Downside of Reusing Stretch Bolts

[dosma]

Just reassembled an AAZ KKK turbo to exhaust header and reused the three stretch bolts.  Didn't realize they were stretch bolts until the last minute.

What's the downside of reusing visually clean stretch bolts the second time?  Is the only downside that they may not stay tight the second time around or is there a strength issue as well?  I.E. - When torqued to spec (33 ft-lb, 45 Nm, in this case), is there also an increased danger of bolt failure during torqing or potentially in the future when the bolts are removed?

I learned my auto repair skills during the days before stretch bolts and have not developed a hard and fast rule around stretch bolt reuse.  Opinions?

[Northern RD]

Don`t,.....
 :cry:  :cry:

[rackley]

Torquing with just torque values such as foot-pounds or newton-meters is at best a guess at getting the torque correct.  Depending on the type of fastener, materials used for both the bolt and the part it screws in to, cleanliness of the thread, lubrication used, TYPE of lubrication used, etc., the torque between two "identical" fasteners can actually vary by as much as 35% (or more) with the torque wrench having the same setting for both fasteners!
   
Many fasteners, such as wheel studs, water pump bolts, etc., are just there to make sure that the bolt is probably tight enough, and probably not too tight.  These are the bolts that you reuse over and over without any problems.

Head bolts, as well as most of the bolts internal to the engine, are what are called stretch bolts (also known as torque-to-yield bolts).  These are designed to be tightened to the point they physically start to stretch, usually by a few thousandths of an inch.  This leaves them in a elastic condition, that allows very equal clamping forces to be applied to the part they're holding down.

When you tighten one of these bolts, the torque setting that is used is designed to get the bolt close to the point that it starts to stretch.  The torque angle guage is then used to put the bolt PAST the point it starts to stretch and actually start to stretch it.  You just can't accurately measure this with a regular torque wrench, thus the torque-to-yield/angle-to-turn method.

The reason for not re-using sttretch bolts?

The reason is quite simple.  Once you have stretched the bolt once, it never returns to its original size.  This weakens the bolt, and if you try to re-use it, you take a risk of snapping the bolt in two, leaving the threaded part in the block, the top in your hand, and a complicated repair required.  It's just not worth the risk to try to re-use old bolts.

You can get replacement bolts that are not designed to stretch at the torque required to hold the parts together.

There are two good reasons for using non stretch bolts.  You want to re-use them in the future, and/or you have an engine that is either forced induction (745i running massive boost for example) or very high compression.  Since the bolt is already at it's maximum stretch without losing strength, you don't want to put undue stress on it from a high compression engine and take the risk of a blown head gasket.

There is also a good reason for NOT using these bolts, and staying with the original OEM style stretch bolts.  Non stretch bolts require retorquing after a few heat/cool cycles (usually specified at around 5-6 cycles), and may have to re-torque them anywhere from every 30k to 70k miles, depending on application.  If you have a normally asperated engine, there's just no reason to have to do the extra work when normally you wouldn't touch the bolts again after they've been done properly with regular OEM stretch bolts.  Also, stretch bolts tend to provide more consistent, even clamping pressure than the solid bolts do.

Blatently lifted from http://www.xmission.com/~kd7olf/torque.html.  :-)

Ray

[QuickTD]

Stretch bolts or "torque to yield" bolts are used to achieve very consistant clamping loads independant of thread friction and tool accuracy. The bolts are brough up to some low (and possibly inaccurate) torque and then turned through an angle that is sufficient to cause the bolt to permanently deform or stretch. Once the bolt begins to stretch it will produce the same clamping force over a very broad range of angle. This is a very cost effective method of getting very consistant results, both in the factory and in the field. TTY bolts should always be replaced. Once they have been stretched they may not deliver quite as high of clamping force and even if they make it through the torquing procedure intact, they may fail in use.
   
 Stretch or TTY bolts are exclusively angle torqued. There is no means by which bolt stretch at the yield point can be controlled by torque alone. If the torque procedure does not include an angle spec you can safely assume that the bolts are not stretch type. In my manual the AAZ turbo/manifold bolts are not noted as stretch bolts. Quite often the manual will specify "always replace" because there is a sealer or threadlocker factory applied to the bolts. Again, if they are not angle torqued you probably don't need to replace them. Just apply your own sealer or threadlocker and reuse them. Use your own good judgement, of course.

 The turbo bolts are some kind of high temp stainless alloy. They cost about $20 each at the dealer. I'm not sure I'd replace them even if they were stretch type...

[rackley]

You can also find them much cheaper at www.mcmaster.com if you know the measurements of the bolt.

[dosma]

Good information.  I'll file it away.

My next question was going to be, how do I know for sure the bolts are stretch or not?  Thanks for answerwing this for me QuickTD.  I had come to the conclusion that the bolts were stretch bolts from my Bentley manual.  For these bolts, the directions were "Replace".  There was no reference to an additional angle torque.  I just looked up the torque procedure for torqing the head and the directions were "Always Replace" and a several stage torque procedure is given with torque and angle. So the big clue is if an additional angle torque is given, it's a stretch bolt. Nice.

Actually, what got me to question whether I should have reused the bolts was that one of the bolts just didn't want to tighten down.  It was like I have felt a few times in the in the past just before a bolt snapped.  I had wire brushed the bolt threads, used 1800 C Copper antisieze and progressively torqued the bolts down in sequence.  The engine has less than 2000 miles on it. This bolt required more turns to tighten than the others.  I did make it to 33 ft-lbs though, so I'm not going to replace the bolts at this time.

Regards, Don

[vwmike]

Everyone already explained this more than adequately, but I'll relay my rules on stretch bolts

-Anything internal to the engine gets replaced with new fasteners or ARP hardware

-Anything in the clutch gets closely scrutinized and loctited into place

-Anything not on a rotating assembly doesn't get replaced unless it is damaged.