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Old 16th May 2019, 17:34   #1
hinged_bap
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Default How much torque to shear an M8 12.9 bolt

Quick question ( I have googled it but couldn;t find an answer ).
I have a job lined up at the weekend that involves removing M8 thread Bolts from an alloy cylinder head, they are 12.9 - ( there are 9 of them and they attach the CAT ). Can anyone suggest a torque where they will start to shear.
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Old 16th May 2019, 17:56   #2
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Assuming the bolts were torqued into the head correctly and the thrads are clean, I'd say they will undo without shearing, however tight they are. On the other hand, if the threads are corroded together, I guess that changes things. There are lots of data online about the shear strength of 12.9 bolts, but how that's translated into a torque wrench value I couldn't guess. Is there anything in these links that you can understand?


Click 1: https://itstillruns.com/129-bolt-she...-11415080.html


Click 2: https://www.wtools.com.tw/STANDARD-B...G-TORQUE.shtml


Click 3: https://www.amesweb.info/Screws/Metr..._Strength.aspx


There are lots more, little of which I understand.


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Old 16th May 2019, 18:04   #3
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I would not worry about that at all. The bolts will come out, and will tear the aluminium before they shear. Provided they are not corroded.
If you are worried, soak them in penetrating oil for 24 hours before you remove them.
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