A Treatise of re-using cylinder head bolts

[a97obw]

Ok, since the 2.5 engine head gasket replacement is a usual topic around here, and it did happen to me as well on my 97, every now and then the topic of re-using the head bolts comes up.

So, with 2 camps of thought being either re-use them or never re-use them with regard to the "Torque to Yield" argument, I figured I'd run it past an 86 year old still practicing consulting structural Civil Engineer, who just happens to be my Pop. I described the procedure of the tightening sequence to him according to the factory manual. So here we go.

 

"Concerning the "Torque to Yield" for Subaru engine head bolts---

 

 

The bolts are tightened by turn of the head. Therefore, in addition to the induced tension due to the elongation produced by turning into the threads, there is also a torsional load on the bolt due to resistance to turning by thread friction. This torsional load is equal to the specified torque required. Torsional yield will result in a permanent "twist" in the bolt which remains after the bolt has been loosened. The bolt may be re-used and will not sustain additional "twist" until the original torsional load has been exceeded.

For tension, the bolt will elongate under load in the elastic range of the steel or aluminum and will return to original length when the load is removed. If, however, the bolt is loaded beyond the elastic limit to the point of yield into the plastic range, elongation after yield will not return. The bolt may be removed and re-used and will sustain the same load as before to the point of yield. Additional load will cause additional elongation in the plastic range."

 

 

Comments?

[uniberp]

I don't agree that a bolt elongated past it's elastic range is reusable. When that occurs, the thread pitch will no longer match it's purchase, and that cause a smaller region of contact, hence, less strong connection. Whether Subaru has enough overbuild in the headbolt/case connection, IDK.

[skibumm100]

In general, I agree with your dad regarding plastic vs. elastic deformation of the bolt. Once you go past the elastic portion of the load curve, the bolt will permanently lengthen. How does this pertain to the re-use of the bolts? Rule of thumb on torque to yield headbolts is to use new ones everytime. It doesn't mean they will fail if you do re-use them. Each time you make the bolt yield, it will get a little longer. The thing you have to be careful about is that once you make them start yielding (stretching), it takes less force each subsequent time to start to make them yield, which means less clamping force on the head which could lead to premature failure of the head gasket. The force curve drops off after the yield point until failure. The yield point is at the top of the force curve. Personally, I wouldn't worry too much about re-using them once, maybe twice if money is really tight. Depends on who is doing the work and how many cars I have running and licensed at the time.

[robm]

You guys aren't reading very closely. He says it will yield in TORSION, so it TWISTS a bit, it doesn't get any longer. He figures that as long as the torque applied doesn't go any higher, it won't twist any further. The tension produced is still in the elastic range, so it doesn't get any longer permanently.

 

It was my impression , many years ago when I took mechanics of solids, that the yield would be along the "transformed principal axes of the stress". SInce we are combining torsion with tension, the yield would be along some weird angle, depending on the relative values of the two. It wouldn't yield purely in either tension or twist. I suspect the torsional deformation might be higher, and it would be easier to measure, but it still yields slightly in length..

 

I am not sure this is valid, but I am no expert, It was my impression that torque to yield bolts exhibit permanent, non-elastic deformation. Bolts that do this should not be reused.

 

What does Subaru say about reusing head bolts? Not all head bolts work this way. Do they use torque to yield bolts? If they recommend new ones, case closed. If they don't, don't bother.

[ron917]

I looked at the factory service manual for my '99 Outback (2.5 DOHC). It doesn't say anything about new bolts. So, for this engine at least, re-using the old bolts is factory approved.

[nipper]

In a nut shell it is said that if you overtorque the bolt, you will damage it and it has to be replaced. In an aluminum engine you with a substantial bolt being threaded into an aluminum block, it is very hard to overtorque the bolt and damage it. What you WILL do is strip the threads in the block and damage the headgasket.

I know working as an inspector/rebuilder for a defense contractor (man i have a busy resume) if there was any signs of torque twist on the bolt it was scraped. I would never reuse any bolt no matter the application that "took a twist".

In some applications, that is one of the reasons for using a washer, the second being to evenly distribute the load on the bolt head. In a HG application, you do it in steps to compress the HG in a controlled manner, and to avoid warping the head.

If someone could give me the size and grade of the bolt, i can give you the max torque that bolt should be apllied (assuming steel threads).

Subaru reuses the head bolts. I thought from previous posts this was settled sheesh

 

nipper

[Legacy777]

Ditto.....reuse the head bolts.

[idosubaru]

Ditto.....reuse the head bolts.

