Torque to Yield Bolts

[redout]

While not quite on the subject of control line models, may I ask a mechanical related question which has intrigued me for the past year odd and which I cannot seem to find a reassuring answer using Google searches.

Does anyone know if "torque to yield" bolts are exactly the same bolts ( metallurgically and construction-wise ) as "conventional" bolts ?

So for example, is a TTY bolt marked with the grade "10.9" exactly the same bolt as a non-TTY bolt marked "10.9" ?

( For context these are ( TTY ) bolts associated with a motor vehicle ( engine and suspension ) and which the manufacturer is stating must be replaced each time they are removed. )

[Dan McEntee]

   Head bolts are like that on modern engines. they are not intended to be reused because they stretch and act like a really strong rubber band to hold the head down. With the advent of aluminum heads they have pretty much become standard. If you have a valve job of similar work done on a head and get a kit   to replace it, it will come with new bolts and the special procedure to torque them down. it IS quite a workout!!
   https://dannysengineportal.com/torque-to-yield-bolts-tty-what-exactly-are-they/
  Type at you later,
  Dan McEntee

[spare_parts]

Always follow the mfg's spec. I've looked into it before. Standard fasteners can be installed TTY, but simply checking the installation process and common torque values for standard sizes would tell you if the bolt is being yielded. Typical TTY install is torque to value, plus "X" number degrees. If a torque value is spec'd which is greater than the typical value, I'd replace it. If it's marked 10.9, the material is defined. That is you can't have a common 10.9 bolt that's somehow different than another 10.9 if the dimensions are the same. How would you know otherwise?

Some "one time use" fasteners are simply due to loctite. Flywheel bolts on M54 BMW are one I ran into. Manual says replace them. Bolts were 12.9. Replacement bolt photos showed pre-applied threadlock/sealant because the hole in the flange is a through hole. Standard torque value for the fastener. I applied new goop and reinstalled.

[Steve Dwyer]

Very common on full size aircraft. You're never supposed to re use prop bolts after torque tightening them.

[Air Ministry .]

Theyed have a intresting load cycle .

Old 50's BSA Rod Bolts were fitted , by measuring ' stretch '  . Castellated nuts & split pins to ' fix ' .
Seeing things  like that are clean & seated properly ( we wont mention piston pin circlips ) if neglected dosnt help .
Debri , swarf , muck , and ' undressed ' machining - in the ' good old days ' where points to observe . & remedy .

[Brett Buck]

   Head bolts are like that on modern engines. they are not intended to be reused because they stretch and act like a really strong rubber band to hold the head down. With the advent of aluminum heads they have pretty much become standard. If you have a valve job of similar work done on a head and get a kit   to replace it, it will come with new bolts and the special procedure to torque them down. it IS quite a workout!!
   https://dannysengineportal.com/torque-to-yield-bolts-tty-what-exactly-are-they/
  Type at you later,
  Dan McEntee

      All bolts work on the principle of "rubber bands", AKA tension, to apply clamping force. What is different in this case is that you tension it to the point of plastic deformation, meaning they are permanently stretched, rather than springing back when the tension is removed.

    Brett

[Bruce Guertin]

TTY is used on cylinder head bolts as it's the accurate way to get a consistent clamping load with a bolt in a blind hole. That's all I know about them. Automotive bolts and race bolts in particular have changed a great deal in recent years. Rod bolts really require special handling. All the bolts in my race engines are ARP brand.

[Paul Smith]

This whole issue has to do with bolting iron and/or steel parts together.

With an aluminum carcass the threads will strip long before this technology comes into play.

[Brett Buck]

This whole issue has to do with bolting iron and/or steel parts together.

With an aluminum carcass the threads will strip long before this technology comes into play.

  Well, no, lots of aluminum engine head bolts/studs are torqued into the plastic region, precisely because the aluminum expands and contracts more than iron. This holds the preload/clamping forces constant over temperature.

         Brett

[Dan McEntee]

      All bolts work on the principle of "rubber bands", AKA tension, to apply clamping force. What is different in this case is that you tension it to the point of plastic deformation, meaning they are permanently stretched, rather than springing back when the tension is removed.

