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Old 26 November 2013, 12:00   #11
Anchorhandler's Avatar
Country: France
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Originally Posted by martini View Post

High tensile bolts are the right ones for the job, stamped with 4.8 on the head rather than 8.8.
Hi Matt, What's your objection to 8.8?

4.8 is generally refereed to as carpentry grade over here used in assembling wooden structures.
In engineering terms we only really start considering a bolt to be high tensile at 8.8 or above.

Of course the Op could opt for A2-80 which is high tensile stainless if he's worried about it.


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Old 26 November 2013, 17:29   #12
martini's Avatar
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Sorry, got that the wrong way round. 8.8 is the higher strength - 4.8 & 8.8 are the 2 that we normally see floating around the UK. Both are ok, one more ok than the other though

Actually this has prompted me to look up tensile strength on a few bolts and it's quite surprising. A2/70 = 700Nmm2, 8.8 HT = 800Nmm2, not much in it

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Old 27 November 2013, 02:18   #13
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Thanks for all your input, I'll be keeping with the standard high tensile bolts.
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Old 27 November 2013, 03:00   #14
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Standard Stainless is tough.... The main issue can be binding of the nut to the bolt when over tightened or the thread damaged. The screw may have to be ground off if it 'picks up' on the thread..

If the bolt is under so much stress as that the difference between 8.8 and stainless is a concern, then maybe the design is too marginal, or you are building a race boat/landrover winch.
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Old 27 November 2013, 05:41   #15
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Don't know how old your vehicle is, but i believe that from 98 onwards all towbars and related parts have to be as homologated for the E mark, which presumably means that the bolts need to be to same or higher specification as tested?
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Old 28 November 2013, 07:25   #16
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For information, the bolt designation for non stainless is as follows. The first number is the tensile strength designator. multiply by 100 to get the tensile strength in MPa. The second number is the yield stress multiplier. Multiply the tensile stress by second number to get the yield, e.g. a 4.6 bolt has a yield of 400MPa and a tensile strength of 240MPa (0.6*400). High tensile bolts are regarded as 8.8 and above (other designations are 10.9 and 12.9).

Stainless ones are a little different in categorisation. Austenitic bolts (A series) come in grades A1 to A5, with the more common grades being A2 and A4. Tensile strength is provided but due to differences in behaviours, yield strength is replaced by the stress at 0.2% permanent strain. Strengths are as follows:-

Grade______Property class____Tensile Strength_____0.2% proof strength
A1 and A2_______50___________500MPa_____________210MPa
A3 and A4_______70___________700MPa_____________450MPa
A5_____________80___________800MPa_____________600 MPa

The equivalent of a grade 8.8 is thus an A5 bolt, not an A2, which are weak as cheese. For a permanent bolted joint tightened to a high (and hopefully specified/measured torque), such as a tow bar, it is important to match the yield and tensile strengths as this will affect the strength of the joint.

Another notable point is the galvanic potential of the materials. Although the stainless bolts are less prone to corrosion, if in direct contact with unpainted mild steel, they will cause accelerated corrosion of the mild steel. I have seen many stainless pipes connected with mild steel bolts which have turned to dust in a matter of only a few years. It is common in industry to use a more corrosion prone bolt as its easier/cheaper to replace a bolt rather than the item being bolted.

It is also worth noting that A4 is more corrosion resistant than A2, hence used more often on boats, even where the strength is not required.

Hopefully these ramblings will be of use to somebody!

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