# Reusing stretch bolts - yay or nay



## happychappy (Feb 9, 2015)

Hi all

One thing that I have been surprised by with my TT is the amount of bolts that need replacing after being undone.

I understand that they are weakened when torqued up and then released, but us it necessary to always replace them?

I have another thread running regarding removing the rear subframe on my FWD 2.0 TFSI for a clean up and paint. Initially I was going to tear down the complete assembly for powder coating and replace all the bolts, but after careful consideration I have decided to wire brush the suspension components and paint while assembled - saving on the considerable cost of replacing so many bolts and bushes.

In that thread another forum member recommended some subframe inserts to stop any subframe/chassis movement and whilst reading about installing the kit noticed that although the manufacturing company recommend replacing the subframe bolts they also say that if you do decide to reuse the subframe stretch bolts to use locktite on the thread.

So to my way of thinking is it ok to reuse the bolts?

Has anyone on here reused any stretch bolts and had problems?

I suspect that if a car taken to a local independent for say replacement calipers/shocks etc I would imagine that most would not replace the stretch bolts, what have forum members experiences been in this field?

Thanks for your input.


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## tttony (Dec 21, 2014)

When stretch bolts are originally installed, they are tightened beyond the limit of their elasticity. In other words they become permanently elongated. If they are then removed, reused and torqued to the original specified value, they are further stretched which may result in them breaking, or, worse, reaching a point just on the verge of breaking. They then run the risk of actually breaking when subjected to forces in use.


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## happychappy (Feb 9, 2015)

I understand and agree.

So why are a respected aftermarket go faster mods company advocating reusing them?


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## tttony (Dec 21, 2014)

Maybe the respect is being misplaced!


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## happychappy (Feb 9, 2015)

I guess as long as their product sells who cares?


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## barry_m2 (Jun 29, 2015)

happychappy said:


> I understand and agree.
> 
> So why are a respected aftermarket go faster mods company advocating reusing them?


Ask them?


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## tttony (Dec 21, 2014)

Who would you rather take notice of? The designer/manufacturer of the car or an aftermarket vendor of go-faster "goodies"?


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## Jasonl (Mar 31, 2018)

Ive used stainless steel on a few bits as you can reuse lots of times


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## tttony (Dec 21, 2014)

Stainless steel fasteners are great in suitable applications but they have their limitations. If you use a stainless steel bolt in place of a stretch bolt how do you know what to torque it up to? The torque setting for the stretch bolt will not be applicable to a stainless steel bolt.


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## Jasonl (Mar 31, 2018)

Used them for front suspension and gearbox mount, torque dont apply if using stainless as their not stretch


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## tttony (Dec 21, 2014)

That response just shows that you don't understand the issues involved.


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## Jasonl (Mar 31, 2018)

I understand perfectly fine the bolt i used came with uprated gearbox mount get torqued up +180°


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## happychappy (Feb 9, 2015)

And herein proves my suspicions...

Are these stretch bolts necessary?

Is it just another money spinner for the manufacturers?

When going to say somewhere life Halfords or KwickFit do they change the bolts?

Has anyone reused a bolt, torques it to the correct figure and it's snapped?

Genuinely interested to know


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## andymar00 (Dec 10, 2006)

i understand all the above point but if they are not required and are more expensive why would manufactures use them at all and not just cheaper option which would be a huge saving, does it make the cars quicker and easier to assemble on the production line by reducing washers and other fasteners ?


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## happychappy (Feb 9, 2015)

I doubt the saving would be that huge tbh, I may be wrong, just a hunch

However, come service time (as we all know, this is where they make some serious coin) it's a double whammy!

A new caliper plus half a dozen "special" bolts

A new shock with half a dozen special bolts again...

Those pounds soon add up 

That's if the aftermarket/independent service agents take the route of replacing the bolts....


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## Jasonl (Mar 31, 2018)

Only stainless ive used is gbox mount and the top mounts on the suspension the 6 bolts other than that i use the stretch bolts audi advise to use and change


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## SwissJetPilot (Apr 27, 2014)

They're called stretch bolts be because they are intended to be stretched and permanently deformed to a given point when properly installed. Unlike standard bolts, they're also designed to take on higher torque loads in order to provide higher clamping forces.

When stretched they actually "neck down" or have a reduced diameter where the metal has been stretched. This reduced area has experienced what is known as 'plastic deformation', and will not hold or maintain the same torque (tension) if removed, reinstalled and re-torqued. In fact it will continue to stretch more, (resulting in a loss of tension or clamping force) and often fracture once torqued past it's deformation limit.

To be clear, all bolts will stretch when they are tightened. But they usually they stay within their 'elastic limit'. So, when they are loosened, they return to their original length.

The required torque value is calculated to apply the correct stretch, and to ensure the correct load (clamping force) on the joint. It's not very accurate because torque is affected by issues such as thread condition, presence of lubricant on the thread, dirt, rust, etc. Stretch bolts are specifically designed to get around these issues and apply the correct amount of tension to the parts being held in place by stretching to a known length.