+1

 

lots of words for little function.

the bolt is loaded beyond the elastic limit to the point of yield into the plastic range

 

saying "plastic range" by default means you went through the elastic range, through the point of yield and into the plastic range....lots of words to say one thing. but no matter, the last thing i want to do is revisit materials classes, they were terrible. actually what your dad mentioned is the most profitable information to be gained in materials engineering. slap the appropriate equations in place for those terms he mentioned and consider yourself well versed in materials. it's all downhill, boring and useless from there!

 

tell your dad i'm not listening to anyone who studied before Tacoma Narrows (that's a joke, ha ha). if you don't know what that is, look it up on the internet and check out the video, it's really cool. it's certainly the highlight of any civil engineering program. not sure if the vid's are copyrighted or available or not, but it's cool.

 

reuse the headbolts. subaru does. not often that head gaskets fail pre-200,000 miles and if they get a head gasket replacement they don't typically re-fail (you turbo guys aren't allowed to respond!!!)

[Tiny Clark]

We re-use bolts all the time on the C130 to hold major sections together, i.e. fuselage to the nose or tail section. We worry more about the lock-nut. Some bolts go thru an NDI check, but our torque values are significantly higher than auto cylinder head bolts.

 

IMO, reuse 'em.

[a97obw]

Yep, I know very well about the Tacoma Narrows bridge. Well, Dad has had only one bridge fall apart in his long history. You've probably seen it in the media in the past several months. It's the long bridge between Biloxi and Ocean Springs on the MS Gulf Coast---the one where you see all the piling and a few of the spans in the water. He shakes his head when he thinks what was built for less than $5 million dollars in 1954 is going to cost in the neighborhood of $280 million today.

 

Here's another on the Northeast corner of the Bay of St. Louis about 7 miles inland of the MS Gulf Coast. Can you spot the problem? Can you say "storm surge"? Fixed in a couple of days!

 

 

[idosubaru]

looks like one of those optical illusions, you're supposed to cross your eyes and everything looks straight again. i figured it impossible for you to not know about tacoma narrows. as an aerospace engineer we touched on that bridge in our vibrations analysis.

 

based on all that yield stress technical discussion assuming the threads were'nt all out of whack or you chased them with a die to straighten them up, maybe even a "stretch to yield" bolt would be reusable if you torqued it to higher than it was previously torqued to, again taking it into the plastic region and re-stretching it somewhat. people on the old generation forum over torque head bolts on the turbo motors to keep the headgaskets happy, can't imagine an extra few pounds would hurt. might be better than reusing the bolts straight up. of course unless you did it, there's no way to easily verify exactly how much it was torqued "last time" but you'd assume it was factory spec since you'd hope to do head gaskets one time only!

[robm]

I can't see even a stretch to yield bolt stretching the threaded portion. The load is distributed over several threads, so in that part of the bolt, there will be no plastic deformation. The long, unthreaded portion of the shank that acts as a spring is what would deform.

 

That is my eyeball analysis. Anyone out there with a better one, backed up by hard data, strain gauge studies, or whatever?

 

It is a moot point anyway, for Subarus. Where these mythical torque-to-yield bolts used, anyway?

[a97obw]

With regard to the discussion, we were focusing on the cylinder head bolts that hold the cylinder heads to the engine case, particularly on the 4 cam 2.5 engines. 6 on each side, the heads of which are apparently "painted" at some point by the factory. Now, if the paint represents a "torque to yield" situation or in my opinion, merely an assembly mark for quality control that the bolts have been installed to specifications is another question.

 

 

The yellow and red marks you see on the bolts where from my own "quality control" process of replacing the head gaskets using the factory torque procedure. Otherwise, the heads of the bolts have a translucent blueish/green marking.

[nipper]

With regard to the discussion, we were focusing on the cylinder head bolts that hold the cylinder heads to the engine case, particularly on the 4 cam 2.5 engines. 6 on each side, the heads of which are apparently "painted" at some point by the factory. Now, if the paint represents a "torque to yield" situation or in my opinion, merely an assembly mark for quality control that the bolts have been installed to specifications is another question.

 

 

The yellow and red marks you see on the bolts where from my own "quality control" process of replacing the head gaskets using the factory torque procedure. Otherwise, the heads of the bolts have a translucent blueish/green marking.

 

The paint is a quick check to make sure the bolts have not moved during assembly and testing. It is also a way to see if the engine has been opened up after it leaves the factory. It's a common procedure to do.

 

If you torque the bolts to yeils on a HG you will DESTROY THE HEAD GASKET! Also a good chance you will warp the cylinder head.

 

nipper