Stripped aluminum threads. Now what?

[brucemc777Participant @brucemc777]

I’ve drilled and tapped a few holes, but other people told me exactly what to get. So far, and as far as i can imagine likely, my predominate use is for when i manage to strip threads in aluminum (which i’ve done more times than i have fixed…).

I want to learn how to determine what to get on my own, and have a case in point to present:

 

My dog, on a long lead line, ripped a couple legs right out of a bench. The studs that secured the legs to the bench are steel, the bench and legs are aluminum.

The studs that now will not grip into the aluminum are 8 x 1.25 (and 3 cm long).

How do i decide what size hole i should drill the formerly threaded hole out to and choose the right tap in order to pick out the right studs to replace the function of the old ones?

 

It is almost a circular problem. I suspect i should first pick out a new stud size that the shank of the new stud would be equal to or very slightly larger than the diameter of the hole in the aluminum, or put another way, equal to or slightly larger than the total diameter (including threads) of the old stud.

 

IF that is correct, then how do i choose the right tap for it? Is it directly translatable? In the actual example should i get a 9 x 1.25 stud and a 9 x 1.25 tap?

 

Is there a more simple way to approach this?

 

THANK YOU for your help,

-Bruce

 

(I’m getting real good at stripping aluminum. Stripped holes that secured a throttle body in a car, stripped holes that held the intake carb plenum to the block in a lawn mower, same lawnmower stripped the holes that secured the muffler. Yes, if anyone wants to employ a professional aluminum hole stripper, i’m right here-)

[David George 1Participant @davidgeorge1]

Hi Bruce Where are you based as I have a few helicoil sets which include 6mm 8mm and 10mm threads. these can be used to restore the original thread and make it potentially stronger than original depending on surrounding material. A few pictures of article with amount and depth of material would be useful. The kits are available at Tracy tools for around £30.00. When at work we would use these in stressed areas in aluminium fixtures especially where screws or bolts were regularly removed or under stress.

helicoil – Taps & Dies, Reamers, Milling Cutters, Tracy Tools

 

David

[DiogenesParticipant @diogenes]

You need a ‘Thread Repair Kit’ – the whole kit comes with it’s own drill-bit, a tap for cutting a new thread, and an insert which you screw in (with the special tool also provided) to end up with a threaded hole that is back to where-it-was before the disaster happened – so you can re-use the original studs / fasteners.

Here is a picture of what’s involved – you can buy the inserts separately – I’d stock up 😁

https://www.diy.com/departments/m8-x-1-25mm-thread-tap-repair-cutter-kit-helicoil-15pc-set-damaged-thread/5056133309461_BQ.prd

 

[JasonBModerator @jasonb]

Or you could just drill it out to 8.5mm and tap M10 x 1.5. Open out the hole in the other part to 10mm and use new M10 fixings.

[Adrian R2Participant @adrianr2]

For your bench, if the tapped hole can be made deeper but the threaded same diameter as original then that might be better, or even a through hole and a conventional M8 nut and bolt.

To re-tap completely you need to go up enough that the new fastener will grip so the damaged hole needs to be drilled out to the starting diameter for the new tap; M9 x 1.25 will start from a 7.75mm hole (thread size minus pitch) so likely too small for a damaged M8; you will also be limited by what fastenings are practically available – e.g. M9 bolts are uncommon compared to M8 and M10 so check that before commencing.

If you can’t increase the fastener size (carb perhaps?) then helicoils as above.

Also stick to open ended spanners for tightening or buy a small torque wrench 🙂

[SillyOldDufferModerator @sillyoldduffer]

If going the next fastener up route, study a Tap Drill Chart to make sure there’s enough meat to take the new thread, and to establish the tap drill diameter.

Note the chart highlights some sizes and not others.  The highlighted sizes are preferred, i.e. easy to find, whilst those in clear exist, but may be tricky/expensive to source.  Generally simpler to stick to preferred sizes.

The chart also refers to tap drills selected for either 75% or 50% engagement.  You might prefer to increase engagement with a slightly smaller tap drill.  Makes it harder to tap the hole and fit the bolt without cross-threading it, but increases the strength.

For Aluminium select a coarse thread rather than fine. (M10x1.5 rather than M10x1.0 etc)

Tapping is more skilled than it appears.  Many Aluminium alloys used to make furniture and castings are gummy, requiring extra care. Vital to keep the tap aligned with the hole and the nature of the job makes it likely a hand operated tap will wander until at least 4 straight turns start steering the tap.  Self-steering in soft Aluminium is unreliable so I’d take positive action to keep the tap straight, not relying on my wobbly hand and dubious eyeball.   Traditionally done with a bell-shaped jig, now more common to hold the tap absolutely straight in a pillar drill, milling machine or lathe.  Lubricate, reverse every couple of turns to break swarf, and remove swarf from the hole frequently.   For a little extra strength, start with an easy cutting taper tap and finish the thread with a plug tap.

Much to be said for Helicoil!

Dave

[bernard towersParticipant @bernardtowers37738]

I think more to the point is thats some dog!!

[Clive FosterParticipant @clivefoster55965]

When dealing with his sort of issue the massive compilation of (nearly) all the threads you are likely to encounter in size order produced by Andy Pugh many years ago is a great help in finding a thread just a tiny bit bigger to replace the original without running out of metal for the hole. Obviously same size thread repair is best but kits aren’t trivial cheap and having full range around just in case isn’t fiscally practical for ordinary folk.

Andys original is here :-

http://www.bodgesoc.org/thread_dia_pitch.html

in a very plain but effective format.