    Brett

     Yes, I am aware of that. i was just trying to keep the explanation simple. I had done head work and such on engines with cast iron heads and blocks on "old school" engines on several occasions in my younger days. Then my daughter's '96 Ford Escort threw a valve guide. A new car was not in the financial picture, and after consulting my ace auto mechanic nephew to get updated on what the more modern engines required, I decided to bust my knuckles one more time and justify the $500 or so that the machine work and parts were going to cost. The procedure to torque the head was really interesting!! My nephew warned me not to take any short cuts, and to have two bowls of Wheaties that day I put the head back on! I don't think I can remember exactly how it went, but it was explained very well in the instructions that came with the kit and new bolts, and it was a 4 step process I think, with a changing pattern and increase in torque value with each pass. There were 10 or 12 bolts I think, more than I thought there would be for a 4 banger, but by the time I was finished, I was plumb tuckered out and spent!! It all came out great and we saved a lot of money. I even got my daughter involved by making here wipe down wrenches and such when I was finished with each session!! THAT was a funny! I told her that it was either that or walk!  Funny how stark reality can really drive a point across sometimes!
  Type at you later,
   Dan McEntee

[Steve Dwyer]

And after torque tightening the prop bolts we secure them with that nasty finger cutting stainless steel safety wire. Glad I'm not doing any of that Repairman stuff anymore, just too old for it.

Steve

[redout]

Always follow the mfg's spec. I've looked into it before. Standard fasteners can be installed TTY, but simply checking the installation process and common torque values for standard sizes would tell you if the bolt is being yielded. Typical TTY install is torque to value, plus "X" number degrees. If a torque value is spec'd which is greater than the typical value, I'd replace it. If it's marked 10.9, the material is defined. That is you can't have a common 10.9 bolt that's somehow different than another 10.9 if the dimensions are the same. How would you know otherwise?

Some "one time use" fasteners are simply due to loctite. Flywheel bolts on M54 BMW are one I ran into. Manual says replace them. Bolts were 12.9. Replacement bolt photos showed pre-applied threadlock/sealant because the hole in the flange is a through hole. Standard torque value for the fastener. I applied new goop and reinstalled.

Yes that is what I was thinking. Some of my stretch bolts are marked with the grade ( and others aren't). So the "10.9" calls out the ultimate strength and where the yield point is in relation to the U.T.S.

Given that the parts shop I deal with won't sell me any of the stretch bolts that the manufacturer says to replace unless I can supply them part numbers, and they won't let me look at their books /screens to get the part numbers, I was considering sourcing matching bolts from my local bolt shop, but due to lack of information of material on many of the original stretch bolts, I gave up on this as too risky to try for a timing belt change, given the consequences of a fastener failure in this area. Also I don't know enough about the quality of generic e.g. "10.9" bolts from a bolt supplier vs. factory supplied bolts.

After looking for a seeming eternity, I eventually found factory parts diagrams with part numbers so have just ordered the o.e.m. bolts. These are what V.W., the engine maker, say have to be replaced for a timing belt change that also involves dismantling the engine mounts at one end to get to. So about $ Aus 200 ( roughly $ US 130 ) later I now have the bolts I need.

I can understand why stretch bolts are used as cylinder head fasteners in modern high performance engines but can't really see the point of making things like idler pulley securing bolts and suspension bolts in this manner, something that ( modern ) V.W. seems to do a lot of.

Thanks to everyone for their comments.

[redout]

Theyed have a intresting load cycle .

Old 50's BSA Rod Bolts were fitted , by measuring ' stretch ' . Castellated nuts & split pins to ' fix ' .
Seeing things  like that are clean & seated properly ( we wont mention piston pin circlips ) if neglected dosnt help .
Debri , swarf , muck , and ' undressed ' machining - in the ' good old days ' where points to observe . & remedy .

Yes and I think any top race engine builder does the same. And I believe also done in piston aero engines. Much more accurate than setting to a torque value.