stretch bolts

[flapjack]

Lets say you're talking to a guy that works at a place like TT or Black Forest or German Auto Parts etc. They say the following. How do you respond?

Stretch bolts are called stretch bolts because the vw diesel head is designed to float and flex. VW tried using head studs but stopped because they put too much pressure on the head and caused problems

[UnderPSI]

Ummm.... *click*

[clbanman]

Long story short, stretch bolts were brought into use because they allow more consistent torques in a high production environment, not because they are a better design.  One difficulty in going to studs or non-stretch bolts and determining proper torque for the application is that most engineers are doing their calculations using stretch bolts and finding good values for non-stretch hardware can be difficult.  You don't torque to the hardware, but rather to the desired clamp load you are trying to achieve.  The hardware needs to be able to hold the clamp load without relaxing or reducing it.

[saurkraut]

Lets say you're talking to a guy that works at a place like TT or Black Forest or German Auto Parts etc. They say the following. How do you respond?

Stretch bolts are called stretch bolts because the vw diesel head is designed to float and flex. VW tried using head studs but stopped because they put too much pressure on the head and caused problems

Stop talking to them about diesel stuff.

They are not stretch bolts.  They are torgue to yeild bolts.  They were incorporated for automated assembly.  A multi spindle machine torques them all at once, and they essentailly allow for slop in the process.

Here's one assembling a transmission:



Go ahead and search this forum for "head studs" and you'll find no posts about any failure atributed their use.

When these heads 'float and flex" bad things happen.

[kibs45]

Floating and flexing on the head have nothing to do with the type of bolt used but rather with the overall torque spec.  I had a Mazda 2.0 that actually was a float & flex head but it was torqued to less than 40 ft/lbs.  As far as what the stretch bolt does I don't know for sure, I do know that, that is what Cummins uses on their ISX engines though... and they actually can be reusable.  Never heard of any other stretch bolts being reusable but they sell a gauge and everything to see if the bolt is reusable or not.

[flapjack]

I know what the guy said was wrong.
But I found it very entertaining. Especially considering where it came from.

[88jetta350]

I've never been a fan of BFI personally. Their prices seem a bit inflated to me.

[arb]

Lets say you're talking to a guy that works at a place like TT or Black Forest or German Auto Parts etc. They say the following. How do you respond?

Stretch bolts are called stretch bolts because the vw diesel head is designed to float and flex. VW tried using head studs but stopped because they put too much pressure on the head and caused problems

His answer shows he's never been at an OEM... studs cost more (parts count) to make and more to install. Cost is king when there is more than one way to skin the cat...

So, when I encounter such comments, the guys saying it has his opinion, and like a butthole, everyone one has one and everyone one is stuck with the one he has ;-)  so, just smile and change the subject and you'll have a more pleasant day.