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Aluminium thread strength

Is it better to use an insert or increase the bolt dimensions

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Absolute Beginner14/08/2018 21:04:13
103 forum posts

Good evening gentlemen.

Based on the success of my last thread, some great ideas. And problem solved. I wonder if I could ask the general consensous ref threads and bolts within aluminium.

I have a need to secure one piece of aluminium to another. I am restricted by the width of the material I need to secure. Eg 8 or maybe 10mm Aluminium bar.

I need to try and create the strongest joint possible.

  1. Would I be better.....Tapping the aluminium to take a 6mm bolt ?
  2. Tapping the aluminium to take a 6mm helical insert or similar and using the same 6mm bolt
  3. Drilling a clearance hole and putting a nut onto the bolt thread?
  4. Moving up a bolt size to say 8mm bolt tapped into the aluminium?

The tapped hole could be up to 30mm in length.

I am sorry if this seems a trivial question, but I am still learning.

Again Many thanks in anticipation of your help.


Michael Gilligan14/08/2018 21:08:57
13780 forum posts
599 photos

Not a trivial question at all, Gary

My selection from your 'multiple choice' is 2

.. explanation to follow, if the discussion gets interesting


richardandtracy14/08/2018 21:20:51
938 forum posts
10 photos

As a stress engineer as part of my daytime job...

It's more down to the aluminium grade and if you are going to remove the thread often. More than 2-3 undoings in 6082T6 and you need an insert. About 2 D insert length is as strong as a grade 8.8 bolt. 3D (18mm) for a 12.9. For no insert, add 6mm (1D) to get the same strength. Much over 3D and not all the engaged thread works, just the first 3D.

So, 3D insert for cap heads, or insert for many removals. Else no insert.

In 6063, the aluminium is weaker, add 0.5D for engagement length, limit 3D again.

For 2014/7075 almost don't need an insert.



Absolute Beginner14/08/2018 21:40:43
103 forum posts


What a fanatic concise reply. Thank you. The material is 6082T-6 not anticipated to have to remove the thread once assembled. I had no appreciation that 2 D insert length + approx 8.8 bolt strength. I wish all structural engineers could produce such readable and understandable answers. Again many, many thanks.


duncan webster14/08/2018 21:59:24
2197 forum posts
32 photos

Option 3 was drill right through and use a nut. This is by far the easiest and completely eliminates the possibility of breaking the tap

richardandtracy14/08/2018 22:00:03
938 forum posts
10 photos

It is a rough approximation, but mostly right most of the time. Must point out it's 2D with insert or 3D without = grade 8.8 bolt strength in 6082-T6 provided the area of the bar is enough. Would tend to go for bigger is better.. But sometimes lots of small bolts is better.

Can't remember offhand the xsectional of an M6 (don't often use bolts smaller than M8 at work). We use 30.6mm^2 for an M8 as we use the minimum minor diameter for the stress area. Ideally the bar will be double the bolt diameter to ensure the normal load distribution within the thread, when less than 2D bar diameter a smaller length of thread seems to be worked harder. 10mm 6082 bar will have a strength of 295 N/mm^2 minimum, and yield of 255 when it starts stretching enough to be of interest.

Hope this helps



Emgee14/08/2018 22:08:28
1157 forum posts
206 photos

Hi Gary

It is best to use a coarser thread in aluminium if removing/fitting occasionally, so instead of M6 use 1/4"20UNC,
if frequently dismantling then I believe the bolt through with nut retention would work best.


Andrew Johnston14/08/2018 22:15:04
4773 forum posts
538 photos

The question needs to be better specified; is the "strongest" joint with the bolt in tension or shear?

Some years ago I did some experiments in 6082 with 12.9 M4 SHCS where I had limited engagement depth for bolting down a 3-phase bridge module for a power inverter. I tried tapped holes with varying percentages of thread engagement. The M4 bolt broke in tension with a hole drilled for 50% thread engagement. Depth of engagement when the bolt broke was about 6mm.


