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Choosing Between Bolt Strength Classes 12.9 and 10.9 for Car Suspension Repair

pawlik118 41541 33
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Is a 12.9 bolt always stronger than a 10.9 bolt for a car suspension repair, and which class should be used in loaded suspension joints?

A 12.9 bolt is stronger than a 10.9 bolt, so it is not “less durable” because of brittleness in normal use; the thread class indicates higher tensile and yield strength, and one reply notes the bolt is surface hardened with a softer core [#16666819][#16666775] For suspension work, the common practice is to use the class specified for that exact joint: 10.9 is most often used in suspension, while 12.9 is used in some very heavily loaded rigid connections such as connecting-rod caps, flywheel bolts, and often brake caliper/yoke bolts [#16666775][#16666881][#16668764] The safest approach is to fit the original or manufacturer-specified bolt class and tighten it to the correct torque rather than choosing the highest class blindly [#16667619][#16666894] For the brake yoke example, one reply says the bolt mainly clamps the yoke, shear load is small, and a 60–90 Nm torque with threadlocker is sufficient for 10.9–12.9 bolts [#16668768]
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  • #1 16666754
    pawlik118
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    The problem arose because of the suspension repair in the car - when unscrewing, due to corrosion, the bolts securing the wishbones broke off. Therefore, the question arises how to practically approach the class of bolts determining its strength. In general, I read that the higher the bolt strength class, the greater the stresses and impacts it can transfer. Following this lead, if you want to have the most reliable screw connection, it is best to use a top-class screw (12.9). Somewhere in turn it was written, the higher the class, the more brittle the bolt is and the more susceptible to cracking under impacts. In places where there are shocks (suspension), is it worth using 12.9 or better 10.9?

    What class of bolts are used in the suspension in the places with the highest loads (e.g. brake pad yoke, mounting of rocker arm pins, etc.)?
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  • #2 16666775
    kkknc
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    You can break seized screws in every class. In the suspension, 10.9 is most often used. Where not where 12.9.
    Yes, it is true that the harder the more brittle. Only the screws are surface hardened. The core is soft. So there is no problem with cracking. On the old ones you have the hardness class stamped on the heads. Harder prices.
  • #3 16666783
    pawlik118
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    price does not matter. Unfortunately, the broken bolts do not have any markings.
    So I wonder how the "fragility" compares to the endurance defined in the class.
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  • #4 16666790
    bearq
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    In addition, there is also some kind of workmanship, thread type, pitch or diameter. It is impossible to clearly answer the question of what bolts, because what brand or model is something else.
  • #5 16666813
    kkknc
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    Apply what is meant to be.
  • #6 16666815
    pawlik118
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    I am asking about the class of screws and not the quality of their workmanship. Pitch and diameter also have nothing to do with the strength class of the bolt.
    The basic questions are:
    - is the 12.9 bolt always more durable than 10.9 or maybe less due to its greater "brittleness" (somehow I doubt) ??
  • #7 16666819
    kkknc
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    She is always tougher.
    After all, the first digit is the information about the minimum tensile strength Rm in N / mm2 divided by 100. And the second digit is the ratio of the yield stress Re to the tensile strength Rm divided by 10.
  • #8 16666834
    bearq
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    pawlik118 wrote:
    I am asking about the class of screws and not the quality of their workmanship. Pitch and diameter also have nothing to do with the strength class of the bolt.
    The basic questions are:
    - is the 12.9 bolt always more durable than 10.9 or maybe less due to its greater "brittleness" (somehow I doubt) ??

    Of course, it is related, because even a fine thread will have a lower breaking strength in the case of the same class as the "normal" screw.
    You ask too general a question, it is like asking what is better steel or cast iron and one cannot be replaced with another. The use of the appropriate class of bolts is primarily related to the forces acting on them.
  • #9 16666850
    kkknc
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    bearq wrote:

    Of course, it is related, because even a fine thread will have a lower breaking strength in the case of the same class as the "normal" screw.

    Doesn't matter, see what I gave above. The class indicates the strength of the bolt in relation to the cross-section. The shear strength of the thread is another matter.
    Don't get confused.
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  • #10 16666881
    Anonymous
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  • #11 16666894
    tata1
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    Also, gentlemen, pay attention to the quality of the screw. For example, the one from Łańcut 8.8 (LF) will withstand more than the unknown brand 10.9.
    Coming back to the "car", these bolts supplied with rocker arms and pins "replacements" are a fear to install - they look like indulgence. In such cases, I prefer to mount it on the old, original screws. Often, mounting such a screw is asking for trouble when unscrewing it again. The original will always unscrew.
  • #12 16666911
    kkknc
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    You buy a good replacement from a well-known company, no problem. And as the cheapest is what you are asking for trouble yourself.
    And somehow the use of acid-resistant screws in the suspension does not make sense, but who will forbid the rich.
  • #13 16666925
    ociz
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    Not necessarily. For example, TRW will have worse screws in the kit than Delphi. Master Sport cheap Chinese food and the screws give a decent ...
  • #14 16666973
    Strumien swiadomosci swia
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    But you can buy only high-end bolts for clutch, flywheel, suspension at automotive wholesalers.

