What is the deal with all "Stretch" Bolts?

Since this is a bolt topic. Where in the heck would I or could I buy the bolts other then the dealer. Sheridan WY has a limited selection of grade 8 metric bolts and the dealer doesn't even carry a lot. I did my wheel bearings and I had to take the ball joint bolts out. I had to go 130 miles to the town with the VW dealer so I figured I would just stop in and get them. They didn't have them so I had to get longer bolts and cut to size grade 8 bolts. I have to get almost all my parts online so an online place would help me. I love my VW's and won't part with them but that means I need a source for these lovely tty bolts.

compu_85 said:

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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Cummins, for their 3.9, 5.9 and 8.3 liter engines, does the same. If bolt exceeds a certain length, then do not reuse. I've done several head removals and the bolts were all within spec, and got reused.

It would be interesting to mic a VW TTY bolt, install and torque it, then remove it and mic it again. Do it a few times and record the numbers. If it continues to stretch, then it is stressed past yield each time. If length does not increase with each use, I would have some confidence in reusing them.

coalminer16 said:

Since this is a bolt topic. Where in the heck would I or could I buy the bolts other then the dealer. Sheridan WY has a limited selection of grade 8 metric bolts and the dealer doesn't even carry a lot. I did my wheel bearings and I had to take the ball joint bolts out. I had to go 130 miles to the town with the VW dealer so I figured I would just stop in and get them. They didn't have them so I had to get longer bolts and cut to size grade 8 bolts. I have to get almost all my parts online so an online place would help me. I love my VW's and won't part with them but that means I need a source for these lovely tty bolts.

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Take a look here:

http://tdiparts.com/catalog/index.php?cPath=36_62&osCsid=1p9uhe0oaaji3ec58h1em7ubk1

Here are your ball joint bolts:

http://tdiparts.com/catalog/product_info.php?cPath=36_111&products_id=702&osCsid=1p9uhe0oaaji3ec58h1em7ubk1

They got other stretch bolts, too, for engine mounts, clutch replacements, etc.

You just gotta look thru their web site.

--Nate

c'mon people, it is an obvious German government cover up to keep bolt industry in business.
Seriously, I just got a handful of bolt from impex for close to $300 to do a 5 speed swap

What still puzzles me is that when a new stretch bolt is torqued up correctly, it stretches and still is good enough to do it's job. Then, when I take it out, without tightening it any more, all of a sudden it is supposedly too weak, and I cannot use it again using that same torque because now it is too weak, although it did fine before I took it out. What am I missing?

rodejetta said:

What still puzzles me is that when a new stretch bolt is torqued up correctly, it stretches and still is good enough to do it's job. Then, when I take it out, without tightening it any more, all of a sudden it is supposedly too weak, and I cannot use it again using that same torque because now it is too weak, although it did fine before I took it out. What am I missing?

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$300 for new bolts.

Well, I read here that a 6-speed swap can run up to $6000 for all the stuff you need, so $300 is nothing compared to this!

--Nate

In Germany, the tty bolts are common... not cheap, but common. They put a lot of stock in the technology.

In situations, they are the ONLY bolt. Head bolts, for example (sorry, but even ARP bit the bullet and makes TTY's for certain diesels).

That being said, there is a limit to the insanity. Once bitten by the TTY bug, the Germans seem to have gone over the top.

For example, I have recently finished the preliminary engineering for reusable bolts in the PD cylinder head. $100 just to look at the camshaft for TTY's is absurd.

Part of the reason is what happens to aluminum threads when using a harder bolt. If a 12.9 bolt is used in place of a 8.8, the 'stretch' is diminished greatly. The torque becomes the critical measure. Where thermal expansion comes into play, the torque becomes even more critical.

The proper torque is a problem due to frictional forces, which can be as much as 60% of the total torque. The TTY bolts overcome the problem of variance in the frictional force when the bolt 'gives' at the proper clamping force.

I ran into an interesting site that helps with much of the issue:

http://www.boltscience.com

It's a primer on bolt technology and application. Although it doesn't get into all the angles of the TTY's, the big issue is in commercial application, rather than having a very finicky and exacting method of installation, you can 'hammer it home' , within limits, and it works great every single time.

But given the aftermarket and especially a few arguably 'damned expensive bolts', I found the proper torque, eliminated the frictional barrier and have a very good solution for the PD cylinder heads with relatively cheap and reusable bolts and a very accurate torque and the use of moly grease.

Btw: another thread mentions "Do you use Anti-seize on lug bolts?" The answer is NO! DO NOT USE IT! That bolt is particularly high frictional load. If you eliminate the friction, you have to reduce the torque a great deal.. at least 30%. Given the importance of the fastener, I wouldn't mess with it. Over-torquing is as bad or worse than under-torquing. In the end a snapped bolt is as bad as one that comes loose.

I've wondered the same question of how to identify a TTY fastener my self, and it's not easy to find an answer. I think this was really the only question asked. It looks like the person who asked that question is pretty knowledgeable about what they're for and what their limitations are, so I won't give them yet another lecture on the science of joint control fastening.

I did find this though, but how useful is it if you're not looking at a head bolt? To my eyes (which need a lot of correction) the suspension bolts have any of the indicated features but I'll have to look again. In fact, the threads on some go all the way up to the washer head, and I don't think the stretch would include the threads. Is there any answer how to identify to TTY bolts?