In short. A stud conversion replaces the bolts that hold the wheels of a BMW, Volkswagen or Audi with studs screwed into the hub and nuts on those studs. It changes how the wheel goes on, not how well it is held: the wheel is still clamped by friction, and the nut, like the bolt, must match the wheel's seat, engage enough thread (at least the stud's diameter, according to the fastener maker McGard) and be tightened to the figure given for the fixings actually fitted: the car maker's bolt figure no longer applies, so get the stud and nut maker's figure in writing. No rule we read in the UK, the US or Australia requires or forbids a conversion; inspectors look at loose or missing fixings, thread engagement and whether the fixings suit the wheel.
Bolts and studs: what actually differs
With bolts, the bolt passes through the wheel into a threaded hole in the hub, and its head presses on the wheel's seat. With studs, a threaded pin sticks out of the hub, the wheel slides over it, and a nut presses on the seat.
BMW, Volkswagen and Audi use bolts; a US BMW parts seller notes that only a select few BMWs were delivered with studs as original equipment (BMW stud and nut page). No maker document we read explains why these makers chose bolts, so we do not offer a reason as fact.
What the conversion changes in practice:
| Wheel bolts (original) | Studs and nuts (conversion) | |
|---|---|---|
| Fitting the wheel | Hold the wheel up while starting the first bolt | The wheel hangs on the studs while you start the nuts |
| Where wear happens | The hub's thread, each time a bolt goes in | The stud and the nut; a worn stud can be replaced |
| Length to choose | Bolt length under the head | Stud length beyond the hub, fixed once fitted |
| Seat | On the bolt head | On the nut |
| Torque | The car maker's figure | The stud and nut maker's figure |
| Hub thread | Used at every wheel change | Used once, when the stud goes in |
The seller treats studs and bolts as wear items: it suggests replacing studs every year for endurance racing, and bolts every three to four years on a street car. That is a parts seller's advice, not a car maker's.
What a conversion kit contains, and how it goes in
A kit is a set of studs (the seller sells 10.9 grade street studs and 12.9 grade race studs, with a bullet nose that guides the nut), nuts to match the wheel's seat, and sometimes jam nuts for fitting. Its fitting method, which it gives as its own and sets aside for any other maker's instructions:
- Clean the threads in the hub before the stud goes in.
- Use threadlocker on the hub end of each stud if none is pre-applied.
- Start every stud by hand.
- Tighten it with two nuts locked against each other (the double-nut method), not with the hex at the tip of a bullet-nose stud.
- Use no anti-seize, oil or lubricant on the outer threads.
The dry outer thread agrees with the fitting instructions supplied with a replacement wheel under UN Regulation 124 that we read: no lubricant on the threads of the nuts or bolts, nor on the face between wheel and hub, because lower friction leads to over-tightening and can loosen the fixing (point 5, our translation from German, fitting instructions, 27 January 2022).
All of this happens with the wheel off. Do it with the car raised and resting on axle stands, never on the jack alone; the steps are in our guide to changing a wheel safely.
Thread engagement: how much is enough
No regulation we read puts a number on it, and DIN 74361, ISO 965 and ISO 4107 (wheel fixings, metric threads) are paid documents we have not read. Two industry rules exist, and they differ:
- McGard, a maker of wheel nuts and locks: "Minimum thread engagement must be equal to the thread diameter of the wheel stud", for example 12 mm for an M12 x 1.5 nut (McGard).
- The BMW parts seller: "M12 bolts must have approximately 10mm of thread engagement" and M14 bolts about 12 mm.
Our design office turned both rules into turns, for the three threads used on these makes. Turns = length engaged ÷ pitch.
| Thread | Used on (from the sources below) | McGard rule (= diameter) | Seller's rule for bolts |
|---|---|---|---|
| M12 x 1.5 | Older BMWs (E30 to E9X, per the seller) | 12 mm, 8.0 turns | 10 mm, 6.7 turns |
| M14 x 1.25 | BMW from the E70 X5 and F models | 14 mm, 11.2 turns | 12 mm, 9.6 turns |
| M14 x 1.5 | Volkswagen, Seat, Skoda (KBA document), Audi (Audi bolt part); a handful of BMWs, per the seller | 14 mm, 9.3 turns | 12 mm, 8.0 turns |
Calculation by the Onyx Forged design office from the two pages cited. Example: 14 ÷ 1.25 = 11.2 turns. Threads: BMW stud and nut page, genuine BMW bolt, KBA variant V7, Audi bolt set. Our PCD and centre bore guide by make lists the thread generation by generation, and our car pages (in French) give the sourced thread and seat per model, for instance the BMW M3 (G80), M14 x 1.25 with a cone seat, and the Audi A3 and S3 (8Y), M14 x 1.5.