    Besides, for example, I collect bolts from the demolition of cars, I currently have about 60 kg of various bolts and nuts, mainly from English.
    They are useful and there are much better niches for thick PLN.

    The question of whether 4.2 versus 8.8 or 12.9 is more durable is childish.
  • #15 16667030
    przemek25l
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    Just let the author of the post not forget about the nuts of the appropriate class because he threads the screws as he releases the thread in the nuts.
  • #16 16667042
    Strumien swiadomosci swia
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    pawlik118 wrote:
    What class of bolts are used in the suspension in the places with the highest loads (e.g. brake pad yoke, mounting of rocker arm pins, etc.)?


    The greatest loads are on the connecting rods feet, head, shaft, satellite basket, joints, wheel hubs.

    There you need cosmic bolts and apply exact moments in order not to stretch the limit of elasticity of the bolt.
  • #17 16667386
    pawlik118
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    kkknc wrote:
    Apply what is meant to be.


    what should be?
  • #18 16667399
    Strumien swiadomosci swia
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    pawlik118 wrote:
    kkknc wrote:


    Apply what is meant to be.




    what should be?


    You know what bolt and where you buy it.
  • #19 16667521
    pawlik118
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    Thanks for the two answers in the topic and the other dozen not related to the question. Main question - is the 12.9 bolt always more durable than 10.9 or maybe less due to its greater "fragility" (somehow I doubt) ??
  • #20 16667602
    Strumien swiadomosci swia
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    pawlik118 wrote:
    is the 12.9 bolt always stronger than the 10.9 bolt


    YES.
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    #21 16667619
    tadkli9743
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    Welcome

    I looked through the Mitsubishi Colt manual, only found the tightening torques of the bolts depending on their class.

    greetings
    Attachments:
    • STANDARD PART.docx (16.09 KB) You must be logged in to download this attachment.
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    #22 16667692
    tadkli9743
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    Hello.

    I found such materials, maybe they will be useful to you.

    greetings
    Attachments:
    • sruby_wl_mech.pdf (87.55 KB) You must be logged in to download this attachment.
    • 5479_Wprowadzenie.,Uszkodzenia,elementow,pojazdow.pdf (1.26 MB) You must be logged in to download this attachment.
  • #23 16667876
    pawlik118
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    I am also concerned about this "brittleness" - that is, what happens when the bolt is subjected to stresses greater than its strength class. Is it not the case that a lower class screw will be able to elongate more than a higher class screw that will break faster (because it is less plastic)?
  • #24 16667989
    kkknc
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    Yes, it will be longer. Although the difference is slight.
    The only question is what will this knowledge give you and how will you use it?
    'Cause you start to go round and round.
  • #25 16668235
    ociz
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    pawlik118 wrote:
    what should be?

    Often, after the number of the original, you can find substitutes or even the original in stores, and the prices are not discouraging at all, not counting the eccentric ones that adjust the geometry of the suspension, because they are usually expensive.
  • #26 16668383
    pawlik118
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    I just want to know what it is like with the class and strength of screws. Knowledge is the main factor that made me start this topic.
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    #27 16668422
    kkknc
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    You got the knowledge almost in the initial post. And then you keep repeating the questions.
  • #28 16668714
    pawlik118
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    The whole discussion in my group was about the broken bolt securing the yoke holding the brake pads. The yoke in the Nissan Almer is fixed with an M10x1.5 bolt (that's all I know). The screw is screwed into the yoke by about 7mm - not enough. The yoke is made of cast iron. According to the comparison from col. Tadkli9743, 12.9 is more stiff. Another complication is the influence of variable temperature - will the stiffness of the bolt greater than the assumed stiffness cause faster fatigue degradation of the threads in the yoke. The safest way is to order the bolts through the service. It is not known what class of screws are better at this point in practice.
  • #29 16668764
    kkknc
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    The yoke screws are practically always 12.9
  • #30 16668768
    robokop
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    But this bolt only "holds" the yoke - the shear moment is minimal as there is still friction between the yoke and the knuckle ears. And you do not have to tighten it until all the juice is released, a torque in the range of 60-90Nm (screws 10.9-12.9) is enough to tighten it. In the absence of a torque wrench - a ring spanner, without an extension, plus a thread braking agent - from the Loctite offer or the much cheaper "Prolok". Not only will it not unscrew by itself then, but the thread will not stick to rust.
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Topic summary

✨ The discussion centers on the selection of bolt strength classes 12.9 and 10.9 for car suspension repairs, particularly in relation to the durability and brittleness of these bolts. Participants note that while class 12.9 bolts are stronger, they may be more brittle, which raises concerns about their performance under shock loads in suspension applications. Class 10.9 bolts are commonly used in suspension systems, and their lower strength is often compensated by their ductility. The conversation also highlights the importance of proper torque application and the quality of bolts from reputable manufacturers, as well as the need to match bolt classes with appropriate nuts to prevent thread damage. Specific applications, such as yoke screws in brake systems, are discussed, with recommendations leaning towards using class 12.9 bolts for critical components.
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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?

  1. 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.
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