Stretch bolt reuse / alternate sources?

shoebear

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We all know that VW loves torque-to-yield (stretch) bolts, and they are supposed to be replaced with new bolts every time they are removed. Although vendors like idparts conveniently packages together the bolts you need for a job, Genuine VW bolts are expensive. I would like to stop paying all that extra money every time I work on my car.

So I have 2 thoughts:
  • When steel is deformed, the initial deformation is elastic. When the stress is removed, the steel will go back to its original shape without damage, This is, for example, how springs work. But if the steel is stressed further, at a certain point, the deformation becomes plastic -- the steel is now permanently deformed and will not go back to its original shape when the stress is removed. My understanding with stretch bolts is that the engineers who specify the torque +degree aim to stretch the bolt to about the middle of the elastic deformation range. If true, when the bolts are removed, they should go back to their original shape with no harm done. So why not re-use them? One possible problem with this theory is that the dynamic loading from normal driving shocks and vibration might further stress the bolts into the plastic range. But this should be easy to check with a thread gauge, or simply by cleaning the bolt and seeing if a new nut will thread onto it easily. Obviously, bolts that are corroded or otherwise compromised should be replaced regardless of whether they are stretch bolts or not.
  • If, for whatever reason, re-use is not a good idea, do we know the specs on the bolts such that we can obtain them from an alternate source? For example, there is a wholesale hardware store near me, Lightning Bolt, that also sells retail for pennies per bolt -- much cheaper than Ace Hardware. But even Ace is cheaper than genuine VW.
Thanks for your ideas and thought on this. I appreciate the discussion!
 
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P2B

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My understanding with stretch bolts is that the engineers who specify the torque +degree aim to stretch the bolt to about the middle of the elastic deformation range. If this is the case, when the bolts are removed, they should go back to their original shape with no harm done.
That is not the case. The installation procedure is designed to take the bolt into the plastic range, in order to ensure the bolt maintains a constant clamping force under varying conditions such as thermal expansion and contraction.

There would be no point in specifying a TTY bolt then only tightening it into the elastic range.
 

Mpaw

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In my experience, VW is actually cheaper than other sources (here - Germany). ebay, for example, is deceptively expensive. But I have picked up some 'bargains' by buying packs of ten or twenty or so.
I've seen high quality subframe / suspension mods from reputable suppliers which use 'non-stretch' bolts as replacements (I think they are grade 10.9 - see here for internationally equivalent standards: https://www.steel-grades.com/metals/85/205121/SAE-Class10.9.html).
I bought some 1m long threads to cut to size, but realized it would be better to fix the nuts on them by welding them. I haven't got around to it yet, but welding such a grade steel is difficult due to high carbon content and other metals. The mods I saw could have been from www.darksidedevelopments.co.uk, or somewhere in the US.
 

shoebear

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That is not the case. The installation procedure is designed to take the bolt into the plastic range, in order to ensure the bolt maintains a constant clamping force under varying conditions such as thermal expansion and contraction.

There would be no point in specifying a TTY bolt then only tightening it into the elastic range.
If the bolts are tightened into the plastic range and thermal expansion puts even more stress on them, the bolts would be deformed even more and would stay that way when cooled. This would also move the elastic range. Eventually, the bolt would stay in the elastic range.

On the other hand, if the bolts were tightened into the middle of the elastic range, the clamping force would stay about the same as the bolt moved through heating and cooling (and thus through the elastic range), as long as the bolt stayed in the elastic range. In a sense, staying within the elastic range allows the bolt to function as a spring.

Maybe VW TTY specs put the bolt at the top of the elastic range instead of the middle. Then temperature ranges will put the bolt slightly into the plastic range, stretching the bolt a little and also moving the elastic range. After initial plastic deformation, the bolt will move around in the (now relocated) elastic range as temp changes. This would be a way to ensure that the bolt remains in the elastic range at all times once it has been through a few temp cycles. By contrast, torquing into the middle of the elastic range might allow the bolt to drop below the elastic range in some temp extremes, loosening the clamp force.
 
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shoebear

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Here's an article from Fel-Pro making a distinction between Torque-To-Angle (TTA) and Torque-To-Yield (TTY) bolts. The distinction they make is that TTA bolts are not torqued into the plastic deformation range, while TTY bolts are. Honestly, I've never heard of TTA bolts before, and I wonder whether the TTY label is being used generically to cover both types.