The diameter rule is the stricter one; we use it to compare, not as a standard. The first, chamfered thread carries little load and we cannot put a figure on it, so a fixing that only just meets a minimum is too short.
For BMW, the seller also sells race studs with M14 x 1.25 in the hub and M14 x 1.5 for the nut, because "The 1.5 is a coarser thread and requires fewer turns to tighten": 14 mm is 9.3 turns at 1.5 against 11.2 at 1.25. Those nuts do not fit a stud with 1.25 at both ends; forced on, they start, then bind and damage the thread.
Stud length and lug bolt length: the arithmetic
For a bolt, the sum is: length under the head = thickness clamped + spacer + thread engaged in the hub. The thickness clamped is the wheel at the bolt hole, from the seat to the face that touches the hub.
Take the series bolt in the KBA document for Seat, Skoda and Volkswagen: M14 x 1.5, 27.5 mm under the head. The thicknesses below are chosen for the arithmetic and belong to no particular wheel:
| Case | Thickness clamped | Thread in the hub | Turns at 1.5 mm | Against 14 mm (McGard rule) |
|---|---|---|---|---|
| Wheel A | 13.5 mm | 27.5 − 13.5 = 14 mm | 9.3 | meets it |
| Wheel B, thicker at the seat | 20 mm | 27.5 − 20 = 7.5 mm | 5.0 | well short |
| Wheel B, 33.5 mm bolt | 20 mm | 33.5 − 20 = 13.5 mm | 9.0 | just under |
| Wheel A with a 15 mm spacer | 28.5 mm | none | 0 | the bolt cannot reach |
Calculation by the Onyx Forged design office.
Wheel B is the trap: a wheel thicker at the bolt hole than the one it replaces, and an old bolt that still screws in without resistance. The seller's spacer rule is the same sum: "if your spacer is 15mm the new wheel bolt must be 15mm longer than your existing wheel bolt". Too long is a fault too: a bolt that bottoms in the hub before the head clamps the wheel makes the torque wrench click on a wheel that is not held. See wheel spacers for spacers and the law.
For a stud, the sum moves to the part that sticks out: thread beyond the hub face ≥ thickness clamped + spacer + thread engaged by the nut, plus the bullet nose, which guides the nut but does not hold it. With the same invented 20 mm wheel and an M14 stud, the McGard rule asks for 20 + 14 = 34 mm of usable thread; with a 15 mm spacer, 49 mm. The seller lists studs from 73 to 92 mm overall but not how much sits in the hub: ask the stud maker for the protrusion.
To check on the car, run each nut down by hand and count the turns from first bite to seat: turns × pitch is the engagement. The wheel offset calculator shows how a spacer or a new offset moves the wheel. Onyx Forged does not recommend a size or an offset.
The seat: cone, ball or flat
The seat is the face that presses on the wheel, and on a conversion it moves from the bolt head to the nut. EUWA, the European wheel makers' association, is plain about the consequence of getting it wrong: "Any incorrect combination of parts (different type, different shape of radius/angle, different length) may cause screw or nut loosening or the wheel failure" (EUWA ES 1.07, 3.3, June 2020).
The documents we read show two seats on these makes. The BMW bolt from the E70 X5 and the F models is a cone seat, according to the seller. The Volkswagen group series bolt in the KBA document has a ball (radius) seat, D = 26 mm. EUWA also describes a third arrangement, flat-seat nuts with a captive washer on wheels centred on the hub spigot (EUWA ES 1.02, 2.2).
So the nut in a kit has to match the wheel you will fit, not the car. A Volkswagen or Audi wheel made for ball-seat bolts takes ball-seat nuts; an aftermarket wheel with 60 degree cones takes cone nuts. A ball nut in a cone seat touches on a line, not a surface. McGard asks you to check the thread size and the seat type "as recommended by the wheel manufacturer"; the wheel's fitting sheet should say which.