Probably the prettiest version is and Excel format spreadsheet put up on a now defunct website and retrieved by Journeyman’s Workshop here:-

https://journeymans-workshop.uk/downloads.php#allthread

Given the number of times this thing has been grabbed over the years its great pity that the internet has no way of directing a small payment to Andy each time the content is downloaded as modest recompense for all the work creating it took.

Clive

[Neil AParticipant @neila]

Stripped threads in aluminium are a common problem, even in industry. At the company I worked for holes tapped directly in the aluminium were only used for studs and always 1.5 x diameter deep. Tapped holes for setscrews were always Helicoiled, again 1.5 x diameter deep.

When we had a failed Helicoil thread our next option was to use another style of thread insert from a company called Tappex. The drilled hole for these was just larger than the outside diameter of a Helicoil tap, so minimum metal removal. The style we used was called an Ensat H type in austenitic stainless steel which has a coefficient of expansion close to aluminium. It is a self tapping insert with three cutting holes, so no special tap required. Probably not cheap though, although you might be able to get a few samples.

You can use a product selector like this example I have done:

Ensat®

I’m afraid some aluminium can be a real headache, but all the advice others have given is what I would have done myself.

Neil

[noel shelleyParticipant @noelshelley55608]

One thought if using a slightly larger thread than original is the cost of the fasteners ! Whilst metric is cheap enough in preferred sizes, odd numbers above 5 can be both more difficult to get and more costly, then there are the imperial sizes, some can be very costly or unobtainable.

In aluminium for high stress threads the Helicoil type insert is used from new. Noel.

[bernard towers]

On bernard towers Said:

I think more to the point is thats some dog!!

What everyone else said + you need a smaller dog.

[brucemc777Participant @brucemc777]

My great appreciation to everyone!!!

I suspect i will go with the Helicoil; though i must admit i want to become more proficient at tap & die – mainly understand the thought process in selecting drill and tap sizing when i do manage to strip something though it seems to me from reading all of your great responses it is as much an art as a science.

 

As to my insane dog, she’s a 25 kg lab mix but has the power of almost twice her size. I’m roughly 93 kg and have made the mistake of getting in the loop of her leader line; she’s ripped my legs right out from under me. Loyal, but crazy.

[old martParticipant @oldmart]

I may have missed something, but the thickness of the aluminium which stripped it’s threads is important. A Helicoil would need to be in at least 1 diameter of thickness, and wouldn’t be much use in thinner. For thin aluminium like box section you would be better off with rivnuts, steel ones are stronger than aluminium ones. Drilling and tapping aluminium requires a tiny ammount of lubrication such as light oil, WD40 or AC90.

An 8mm thread 30mm long in aluminium would hold a ton easily.

[brucemc777Participant @brucemc777]

You didn’t miss anything at all! I will inspect the area to determine thickness. Thank you!

[jimmy bParticipant @jimmyb]

Rivnut sounds most promising to me

 

Jimb

[brucemc777Participant @brucemc777]

Hmmmmm. OK, i have a “thread repair kit” on order from Amazon. Hopefully the set i ordered doesn’t used cardboard instead of metal, but while Helicoil was around £27 for the one, i found a set from Bilitools for about £11.

I also am learning more from the posts that addressed the considerations when using tap & die – i like the idea of being able to have everything i need in place in the event that i happen to have the correct tap and replacement bolt. And yes, i do see the absurdity of it these days with one-day delivery available from Amazon, but heck, i’m getting old. That’s my excuse.

And now i am learning of these Rivnuts. They appear to be a solid inexpensive solution. I do like the idea of saving money…

[EmgeeParticipant @emgee]

The drill chart above seems to have some errors, unless I am mistaken which is quite possible, in the 50% engagement column the first 3 sizes are greater than the thread max diameter, I didn’t check the other stated sizes.

Emgee

[bernard towersParticipant @bernardtowers37738]

sorry brucemc777 I can see why you are mending the bench and not getting rid of the dog!!

[Howard LewisParticipant @howardlewis46836]

What length are the bolts?

You need an ABSOLUTE minimum thread engagement of one diameter.  But 1.5 or 2 would be even better.

If the original thread is M8, a Helicoil is likely to drill out to something likely 9 or 9.25 mm diameter and tap M9. If there is plenty of material on the legs, you could always drill and tap out to a larger size, probably M11 or M12 and make a bush to return the tapping to M8.

Howard

[brucemc777Participant @brucemc777]

I looked at the stripped out holes – they might be a hair over the width, 8mm. (Only the highest quality of furnishings…)

[jimmy b]

On jimmy b Said:

Rivnut sounds most promising to me

 

Jimb

Rivnuts are not a thread repair system, they are a means to achieve a threaded attachment point in a sheet metal assembly, such as an aircraft wing skin.

[Bill PhinnParticipant @billphinn90025]

It’s hard to say what the best repair method(s) would be without seeing the bench and leg. A wire insert or threaded bushing or just drilling and tapping a size bigger are obvious solutions, as others have said, but these might be no more appropriate than a rivnut, pop-rivet, or clearance hole and back nut.

The OP talks about a stud, not a screw or a bolt. I confess I can’t really picture what we’re dealing with.

[howardb]

On howardb Said:

On jimmy b Said:

Rivnut sounds most promising to me

 

Jimb

Rivnuts are not a thread repair system, they are a means to achieve a threaded attachment point in a sheet metal assembly, such as an aircraft wing skin.

Agreed, in principle, not sure about aircraft wings though….but the question was about a suitable thread repair. From the sound of it I would give it a go.

 

Jimb

[Author]

Posts