One test I think I'll start doing in the future: I will (for now) continue to use new bolts whenever Bentley calls for them. But before installing the new bolt, I'll measure the old and new overall bolt lengths with calipers. I'll keep a log of this as well. If a bolt is stretched past the elastic zone, the used bolt will be longer than the new one, and over time and repeated samples, I'll be able to tell whether the bolt is TTA or TTY.
 

shoebear

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In my experience, VW is actually cheaper than other sources (here - Germany). ebay, for example, is deceptively expensive. But I have picked up some 'bargains' by buying packs of ten or twenty or so.
I've seen high quality subframe / suspension mods from reputable suppliers which use 'non-stretch' bolts as replacements (I think they are grade 10.9 - see here for internationally equivalent standards: https://www.steel-grades.com/metals/85/205121/SAE-Class10.9.html).
I bought some 1m long threads to cut to size, but realized it would be better to fix the nuts on them by welding them. I haven't got around to it yet, but welding such a grade steel is difficult due to high carbon content and other metals. The mods I saw could have been from www.darksidedevelopments.co.uk, or somewhere in the US.
Thanks -- good points. Understanding the rationale behind TTY bolts, I don't want to change to non-stretch bolts unless I were sure that it was OK for specific uses. For example, the suspension/subframe would generally see less thermal range than an engine part, so maybe it's OK in that case. I'm sure VW engineers want "engineering level" precision and may have specified TTY more than really needed.
 

shoebear

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That is not the case. The installation procedure is designed to take the bolt into the plastic range...
I concede this point. Here's a good video explaining TTY bolts. With more research, including @P2B 's response and this video, I agree that actual TTY bolts (as opposed to TTA bolts) cannot be re-used safely. Then the other question -- can we buy replacement bolts safely from a less expensive source than VW?

 

P2B

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Here's an article from Fel-Pro making a distinction between Torque-To-Angle (TTA) and Torque-To-Yield (TTY) bolts. The distinction they make is that TTA bolts are not torqued into the plastic deformation range, while TTY bolts are. Honestly, I've never heard of TTA bolts before.
That's because TTA is not a bolt type, it's an installation method intended to reduce the clamping force variations which can be introduced using torque value only.
 

mittzlepick

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Mercedes manuals show measuring bolt length to determine reusability. Not sure where or whether it was unimog or 300d or where it was used.
 

kf6eml

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The important thing about torque is preload.
Under-torquing a bolt allows stress to overcome preload, which stretches the bolt. That causes fatigue, and eventually, failure.
Dry, rough, or dirty threads can make the recommended torque less effective at giving the prescribed preload. A torque-to-yield bolt ensures that those factors don't result in too low a preload.
If you can ensure the proper preload, then any old bolt should work just fine - as long as it doesn't cause other problems, like slippage, galvanic activity, over-expansion with heat, etc. (This assumes that the bolt has the tensile strength to handle the extreme loads from normal use.)
A little math can tell you the theoretical preload force based on torque recommendation, friction coefficients, and thread pitch. Typically, over-torquing causes problems with worn threads, stripping, and breaking or distorting parts. Under-torquing is worse. If the parts will take it without unacceptable distortion or breakage, and you're not going to have the part on and off frequently, then overtorquing slightly, just to be sure, is the safer option.
 

Abacus

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Interesting discussion, this is why I love this board. For what it's worth...

I went and measured (using a calibrated Mitutoyo Digimatic caliper) the crank sprocket bolt from a B4 I just replaced the timing belt and crank sprocket on (torque to 66 ft-lbs, then 90°):

New Bolt-1: 2.5345"
New Bolt-2: 2.5370"

Old bolt: 2.5550"

Bolt difference=0.0192"
 

KrashDH

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The important thing about torque is preload.
Under-torquing a bolt allows stress to overcome preload, which stretches the bolt. That causes fatigue, and eventually, failure.
Dry, rough, or dirty threads can make the recommended torque less effective at giving the prescribed preload. A torque-to-yield bolt ensures that those factors don't result in too low a preload.
If you can ensure the proper preload, then any old bolt should work just fine - as long as it doesn't cause other problems, like slippage, galvanic activity, over-expansion with heat, etc. (This assumes that the bolt has the tensile strength to handle the extreme loads from normal use.)
A little math can tell you the theoretical preload force based on torque recommendation, friction coefficients, and thread pitch. Typically, over-torquing causes problems with worn threads, stripping, and breaking or distorting parts. Under-torquing is worse. If the parts will take it without unacceptable distortion or breakage, and you're not going to have the part on and off frequently, then overtorquing slightly, just to be sure, is the safer option.
Fellow engineer it seems....
 

kf6eml

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Fellow engineer it seems....
Aircraft mechanic, actually.
There have been a few catastrophic engine failures due to fatigue from undertorqued engine nuts and bolts. It's a big deal.
Once in a while, we can use whatever bolt works, but the vast majority of the time, we must use the hyper-expensive fasteners that the manufacturer specifies. (The bolts we use on the propeller of a Cessna 172 cost something like $70 each. There are 6 of them. Thankfully, they can be reused.)
Once you go down the rabbit hole of fasteners, you come out a changed man. I could easily give an hour-long lecture on the subject without even getting too deep into the weeds.