Torque: whose figure applies
This is the change people miss. The car maker's figure is for the car maker's bolt. Once studs and nuts are fitted, the figure is the stud and nut maker's. The seller puts it in five words: "Bolts are not Studs! Or vice-versa!"
| Fixing | Source | N·m | lb ft |
|---|---|---|---|
| BMW M12 wheel bolts (E30 to E9X) | BMW figure as quoted by the seller | 120 | 88.5 |
| BMW M14 wheel bolts (F and G models) | BMW figure as quoted by the seller | 140 | 103.3 |
| BMW original studs into the hub (the few models delivered with studs) | BMW figure as quoted by the seller | 80 | 59.0 |
| Seller's M12 nuts on its studs | Seller, 75 lb ft | 101.7 | 75 |
| Seller's M14 nuts on its studs | Seller, 90 lb ft | 122.0 | 90 |
| Seller's studs into the hub, double-nut method | Seller, 50 to 60 lb ft | 67.8 to 81.3 | 50 to 60 |
| VW, Seat, Skoda series bolt M14 x 1.5, ball seat | KBA variant V7 | 120 | 88.5 |
| Porsche 911 (992) wheel bolts | Porsche bulletin 38/25 | 160 | 118.0 |
Conversions by the Onyx Forged design office with 1 lb ft = 1.355818 N·m (NIST SP 811). We did not read a BMW manual: the BMW rows are the seller's quotation, not a BMW document.
On the seller's M14 parts, the nut figure (122.0 N·m) is below the BMW bolt figure (140 N·m), and it says to torque only cold, never hot. The same page also says "Do not exceed 75ft-lbs with our wheel nuts on our wheel studs", without saying whether that covers the M14 nuts: a page that gives two figures is a reason to ask the maker which one applies, not to pick one. Those values belong to its parts; another maker's studs may carry another figure, so read the sheet that comes with yours. EUWA ES 1.02 says the torque prescribed by the vehicle maker must be adhered to (4.3.2), and the vehicle maker publishes no figure for parts it did not supply: after a conversion, ask the stud maker to confirm in writing that its figure applies to your car, and keep that document.
The tightening does not change: by hand first, in stages, crosswise, a torque wrench for the final figure, and a second check after 50 to 100 km, which the fitting instructions above call essential at every wheel or tyre change (point 8). Our guide to wheel nut torque explains why friction changes the clamp load.
Locking nuts, track days and broken studs
Locking nuts. A conversion swaps the locking bolt for a locking nut with the right thread and seat; McGard's engagement rule applies to its locks too. If the old key is lost, deal with that first: see lost locking wheel nut key.
Track days. Studs pay off most when wheels come off often. Our track day checklist covers the rest of the car, and our guide to center lock wheels covers the other conversion, to a single central nut.
A stud that breaks or seizes. A stud that turns with a seized nut, or snaps, is a separate repair: see stuck lug nut or broken wheel stud. One loose, missing or broken fixing is enough to stop: have the wheel checked before you drive on.
What the rules say, country by country
We found no text in these countries that names a stud conversion. The rules look at the result.
| Country | Text | What it says |
|---|---|---|
| Great Britain | MOT inspection manual, 5.2.1 and 5.2.2 | "A wheel with a loose or missing wheel nut, bolt or stud": major; more than one: dangerous; "A wheel and its fixings not compatible": major |
| Great Britain | Construction and Use Regulations 1986, reg. 100 | Parts and accessories kept in such condition that "no danger is caused or is likely to be caused" |
| United States (Pennsylvania) | 67 Pa. Code § 175.80, (e)(1) | Reject if "The wheel nuts or bolts are missing, loose or have improper thread engagement" (xi), or if spacers over 1/4 inch are used to increase wheel track (xxi) |
| Australia (New South Wales) | Transport for NSW, VSI 09, revision 5, 28 February 2019 | "The wheel must not prevent the wheel nuts from fully engaging their studs"; the wheel must suit the hub "in respect to bolt pitch circle diameter and wheel nut tapers"; no spacers unless fitted by the vehicle maker |
What we do not know: we found no US federal rule on the fixings of passenger car wheels, and each inspecting state has its own code, so Pennsylvania is only an example. We read no Canadian text on this point; in Canada or another Australian state, read your own inspection standard.