Jon14/08/2018 22:47:08
988 forum posts
46 photos

Need more info on the specific application.

Fine threads can and will take serious pressure measured in tonnes.

Next time you whip the head off your cars engine, have a look its more than likely an aluminium casting weaker than 6082T6.

Jon14/08/2018 23:18:37
988 forum posts
46 photos
Posted by Absolute Beginner on 14/08/2018 21:04:13:

I am restricted by the width of the material I need to secure. Eg 8 or maybe 10mm Aluminium bar.

I need to try and create the strongest joint possible.

  1. Would I be better.....Tapping the aluminium to take a 6mm bolt ?
  2. Tapping the aluminium to take a 6mm helical insert or similar and using the same 6mm bolt
  3. Drilling a clearance hole and putting a nut onto the bolt thread?
  4. Moving up a bolt size to say 8mm bolt tapped into the aluminium?

The tapped hole could be up to 30mm in length.

Take it the 8 or 10mm is the thickness of flat bar or plate. May find off the shelf comes in imperial so would be a nominal 7.9mm or 9.5mm thick.
All we can decipher here is two aluminium parts bolted or screwed together. This could be end on? Or the two parts lying on top of each other one with a through hole the other tapped?

Either way better looking at cap headscrews, will look better counterbored. Can get shallow head cap screws.
M4 head dia usually 6.82mm x 4mm deep (2.9mm shallow head)
M5 head dia usually 8.45mm x 4.8 deep (3.5mm shallow head)
M6 head dia usually in excess of 9.3mm

If end on M4 my choice as leaves decent strength for counterbore, may need more than one screw.

To give an idea 4 off M3 grade 8.8 with 6.8mm thread engagement will take well over a tonne. Likewise M30X0.75 with 7mm engagement will take several tonnes of pressure, its not all about thread engagement.

pgk pgk15/08/2018 02:01:08
1425 forum posts
278 photos

I like the question. I have no background in material stress matters (except how it pertained to soem aspects of fractured bone repair) and was still pondering my own thoughts when the replies above came in.

I also wondered about the differences in tensile stress compared to shear stress and the bolt size compared to bar width. I freely admit not having considered multiple fixings at all. It did strike me that one factor to consider is the core diameter of any thread compared to using a bolt with only a limited threaded portion on a full diameter shank and the implied access from the back of the part (30mm thick?) allowing one to countersink a nut-like fixing from that side. Since there is a thread then in tension I guess it should fail just as easily as a longer thread but in shear it might be stronger. Also I guess (and wait to corrected) that friction from compression across the joint may have a bearing?

Lastly if there is no likelihood of needing to undo that joint then perhaps bonding a full thickness pin of equal stength to a bolt bypasses the matter of a thinner core of a thread?


Barrie Lever15/08/2018 08:05:51
323 forum posts
1 photos


I am with Emgee on this, the UNC thread form feels much nicer in aluminium and his suggestion of 1/4 UNC gives a somewhat corser thread at 20 TPI versus the M6 x1 at 25.4. Both having a 60 degree thread angle then you can see a somewhat deeper radial thread engagement with UNC.

We dont see many UNC threads in aluminium, not because it is not good but rather our global manufacturing (everything apart from the USA) is very wedded to the metric system, which is good in almost all cases but not as good as UNC in this case IMO.

Given a free choice, every time it would be UNC into aluminium for me.


Edited By Barrie Lever 1 on 15/08/2018 08:06:26

richardandtracy15/08/2018 08:23:48
938 forum posts
10 photos

With most metals the ratio of shear to tensile stress is in the range 65% to 70%. There are one or two oddities as low as 63% and some as high as 72% (According to the US document MIL-HDBK-5J [now obsolete as free standards become less obtainable]). The general figure used is 65% unless specific information is available.