This is why you should NOT lubricate spark plugs with copper grease prior to installation. Most modern spark plugs are already pre-lubricated by the manufacturer. Therefore the torque value for spark plugs is based on a 'dry' fit. In fact, the Audi workshop manual states not to use additional lubricant when installing and torquing spark plugs.

Stretch bolts are made of steel which has a relatively long horizontal section of the load/extension curve past the yield point. In use they are tightened by stretching them to specific length, with consideration to the 'necking' effect. This is calculated to take them just past their yield point. In this way the load on the bolt, and the load on the joint, is accurately known. Because they work only when deformed and are not designed to return to their original length, they must be replaced.

To the point of using stainless steel, unless you know the hardness, ductility and toughness, you have no idea how that bolt will perform under a given torque load or whether or not it can apply sufficient clamping force. They may even stretch without breaking, resulting in a loss of clamping force.


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## happychappy (Feb 9, 2015)

A fantastic explanation Swiss

However I suspect a lot of the independants etc do reuse these bolts, what I am wondering is who has actually has a boltsnap on them?

Obviously it's not something that I would like to experience, but I am wondering if it's a little "overstretched" this "must replace/do not reuse" advise?


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## SwissJetPilot (Apr 27, 2014)

Most engineering designs are intentionally over designed for safety and idiot factors. Especially in automotive where any fool with a wrench can do what they want. There's no laws against stupid. But the results are certainly predictable.

As you say, it's very likely some mechanics (professional and shade tree) who know better will cut corners because the odds are a failure will not be catastrophic. Plus most owners (customers) have no idea nor do they care about these sorts of 'details'. They just want the lowest price they can find.


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## qooqiiu (Oct 12, 2007)

My advise to you my TT friend would be buy new bolts. Observe the Torque to yeald value.

That way you have peace of mind. That is worth a lot!

You are undertaking a not exactly essential bit of work because presumably you love this car. So buy new bolts. I know it's a bit painful laying out £5 a time on a bolt but you will have peace of mind. So buy new bolts. Go to your dealer and ask for some discount they usually do on items like this. Buy new bolts.


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## qooqiiu (Oct 12, 2007)

In answer to your question. Nay. Unlock!


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## commuter (Nov 6, 2017)

Some very confusing comments here. As an apprentice , I was lucky enough to work in a metallurgy lab. If a metal sample endures sufficient force to exceed its elastic limits, it will reach its yield point whereupon it becomes significantly weaker due to permanent deformation. "Stretch" bolts must not go beyond their elastic limit in order to retain their strength and so if there is a danger that the yield point has been exceeded whereby permanent deformation occurs, the metal is past its strongest and therefore of little use whilst in service whether re-used or not disturbed. So to conclude, for "stretch bolts to retain their usefulness, they must not be subjected to forces which exceed their elastic limit and therefore , may be re-used.
A coil spring is a piece of metal which is subjected to forces which deform it temporarily. those forces do not exceed the metal springs yield point and therefore it returns to its original shape. Staying the the coil spring analogy , if you cycle the coil spring a sufficient number of times, it will become work hardened and more brittle. The same could be said of a stretch bolt holding the coil spring in position.The stretch bolt is subjected to repeated loading just as much as the spring and will also become susceptible to becoming brittle. 
So..... how old are your stretch bolts? How much shock loading have they been subject to? Are you beginning to see why the dealers recommend replacing the stretch bolts? The dealer simply doesn't have a metallurgy lab tucked away round the back. By the way if you thought that the likes of Ferrari and Porsche might have access to metallurgy labs, they do! and that independent lab is who I was working for all those years ago. I suspect the factories have moved on a long way since then and now have their own metallurgists. You might be surprised at the amount of steering components which had to be scrapped due to chilling during the casting process. I guess they're all made of carbon fibre these days!


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## happychappy (Feb 9, 2015)

Some fantastic science fact based explanations going on here, but none showing hard evidence of the repercussions of reusing the stretch bolts in the real world.

My understanding of the stretch bolts as a layman was not far off, but what confuses me is why they are used so often on the MK2, does the MK1 TT use as many stretch bolts?

If it's due to pioneering cutting edge technology why the hell did Audi not put a decent coat of paint under the boot area and perhaps isolate the aluminium rear heat shield from the effects of galvanic corrosion from the steel rear end?

Getting more confused as I try to understand how the minds at Audi work.


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## happychappy (Feb 9, 2015)

qooqiiu said:


> My advise to you my TT friend would be buy new bolts. Observe the Torque to yeald value.
> 
> That way you have peace of mind. That is worth a lot!
> 
> You are undertaking a not exactly essential bit of work because presumably you love this car. So buy new bolts. I know it's a bit painful laying out £5 a time on a bolt but you will have peace of mind. So buy new bolts. Go to your dealer and ask for some discount they usually do on items like this. Buy new bolts.


I am on first name terms with my local TPS due to my fetish with replacing bits on my car. 