In Europe, a replacement wheel sold under UN Regulation 124 comes with papers that name its fixings. The KBA document we read sets a condition on its fixing S01: "Only the serial fixing components No S01 (see page 1) are to be used to attach the replacement wheels." For that wheel, the car's own bolts are the allowed fixings and studs fall outside its papers. Other wheels may differ: the wheel's papers decide.
Wheels from Onyx Forged
We do not tell you whether to convert. Onyx Forged wheels are designed to JWL/VIA standards for the car you describe, and the seat and the thickness at the bolt hole are part of that design: if you run studs, say so, and ask us for both before you buy fixings. You can start a design from customise.
Frequently asked questions
Is a stud conversion worth it on a BMW, VW or Audi?
It makes fitting the wheel easier and spares the hub thread, which counts when wheels come off often, on track for instance. It does not hold the wheel any better: the clamp still comes from torque and friction.
What torque for wheel studs and nuts?
The figure of the maker of your studs and nuts, not the car maker's bolt figure. One BMW parts seller gives 75 lb ft (101.7 N·m) for its M12 nuts and 90 lb ft (122.0 N·m) for its M14 nuts, for its parts only, and the same page also caps its nuts at 75 lb ft: get the figure for your parts in writing.
How much thread engagement do lug nuts need?
McGard asks for at least the stud's diameter: 12 mm on an M12 stud, 14 mm on an M14. A BMW parts seller gives about 10 mm for M12 bolts and 12 mm for M14 bolts. Count the turns by hand and multiply by the pitch: 11.2 turns of an M14 x 1.25 nut make 14 mm.
How long should wheel bolts be?
Length under the head = thickness clamped + spacer + engagement in the hub (at least the thread diameter, per McGard). A 15 mm spacer needs a bolt 15 mm longer. Too long is a fault too, if the bolt bottoms in the hub.
Do I need new nuts with new wheels after a conversion?
Only if the seat changes. A Volkswagen group wheel made for ball-seat bolts takes ball-seat nuts; a wheel with 60 degree cones takes cone nuts. EUWA warns that a wrong radius or angle may loosen the nut or make the wheel fail.
Can I use M14 x 1.5 nuts on a BMW?
Only on studs made with a 1.5 outer thread, or on the handful of BMWs with M14 x 1.5 in the hub. Most BMWs from the F models have M14 x 1.25 in the hub, and some race studs use 1.25 in the hub and 1.5 for the nut. A 1.5 nut on a 1.25 stud starts, then binds and damages the thread.
Will a stud conversion fail the MOT?
The MOT manual does not mention conversions. It fails a loose or missing nut, bolt or stud as major, more than one as dangerous, and a wheel and fixings that are not compatible as major.
Should I put grease or anti-seize on wheel studs?
Not on the outer thread. The replacement wheel instructions we read ban lubricant on the threads and the hub face; the BMW parts seller says no anti-seize or oil outside, threadlocker on the hub end only.
Sources
- EUWA, ES 1.07, Safety and Service Instructions for the use of Wheels in Tyre Workshops (June 2020), 3.3; EUWA, ES 1.02, Using Description Including the Fixation (May 2011), 2.2, 2.3 and 4.3.
- Kraftfahrt-Bundesamt, communication no. 1751 under UN Regulation No. 124, variant V7 (28 February 2020); fitting instructions for a replacement wheel (27 January 2022), in German.
- McGard, installation and removal instructions.
- A US BMW parts seller: BMW wheel stud and nut install and genuine BMW wheel bolt M14 x 1.25.
- Audi Original Zubehör, anti-theft wheel bolts M14 x 1.5; Porsche AfterSales, bulletin 911 (992) 38/25.
- GOV.UK, MOT inspection manual, 5.2.1 and 5.2.2; Road Vehicles (Construction and Use) Regulations 1986, regulation 100.
- Pennsylvania Code, 67 Pa. Code § 175.80; Transport for NSW, VSI 09, Guidelines for alternative wheels and tyres.
- NIST Special Publication 811, Appendix B.9, conversion factors.
All pages were read on 16 October 2026.
The Onyx Forged design office.