uniform appearance, solderability	Carbon-steel or brass fittings, or a part that must not show tarnish
Phosphated	Conversion coating that retains oil and reduces galling	Threaded parts that must run in without seizing
Electropolished	Brightens the surface and improves Ra by roughly 30–50%	Visible or clean-service parts

Passivation, electropolishing, plating and phosphating are subcontracted processes for us and are disclosed as such on the quotation. See also typical Ra values by process.

Balford produces these hex nuts on presses from 25 t to 350 t, with the dies designed and built in its own tool room.

Frequently Asked Questions

Are these standard parts or made to drawing?

Made to drawing. The thread, hex size, bore and sealing face are all set by your drawing; we do not hold a catalogue of standard hex nuts. The table above shows the families we turn most often, with the sizes that recur.

What tolerance can you hold on the thread and the bore?

Thread and bore tolerance depend on the callout and the sealing method. Dimensions to 0.01 mm are possible on selected critical machined features where process, geometry, material and inspection conditions allow and the drawing confirms it - that is a drawing-confirmed exception, not a default tolerance.

How do you keep the hex concentric to the bore?

By producing the hex, the thread and the bore from the same datum, usually in one or two setups on a CNC lathe rather than across separate operations. Concentricity and runout are checked with a concentricity gauge and a 2.5D optical measuring projector.

What about the plastic parts?

No - they are bought out. The metal parts on this page are turned at Balford; the PTFE insulators are purchased to drawing from qualified suppliers and supplied with the metal fittings, either separately or as an assembly. We do not machine plastics in our own plant, and we say so rather than implying it.

Send a drawing for a DFM review and a quotation All Balford catalogues and data files are collected on the downloads page.