Short answer: your wheel bolt has a shaped face where it meets the wheel. It has to match the shape of the hole in the wheel. A 60 degree cone, a rounded ball, or a flat washer. Mismatch them and the bolt only touches at one small point, which means it cannot hold proper clamping force, works loose, and can take the wheel off the car.
This is one of the most dangerous and least understood parts of fitting wheels, and the parts cost about £20.
The three seat types
| Seat | Looks like | Typically found on |
|---|---|---|
| Conical / tapered | A 60 degree cone | Most aftermarket wheels, VW, Audi, SEAT, Skoda, Ford |
| Ball / radius | A rounded dome | Many Mercedes, Honda, some Renault |
| Flat / washer | Flat face with a captive washer | BMW, Mini, some Porsche |
Look at the bolt holes in the wheel. A tapered hole wants a conical bolt. A rounded hole wants a ball seat. A straight hole with a flat face wants a flat seat with a washer.
They are easy to tell apart once you have seen them side by side, and impossible to spot if you never look.
Why a mismatch is genuinely dangerous
A wheel is not held on by bolts passing through holes. It is held on by friction, created by clamping the wheel hard against the hub face. The bolt is a very stiff spring, stretched slightly when you torque it, and that stretch is what generates the clamping force.
For that to work, the bolt's seat has to make full, even contact with the wheel. Matched correctly, the cone sits in the cone and contact is spread all the way round.
Mismatched, you get a thin ring of contact, sometimes a single point. Three things follow:
- The torque wrench lies to you. It reads correct torque, but most of that is the bolt deforming the wheel rather than stretching the bolt. You have far less clamping force than you think
- The contact point beds in. Over the first fifty miles the metal deforms, the bolt loses tension, and the wheel is now loose
- The wheel starts to move. Once it moves at all, the holes elongate rapidly and it gets worse quickly
Wheels coming off cars is rare. When it happens, the wrong bolts are one of the usual causes.
Three other things to get right
Thread size and pitch
Common UK and European sizes:
- M12 x 1.5 — Ford, many VAG, older BMW
- M14 x 1.5 — Audi, VW, Range Rover, Porsche
- M14 x 1.25 — newer BMW, including all G chassis
- M12 x 1.25 — some Japanese
M14x1.5 and M14x1.25 will both start to thread into the same hub. They will feel wrong after a turn or two and will strip the hub if you force them. If a bolt does not spin freely by hand for its full length, stop.
Shank length
The bolt must engage the hub by at least 1.5 times its own diameter. For an M14 bolt that is about 21mm of thread in the hub.
Too short and you do not have enough thread holding the car on. Too long and the bolt bottoms out in the hub, which feels exactly like correct torque while clamping nothing at all. Both failures feel fine on the driveway.
This is why spacers need longer bolts, which we covered in the BMW spacer guide.
Torque, and re-torque
Use a torque wrench, not a windy gun and a guess. Typical road car figures run 110 to 140Nm, but use your car's figure.
Tighten in a star pattern, never round the circle, so the wheel pulls onto the hub squarely. Then re-check after about 50 miles. New wheels bed in slightly and almost always need a nip up. Every race team does this. Almost no road driver does.
When you need new bolts
- Moving from steel wheels to alloys. Steel wheels are thinner, so the original bolts are usually too short
- Changing wheel brand. Seat shape and the depth of the bolt hole both vary
- Fitting spacers under 20mm. The spacer thickness comes straight off your thread engagement
- Any bolt that is stretched, rounded or rusted. They are a consumable, not a lifetime part
Our OEM style wheels are machined for the standard bolt seat of the car they are listed against, so your original bolts normally carry over. Branded wheels from our premium range are more often conical and may need a bolt change, which is worth asking about before you order rather than discovering on fitting day.
Locking bolts
Worth two sentences. Locking bolts have the same seat requirement as every other bolt, so the same rules apply. And keep the key somewhere you will find it, because a locking bolt with no key is a specialist removal job and it is never convenient.
Common questions
What happens if you use the wrong wheel bolts?
If the seat shape does not match the wheel, contact is reduced to a thin ring or a point. The bolt cannot generate proper clamping force, it beds in over the first miles and loses tension, and the wheel can work loose. A torque wrench will still read correctly, which is what makes it dangerous.
How do I know if I need conical or ball seat bolts?
Look at the bolt holes in the wheel. A tapered, cone-shaped hole takes conical bolts. A rounded hole takes ball seat. A straight hole with a flat face takes flat seat bolts with a washer.
Do I need new bolts when fitting alloy wheels?
Often yes, particularly when moving from steel wheels, since alloys are thicker and the original bolts may be too short. Changing wheel brand can also change the seat type or hole depth.
How much thread should a wheel bolt have in the hub?
At least 1.5 times the bolt diameter, so about 21mm for an M14. A bolt that bottoms out in the hub feels like correct torque while clamping nothing, which is as dangerous as one that is too short.
Should I re-torque wheel bolts after fitting?
Yes. Check them after about 50 miles. New wheels bed in slightly and bolts commonly need a small amount of retightening. Always tighten in a star pattern rather than around the circle.
Are M14x1.5 and M14x1.25 bolts interchangeable?
No. Both will start to thread into the same hub and feel wrong after a turn or two. Forcing one will strip the hub thread, which is an expensive repair. If a bolt does not turn freely by hand for its full length, stop.
Not sure what your car takes? Send us your make and model and we will confirm the thread size, seat type and whether your existing bolts will carry over, before you order. sales@omiwheels.com or WhatsApp +44 7753 828059.
Related reading: do I need spigot rings and what size wheel spacers you need.