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TDI 101 Got a simple/basic TDI question? Are you a newbie (new to the forums). Feel free to post your question here.

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Old November 21st, 2009, 23:53   #1
biopete
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Default What is the deal with all "Stretch" Bolts?

Man, what a pain to work on your car on the weekend and then never be able to put anything back together because you do not have brand new "always replace" bolts. They are also known as "stretch bolts", or "torque to yield". Since the A4s VW uses them everywhere it seems :

Engine mounts,
Flywheel,
CV Axle to tranny,
Rod bearing cap,
Main bearing cap,

...
...

It is such a long list. There are major problems with this engineering decision for the home mechanic.

1) Only dealers carry them and they are never convenient to get to or available when you need them, like on saturday afternoon or sunday.
2) They are expensive, often 4.00 or 5.00 a bolt. An engine swap costs 50.00 just in bolts.
3) They are a waste of metal

What is the purpose of this engineering trend? Older VWs did not have these. Or if they did they were not written about in The Idiots Guide to Volkswagens. I don't remember them on my 1991 Jetta Diesel when doing a timing belt.

Having not researched it at all, i would venture the main motive is quick assembly on the line. You do not need locktite and that slows the worker down. Just push the button on the torque gun three times for initial tighten, the torque, and then the extra quarter turn or "torque to yield".

Anyone know anything thing about these torque to yield bolts used in way too many places on these cars? Do you resuse yours? In protest I'm thinking of just using locktite and torquing a little less than spec and seeing how it goes.

Has anyone ever reused one and it broke? Where?
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Old November 22nd, 2009, 00:36   #2
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The bolts deform permanently when tightened. The rationale is simple:

-More consistent clamping force
-A given amount of clamping force can be achieved with fewer fasteners.

Don't reuse the deformed bolts. They're weakened and will break sooner or later. A lot of the one time use screws can be bought with better pricing as sets for a given application from the specialty parts dealers as opposed to the VW dealership, which can have pretty horrible pricing.

One of the most common things (you can search for) that happens is the occasional engine that falls down due to someone somewhere not replacing the engine bracket to mount bolts when changing timing belts. A lot of the other single use screws don't get taken apart as much.
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Old November 22nd, 2009, 00:57   #3
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Interesting. Fewer fasteners. Hadn't thought of that. Just googled a bit and read about the bolt science.

Unfortuneately specialty parts dealers are not available on Sunday usually either. It is such a one sided rationale. I guess i wont be reusing any TTY bolts on important things.

But can you tell a difference between tty bolts and non tty , like if someone replaced them with not tty? My flywheel bolts have blue locktite and look like just regular steel black allens. I'm wondering if someone did a clutch job and replaced the tty ones.


I suppose i could just get a matching bolt that is not TTY and lock tite it?
Can you replace these bolts with not tty bolts?
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Old November 22nd, 2009, 01:47   #4
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Opinions are cheap but I will give my opinion as to why the tty bolts got into use. Long long ago when you put on a head gasket things tended to settle in and you needed to have your head bolts re-tightened in 50 (?) miles. The head gasket itself compressed and the bolts stretched a little more, leaving the head not pulled down enough and risking a blown head gasket. The industry worked on the head gaskets, and that helped. Finally, they found a way to help stop the head bolt stretch. Now you buy a car and within reason the head gasket just goes forever and ever without blowing. This all might allow the use of fewer bolts, but over the years they have found that the number of bolts you use is more stable with tty bolts.

I wrote a little on tty bolts here yesterday.
http://forums.tdiclub.com/showthread...241279&page=42

There is a little difference of opinion on why, but that is why we come here to discuss at times. There are some non tty bolts you can get to allow multiple removals of parts. When you get into standard stretch bolts you have to watch thread lubrication, and there is always a limit to the number of times aluminum will allow bolts to be retightened in it. You have to go back to re-tighten some areas as mentioned before. Tty bolts might be a way to keep folks from stripping out bolts threaded into aluminum. They just do not work on things at the house as much. Heli-coils will fix that, but it might cost the company a USD to do 3 bolts with coils, and they want to save that money.

Progress, got to love it.

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Old November 22nd, 2009, 05:33   #5
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Quote:
Originally Posted by biopete

It is such a long list. There are major problems with this engineering decision for the home mechanic.

