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?
Do nuts need to match the bolt class?
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?
Generated by the language model.