Recently I managed to break a bracket that secures the front of the rear bumper to the bodywork and was amazed that the whole bracket was only £8'ish, they were over £20 on eBay! Piece of advice to anyone reading this - Always contact TPS/V.A.G dealer for a price first!

But funnily enough even the TPS chaps wince at the cost of some of the smaller bolts etc, it often makes no sense.


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## SwissJetPilot (Apr 27, 2014)

Since stretch bolts can apply more clamping force, you need fewer of them than standard bolts to achieve the same level of fixation between parts. Less bolts means less holes to drill/tap and fewer bolts to tighten. Production cars (and consumer products in general) are not designed around ease of access during maintenance, but around ease of assembly in order to lower production costs.


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## John949 (Apr 12, 2017)

If you want to understand the metallurgy start by Googling Hooke's Law. Once a material is taken past it's elastic limit the slope of the stress/strain curve lessens so further movement of the bolt still produces an increase in clamping force but the amount of increase for a given angle reduces. Manufacturers are taking advantage of this property to lessen the variation in clamping force and (as SJP says) get away with less fasteners. You can think of it this way, once the bolt has gone past it's elastic limit then it becomes progressively less sensitive to incorrect torque settings because the stress/strain curve flattens. You will also note that an awful lot of tightening torques are now given as X ftlbs plus so many degrees. This is again a means to reduce the variation in clamping force as it removes most of the vagaries of friction between the clamping surface and the bolt head.

As to re-using them: There is most definitely a limit on the number of times you can re-use them as they will eventually break if re-used many times. How many that number is depends on how far past the elastic limit they are taken each time they are installed. One of the most notorious stretch bolts (the term yield bolt would be technically more accurate) is the head bolt for a Rover K series engine. These engines have a fearsome reputation for blowing head gaskets. The bolts are about 20 inches long and not only clamp the head to the block but also the main bearing shells to the block. The spec for these bolts says you can re-use them 3 times and I have re-used the bolts on my K series. It has lasted over 4 years without a problem. I have also re-used the hub driveshaft bolt on my TT (largely because the bolt type mine originally use is no longer available.) It has only been in a few weeks but so far so good.

One should also remember how sensitive Audi is to being sued. Given the choice between recommending that stretch bolts can be re-used 3 times and recommending that they should be replaced every time, if you were Audi what would you say?

You'll have to make up your own minds but I would definitely replace stretch bolts if I had no idea how many times they had been re-tightened. If I was pretty sure they had only been used once I would be happy to re-use them at least once more.


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## happychappy (Feb 9, 2015)

Thanks John

A brilliant explanation to many of my questions, interesting point regarding using a bolt more than once, but only if you know the history that brings another question - does age affect the performance of these bolts?

Interesting and poignant mention of the "K Series" engines. I was an owner of one of these in a MGTF160 some years back. I was lucky as my HG was fine, however many people did suffer failures and often another after replacing the HG. I made reference to them reusing these bolts and some had. I can't be sure if it was operator or component failure that attributed to the re-failures.

The litigation consequences make complete sense regarding Audi protecting their market position and also brings in another healthy stream of revenue to boot.

We can take a belt and braces approach to almost everything in life, I generally am a very careful and concise person, but don't like to be taken for a ride, however safety is paramount and so I would replace bolts that are weight bearing and strategic.


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## John949 (Apr 12, 2017)

Another point I forgot to mention is the question of aluminium bolts. Everyone re-uses these without question but, as aluminium yields at minimal stress, these bolts are permanently stretched every time they are used. In general aluminium bolts are not used for high stress applications and so the 'stretch' is not excessive and the bolts may be safely re-used a number of times.


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## TT-driver (Sep 14, 2010)

Additionally to all that was posted above, I once came across the advise to even replace the wheel bolts after they've been tightened 10 or 20 times or so. This advise was also based on the fact that iron looses its stretch at some stage. And that when the stretch is gone, the bolts kind of loose their grip and could even work themselves loose, even when torqued up to spec. Not sure if this all is true though. But I can imagine that it is true.

Perhaps new bolts also don't rust as fast as reused bolts. After all they seem to have a weird grey coating. And that coating gets damaged as the bolt is being tightened.


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## Wolvez (Jun 22, 2018)

The aluminum body frame is the reason why they recommend using new special coated bolts. Severe Contact/Galvanic Corrosion will occur when steel (bolt) makes contact with aluminum (frame) when it get wet. The zinc coating on the bolts don't prevent corroson. It only slow down the process.

Read Page 6
http://www.volkspage.net/technik/ssp/ssp/SSP_383.pdf

Don't confuse Torque To Yield Bolts (Stretch Bolts) with Normal bolts. They look different from normal bolt. You can vissualy see if the bolt is stretched.


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## happychappy (Feb 9, 2015)

The bolts I was originally referring to were the ones clamping the rear suspension together, so steel to steel.

Yes, galvanic corrosion is a huge problem for the TT, but not in the rear subframe/suspension component areas as I believe that's all made from steel, other than the bearing carrier.


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