1) Only dealers carry them and they are never convenient to get to or available when you need them, like on saturday afternoon or sunday.
I would add that many times, the dealer does not have them in stock at all, even though they are REQUIRED for many common repairs. Makes you wonder if their own mechanics are following the procedures for replacing tty bolts.

Last week, I needed 4 tty bolts to attach the rear caliper carrier frames to the car. Impex was only able to find 2 for me and I had to order 2 more from the dealer. I wonder how the dealer is able to replace rear rotors in a day when they do not stock key parts
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Old November 22nd, 2009, 05:35   #6
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Another way to look at it; ferrous alloys don't fatigue if they don't cycle through zero stress, like a spring with plenty of internal stresses or a proper bicycle spoke.
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Old November 22nd, 2009, 05:54   #7
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"Man, what a pain to work on your car on the weekend and then never be able to put anything back together because you do not have brand new "always replace" bolts. They are also known as "stretch bolts", or "torque to yield". Since the A4s VW uses them everywhere it seems :

Engine mounts,
Flywheel,
CV Axle to tranny,
Rod bearing cap,
Main bearing cap,"

Don't forget the cylinder head, shock and strut mounting, subframe, tranny dogbone mount, and ball joint bolts. Give me a few minutes and I will think of a bunch more!

Yea, they are used in many places on VWs. I looked at the Honda service manual for my son's car and you see no mention of stretch bolts. I wonder if this is an European tradition. It is a pain.

--Nate
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Old November 22nd, 2009, 07:14   #8
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Can you identify them by a quick glance? Or do you just "have to know'?
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Old November 22nd, 2009, 08:16   #9
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you have to know. Yea they're a pain, planning... I have had the same issues. Comes with the territory.
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Old November 22nd, 2009, 09:09   #10
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Quote:
Originally Posted by PDJetta
Yea, they are used in many places on VWs. I looked at the Honda service manual for my son's car and you see no mention of stretch bolts. I wonder if this is an European tradition. It is a pain.

--Nate
It would be interesting to know how many other car manufacturers use TTY bolts. Personally I don't like them and consider it poor engineering. No doubt some bolt meister will come along to teach me the error of my ways but as far as I am concerned it's a racket.
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Old November 22nd, 2009, 09:18   #11
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My opinion is that not all "Torque+Angle" fasteners need to be replaced. Torque+Angle does not always mean "Torque to Yield" and without dong the calculations you don't know which is which. I think that Bentley/VW lean to the conservative side on telling you which ones to replace (all of them).

I have re-used many "Always Replace" bolts on my car and have not had any issues. (motor mount, dogbone mount, Axle flange, Flywheel) Many of the times I've done this I was in a similar situation that the OP describes where a new replacement was not available and the car had to go back together. Many of them I have since replaced, however, I ran my flywheel bolts 20+k miles on a 2nd use. Do this at your own risk - I am not recommending it - Just sharing my experiences.

Here's another "fix" I did on my left side motor mount

There's a few that I wouldn't risk though - head bolts, rod caps, main caps - basically anything that is a real big PITA to replace in the future and/or could cause significant other damage.

The official stance is to replace what it recommends.
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Old November 22nd, 2009, 12:10   #12
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IMHO the auto manufacturers are try to save money. You can put in a lower strength bolt, stress it just shy of permanent deformation, and it will hold find for one use. Or maybe it is more noble and the auto manufacturers are trying to lower weight, I doubt it though.

The left side motor mount aka bracket is a classic case of what were those engineers thinking!
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Old November 22nd, 2009, 15:01   #13
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Quote:
Originally Posted by Lee_Taylor
It would be interesting to know how many other car manufacturers use TTY bolts. Personally I don't like them and consider it poor engineering. No doubt some bolt meister will come along to teach me the error of my ways but as far as I am concerned it's a racket.
GM and Subaru uses tty head bolts.
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Old November 22nd, 2009, 17:48   #14
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Quote:
Originally Posted by Doug Huffman
Another way to look at it; ferrous alloys don't fatigue if they don't cycle through zero stress, like a spring with plenty of internal stresses or a proper bicycle spoke.
Yes, higher initial tightening force helps keep the fastener or thread below the fatigue limit which can be well above zero. Also, tightening by torque is very dependent upon thread lubrication and fastener coatings. Tightening force can vary outside the safe range very easily, thus permanently deforming the fastener.
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Old November 22nd, 2009, 18:06   #15
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Mercedes specifies a length of their stretch bolts... too stretched and you can't re-use them. I don't know if I entirely trust that...

-Jason
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