That's why I like working on cars. Torque the critical stuff and tighten the rest as is reasonable. No logbooks. No car mechanic license to lose. And parts are a fraction of the price. I can buy a whole (used) engine for what a single cylinder costs for an airplane.
 
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KrashDH

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Aircraft mechanic, actually.
There have been a few catastrophic engine failures due to fatigue from undertorqued engine nuts and bolts. It's a big deal.
Once in a while, we can use whatever bolt works, but the vast majority of the time, we must use the hyper-expensive fasteners that the manufacturer specifies.
Once you go down the rabbit hole of fasteners, you come out a changed man. I could easily give an hour-long lecture on the subject without even getting too deep into the weeds.

That's why I like working on cars. Torque the critical stuff and tighten the rest as is reasonable. No logbooks. No car mechanic license to lose. And parts are a fraction of the price. I can buy a whole (used) engine for what a single cylinder costs for an airplane.
I know it all too well, I design tooling (aerospace), so fasteners are part of the bread and butter of it. Lots and lots of specs and design considerations.
 

325_Guy

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Then the other question -- can we buy replacement bolts safely from a less expensive source than VW?
You can shop around and find dealers that offer a steep discount off MSRP.

https://parts.vw.com/ then "Change Dealer" at the near-top left.

Once the dealer list opens up, what I'll do is open each dealer link in it's own window and search the parts I'm looking for.

I routinely get my parts at 40-45% off list this way from a dealer a few hours from me.
 

kf6eml

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I know it all too well, I design tooling (aerospace), so fasteners are part of the bread and butter of it. Lots and lots of specs and design considerations.
You could make a killing designing special tools for aircraft mechanics.
I don't know a single mechanic who doesn't have a $50 tool for that one screw that can't be reached any other way.
I actually bought an angled driver and use two different bits to get at one particular screw during routine maintenance. That's literally the only use I have for that $20 tool.
We use shaved sockets, thin wrenches, wrenches that have been cut and re-welded into grotesque shapes... because engineers hate mechanics.
 

KrashDH

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You could make a killing designing special tools for aircraft mechanics.
I don't know a single mechanic who doesn't have a $50 tool for that one screw that can't be reached any other way.
I actually bought an angled driver and use two different bits to get at one particular screw during routine maintenance. That's literally the only use I have for that $20 tool.
We use shaved sockets, thin wrenches, wrenches that have been cut and re-welded into grotesque shapes... because engineers hate mechanics.
Lol. I'm one of those people that grew up taking things apart and working on vehicles and everything else. Before I even started my design part of my career, I was a machinist, then a manufacturing engineer. Needless to say, I have great relationships with my guys on the floor. They get the tools to make their lives easier. It's a win for everyone honestly.

But I understand your pain, 80% of designers haven't had hands on experience nor can they communicate well, so that leads to the situation you're eluding to.
 

shoebear

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Interesting discussion, this is why I love this board. For what it's worth...

I went and measured (using a calibrated Mitutoyo Digimatic caliper) the crank sprocket bolt from a B4 I just replaced the timing belt and crank sprocket on (torque to 66 ft-lbs, then 90°):

New Bolt-1: 2.5345"
New Bolt-2: 2.5370"

Old bolt: 2.5550"

Bolt difference=0.0192"
So that bolt, apparently, was torqued into plastic deformation. Thanks!
 

shoebear

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You can shop around and find dealers that offer a steep discount off MSRP.

https://parts.vw.com/ then "Change Dealer" at the near-top left.

Once the dealer list opens up, what I'll do is open each dealer link in it's own window and search the parts I'm looking for.

I routinely get my parts at 40-45% off list this way from a dealer a few hours from me.
Good idea! I'll try that.
 

shoebear

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The important thing about torque is preload.
Under-torquing a bolt allows stress to overcome preload, which stretches the bolt. That causes fatigue, and eventually, failure.
Dry, rough, or dirty threads can make the recommended torque less effective at giving the prescribed preload. A torque-to-yield bolt ensures that those factors don't result in too low a preload.
If you can ensure the proper preload, then any old bolt should work just fine - as long as it doesn't cause other problems, like slippage, galvanic activity, over-expansion with heat, etc. (This assumes that the bolt has the tensile strength to handle the extreme loads from normal use.)
A little math can tell you the theoretical preload force based on torque recommendation, friction coefficients, and thread pitch. Typically, over-torquing causes problems with worn threads, stripping, and breaking or distorting parts. Under-torquing is worse. If the parts will take it without unacceptable distortion or breakage, and you're not going to have the part on and off frequently, then overtorquing slightly, just to be sure, is the safer option.
I understand all of that -- but to use alternate bolts, I would need to run that analysis on each bolt I want to replace with non-VW. That will take a lot of time and tools I don't have. If I wanted to set up a company specializing in selling aftermarket bolts for VW, the time and tools might be worth it, but being a guy who still works in IT full time and doesn't have a great place indoors to work on my car, much less do all that analysis, it's not a workable plan of action.

