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Aluminium thread strength

How does the depth effect strength?

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Luke Parsons13/07/2020 12:50:19
1 forum posts


Can anyone help me?

I have an aluminium block that I need to tap an M8 hole in to hold 2 parts together, the joint will need to support a reasonable large weight. I have 2 options, either and 18mm long thread or a 12mm thread.

Can anyone tell me how much tensile strength I will lose if I use the 12mm instead of the 18mm?



old mart13/07/2020 13:13:23
2912 forum posts
184 photos

There would not be much advantage of the longer thread, but if you are concerned, then follow the aircraft industry proceedure with almost all threads in light alloys and use Helicoil type wire thread inserts. 1 1/2 diameter coils will be 12mm deep and will be stronger than any ordinary depth straight into the aluminium.

Tim Stevens13/07/2020 13:14:49
1423 forum posts

The answer depends on the exact 'recipe' for the alloy you describe as 'aluminium', the way it was formed, particularly whether cast or forged or extruded etc, the way it has been heat treated and how long ago, and that is only dealing with the block. (Similar questions apply to the threaded component - what sort of steel etc, and how was the thread formed? And then we come to how the weight applies force to the block - in tension, or shear, etc - but perhaps we can deduce that the load is tensile only.)

If everything is done to reasonable engineering standards, a thread 18mm long in an aluminium alloy component is not likely to be stronger in tension that a similar 12mm thread. But as I have hinted above, it all depends.

If you need an answer that you can rely on (in court, etc) you need to offer much more detail and expect to pay a serious sum for insurance cover by an expert - which I am not.

Cheers, Tim

Paul Lousick13/07/2020 13:36:21
1720 forum posts
627 photos

Hi Luke, My engineering theory is a bit rusty but common practice for depth of hole for bolt pullout is:

Grade 5 steel bolt in mild steel threaded hole allow 1 diameter actual engagement,

(Head and main bolts in cast iron engine components commonly have 1 1/4 diameters engagement)

Grade 5 bolts in aluminum or stainless steel 1 1/2 diameters, (what grade bolts are you using ?)

Grade 8 bolts in aluminum 2 diameters.

(For extra pull out strength use a Heli-coil insert)


old mart13/07/2020 13:41:02
2912 forum posts
184 photos

While you are considering options, make sure you eliminate the bolt factor by using 12.9 high tensile bolts, they are much stronger than 8.8 or stainless steel.

Cabinet Enforcer13/07/2020 13:44:54
102 forum posts
3 photos

Hi Luke, do you mean 12mm of thread on the bolt, or do you mean 12mm of thread engagement?

If you mean thread engagement, then you are in over your head, and you should probably start from the beginning to explain.

If you mean 12mm of thread on the bolt, then you are in waaay over your head, and definitely need to start from the beginning.

The fact that you have sought help on this shows you feel it is important, and I am sure people here will be willing to help, there are potentially very straightforward answers to your question, but without context it is impossible to know.

Clive Hartland13/07/2020 14:40:01
2667 forum posts
40 photos

My experience of threads in Aluminium come from a military aspect. Threads to hold the flotation attachments onto the aluminium hull used thread inserts locked in place wuth 3 spear like pegs that prevented rotation and completely secured the thread insert. Also that fine threads in alu. are not as strong as coarse threads, hence the thread inserts of a finer thread and the inserts in a coarser thread. Depth was over 2 dia.

Michael Gilligan13/07/2020 22:23:16
17864 forum posts
825 photos
Posted by Tim Stevens on 13/07/2020 13:14:49:

The answer depends on the exact 'recipe' for the alloy you describe as 'aluminium', the way it was formed, particularly whether cast or forged or extruded etc, the way it has been heat treated and how long ago, and that is only dealing with the block. […]


I’m a little late to this thread, but I feel I must just endorse Tim’s observation ^^^

Sorry, Luke, but “Aluminium“ is simply not an adequate description of anything other than the pure metal.

... Nothing personal; it happens a lot !


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