FAQ
TL;DR: Class 12.9 bolts provide ≥1220 MPa tensile strength—about 17 % higher than 10.9—"She is always tougher" [ISO 898-1; Elektroda, kkknc, #16666819]. Use 10.9 for most suspension links; switch to 12.9 where OEM specifies or where shear is critical.
Why it matters: Matching bolt class to load keeps joints safe, serviceable, and corrosion-free.
Quick Facts
• Class 10.9: ≥1040 MPa tensile, ≥940 MPa yield strength [ISO 898-1].
• Class 12.9: ≥1220 MPa tensile, ≥1100 MPa yield strength [ISO 898-1].
• M10×1.5 torque: 60–70 Nm (10.9) or 70–90 Nm (12.9) [Elektroda, tadkli9743, post #16667619]
• Suspension bolts are mostly 10.9; brake-caliper yoke bolts often 12.9 [Elektroda, kkknc, #16666775; #16668764].
• ISO 898-1 lists typical fracture elongation: 12 % (10.9) vs 9 % (12.9) [ISO 898-1].
What do the numbers 10.9 and 12.9 on a bolt mean?
The first number (10 or 12) ×100 gives minimum tensile strength in MPa; the second number (9) × first/10 gives minimum yield strength. A 10.9 bolt therefore guarantees ≥1040 MPa tensile and ≥940 MPa yield strength; 12.9 guarantees ≥1220 MPa and ≥1100 MPa [ISO 898-1; Elektroda, kkknc, #16666819].
Is a 12.9 bolt always stronger than a 10.9 bolt?
Yes. Both tensile and yield limits are higher for 12.9, so it resists greater axial load before plastic deformation or fracture [ISO 898-1]. "She is always tougher" confirms forum experience [Elektroda, kkknc, post #16666819]
Which class is normally used in car suspension?
Manufacturers specify class 10.9 for most wishbone, hub and link fasteners; some high-shear spots like brake-caliper yokes use 12.9 [Elektroda, kkknc, #16666775; #16668764]. OEM markings on old bolts tell you the class.
Does the higher class make the bolt brittle?
Higher class means higher hardness and slightly lower elongation (9 % for 12.9 vs 12 % for 10.9) [ISO 898-1]. That reduces stretch margin but does not cause sudden shattering in normal service because the core remains tougher than the case, per surface-hardening practice [Elektroda, kkknc, post #16666775]
Will a 12.9 bolt damage cast-iron threads in a brake yoke?
No, provided you torque to spec (≈70–90 Nm for M10×1.5) and use thread locker. Load goes through the clamp interface; the cast iron sees mainly shear, which stays below its limit [Elektroda, robokop, post #16668768] Over-tightening, not class, strips threads.
Can I substitute stainless A4-90 for class 10.9?
A4-90 gives 1030–1100 MPa tensile and 900 MPa yield, close to 10.9, but its lower modulus and different galling behaviour make it unsuitable for high-dynamic joints like suspension [Elektroda, Anonymous, post #16666881] Use only where OEM approves.
What torque should I apply to an M10 brake-caliper yoke bolt?
Service data list 60–70 Nm for class 10.9 and 70–90 Nm for 12.9, ensuring 70 % of yield without creep [Elektroda, tadkli9743, post #16667619]
Do nuts need to match the bolt class?
Yes. A softer nut will strip before the bolt reaches preload. Use 10.9 nuts with 10.9 bolts and 12.9 nuts with 12.9 bolts [Elektroda, przemek25l, post #16667030]
Do fine threads weaken a fastener?
Shear area is lower, so thread-strip strength drops slightly, but tensile capacity set by class and core area remains unchanged [Elektroda, bearq & kkknc, #16666834; #16666850].
When is thread locker recommended?
Apply medium-strength locker on suspension bolts that see vibration yet should be serviceable, such as brake-caliper yokes. Avoid it on wheel studs where frequent removal is required [Elektroda, robokop, #16668768; kkknc, #19574374].
How can I avoid breaking corroded suspension bolts?
- Drench threads with penetrating oil and wait 10 min. 2. Use gentle heat (≈200 °C) to expand the nut/joint. 3. Apply steady torque; avoid impact until last resort. This limits torsional shock that snaps rust-seized shanks [Workshop best-practice].
Are cheap replacement bolts risky?
Yes. Forum users report branded 8.8 bolts outperforming unbranded 10.9 due to poor metallurgy [Elektroda, tata1, post #16666894] Buy reputable OE or ISO-certified fasteners.
What happens if a bolt is loaded beyond its yield?
It elongates permanently; above ultimate strength it necks and snaps. Class 10.9 can stretch ~12 % before fracture; 12.9 only ~9 % [ISO 898-1]. This smaller margin means sudden failure is an edge case when over-torqued or impacted.
Why isn't thread locker used on wheel studs?
Studs rely on friction between wheel and hub, not thread retention. Locker can act as lubricant during tightening, reducing clamping force and risking wheel loss [Elektroda, Strumien swiadomosci swia, #19574415].