Thanks to everyone to all your input! I think I will just continue paying for VW bolts, although I will try 325_Guy's tip for shopping around.
 

turbodieseldyke

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I went and measured (using a calibrated Mitutoyo Digimatic caliper) the crank sprocket bolt from a B4 I just replaced the timing belt and crank sprocket on (torque to 66 ft-lbs, then 90°):

New Bolt-1: 2.5345"
New Bolt-2: 2.5370"

Old bolt: 2.5550"

Bolt difference=0.0192"
"Hey youtube, today i'm going to show you how to restore your TTY bolts by SMASHing them with a hammer back to their original length. And while i'm SMASHing this bolt, don't forget to SMASH that Like button."
 

turbodieseldyke

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I understand all of that -- but to use alternate bolts, I would need to run that analysis on each bolt I want to replace with non-VW. That will take a lot of time and tools I don't have.
Instead of analyzing each bolt, wouldn't it make more sense to use the general torques for similar sized bolts, as specified by saner manufacturers who don't use deforming hardware?
 

shoebear

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Instead of analyzing each bolt, wouldn't it make more sense to use the general torques for similar sized bolts, as specified by saner manufacturers who don't use deforming hardware?
Deforming a bolt neutralizes the effect of thread friction and gets the maximum clamping force from that bolt. In order to get similar performance from a non-deforming bolt, say a torque-to-angle one instead of torque-to-yield, you would generally need to either increase the bolt diameter (impractical in most cases) or use a higher grade bolt. Higher grade bolts are stronger, but also more brittle -- probably safe in many applications, but not in some. Again, more engineering needed to know where they can be used safely. Easier just to pay the extra bucks and use VW bolts.
 

KrashDH

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Too bad VW doesn't at least give a preload value, would be easy to back that value out into a torque for a standard bolt of the same size.
 

burpod

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so where are all these TTY bolts on the mk4? i can't think of any one i've replaced in my mk4s over all the years i've had them, aside from internal engine/flywheel bolts... none on my mk3 or mk1
 

KrashDH

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so where are all these TTY bolts on the mk4? i can't think of any one i've replaced in my mk4s over all the years i've had them, aside from internal engine/flywheel bolts... none on my mk3 or mk1
Most of them (OEM) are.
Any of the hardware with the light grey color with a tinge of green. Not all of the specs call out the utilization of the TTY feature (or some are just called out as standard torque without the additional angle degree).
 

Matt-98AHU

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VW lists a whole lot of bolts as "one-time use" even if they aren't technically stretch bolts.

But, even if they aren't getting fully into the TTY point, the threads do still deform, and it is an effect that you can see with your naked eye, and when re-using those bolts, they take less torque to get to the same level of tightness.... You can absolutely feel the difference if you're attempting to torque them to the original spec.
 

AndyBees

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so where are all these TTY bolts on the mk4? i can't think of any one i've replaced in my mk4s over all the years i've had them, aside from internal engine/flywheel bolts... none on my mk3 or mk1
Until I purchased my 2000 Jetta, I had never heard of a stretch bolt. Now, mind you, my career was not in mechanics or anything related. Anyway, my first TB job on the car was without replacing those bolts. , In fact, I don't think I replaced them next time (just laid them back for future use). But, on every other TDI belonging to someone I did a job for, new bolts were installed. I do have a pile of old bolts.

I'll share a story or two here about some YTT bolts. In about 2008, I removed one rod cap on a 2001 Passat 1.8 gasser engine for the owner to see the bearing. He said put it back on ...... no new bolts or bearing. He's still driving the car. I've never replaced the suspension bolts on any TDI that I've done work on. And, in 2017, I installed a DMF on the ALH engine in my Vanagon. I could only turn the bolts thru about 50% of the 90 degree final torque procedure with my little skinny arms. That was at least 20k miles ago............... yes, likely on borrowed time of which I plan to address later this winter/spring.
 

Nuje

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so where are all these TTY bolts on the mk4? i can't think of any one i've replaced in my mk4s over all the years i've had them, aside from internal engine/flywheel bolts... none on my mk3 or mk1
Engine mount to frame rail; transmission mount to transmission; ball joint bolts and plate; timing belt large roller; flywheel (of course) and pressure plate (never understood why on that - 15 or 20Nm and "always replace"?).
 
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