For a bolt in tension you should always use a stress area based on the minimum minor diameter if the thread as it's conservative. In shear, provided the bolt shank passes through the possible slip plane between two parts, the shank area may be used (which is larger) otherwise use the minor diameter based cross sectional area.

When there is shear, the bolt shank may abut on the metalwork on either side. The area needed depends on the bearing/bruising stress of the base metal, which (again from a survey of MIL-HDBK-5J) is between 1.9 and 2.1x the tensile strength. I conservatively use 1.9x. This then gives a handle on how far the centroid of the abutment load acts from the slip plane. This in turn allows a calculation of any bending stress that may occur in the bolt during shear loads. All this depends on the conservative assumption that there is no pre-load on the bolts and therefore no friction to overcome, all of which make the joint stronger than calculated.

Preload in the bolts is not additive to any tensile load unless you are going to analyse the joint to the nth degree. A reasonable working assumption is that as the joint is pulled apart, the pre-load at the joint is relaxed at a ratio of 1:1 while the bolt tension remains the same until the bolt pre-load is exceeded and the joint can gap. In practice the bolt tension goes up a little, the amount being dependant on a complex ratio between the bolt stiffness and joint stiffness. This is a reason why many cylinder head bolts are waisted, to make them stretch more, therefore see a lower load variation as the cylinder fires, and therefore have a higher fatigue life.



Carl Wilson 415/08/2018 08:30:53
668 forum posts
53 photos
It's nice to see a reference to MIL HDBK 5J. I thought it was just me that used it.

The case of the bolt threaded directly into aluminium is really dependent on what you want to do with it.

If you are building air or spacecraft or a nuclear reactor, then use an insert. If it is for a jobbing task in the workshop or home then threads into the aluminium will be fine.

I've seen threads directly into 6082 T6 take loads measured in tonnes.

Edited By Carl Wilson 4 on 15/08/2018 08:31:49

JasonB15/08/2018 08:32:36
15988 forum posts
1680 photos
1 articles

I think I would be worried about the lack of parent metal left if putting an M6 insert into 8mm thick material. A Helicoil would be about 7.3mm OD and other inserts even more, that leaves only 0.35mm or less of metal either side of the thread.

Carl Wilson 415/08/2018 08:41:44
668 forum posts
53 photos
As others have said, on 8mm thick material a nut and bolt is the answer.
Michael Gilligan15/08/2018 09:00:13
13780 forum posts
599 photos
Posted by Absolute Beginner on 14/08/2018 21:04:13:

I have a need to secure one piece of aluminium to another. I am restricted by the width of the material I need to secure. Eg 8 or maybe 10mm Aluminium bar.


Reference Jason's comment ... I am struggling to understand the geometry of the job

Gary's opening post refers to a width of '8 or maybe 10mm'

... and Jason refers to '8mm thick'

Neither makes reference to the dimensions of the other component [which is what I thought was the one to be tapped], but Gary did mention that "The tapped hole could be up to 30mm in length."

I'm probably being dim, but a rough sketch by Gary might clarify.



Edited By Michael Gilligan on 15/08/2018 09:05:35

Hopper15/08/2018 09:17:27
3651 forum posts
72 photos

Need context. What is the job/application?

Answer may be influenced by whether you want the bolt to hold the handle on a barbecue fork, or the big-end cap on a racing motorcycle engine con-rod.

Gordon W15/08/2018 09:54:53
2011 forum posts

I've worked in industry most of my life. General rule is don't specify a tapped hole unless it is needed. Tapping is expensive.

Farmboy15/08/2018 10:29:02
116 forum posts
8 photos

I have little experience of aluminium but even in steel drilling a 6mm hole in a bar 8mm wide would seriously weaken it, depending on the final use. If there's space I would have thought two or three 4mm would be better. And, as others have said, nut and bolt would be quicker, easier and probably stronger than tapped holes.

I certainly wouldn't try 8mm teeth 2


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