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Discussion Starter · #1 · (Edited)
I'd greatly appreciate opinions on this:

I've replaced a few axle bolts over the years and generally followed the Bently Manual instructions to torque to 140 ft-lbs and then the additional half turn (180 degrees) after. But I usually would only do an additional 1/4 turn (90 degrees) because I felt that if I turned any more, then something would break. Some axle suppliers for the Passat recommend only doing the additional 1/4 turn, not the full half turn. I never had any problem doing axle bolts this way.

But today I replaced the Crankshaft Bolt for the first time, and am very worried that something went wrong, and I don't know what to do now. Here's what happened:

Similar to the axle bolt, the Bently spec says to pre-torque the crankshaft bolt to 148 ft-lbs and then do an additional 180 deg. So I torqued it to the 148, and then started to turn further with a long pipe for leverage. (btw, I oiled the bolt threads as Bently recommended. But I also put some grease at the contact area between the bolt head and the washer, which may have been a huge mistake, because it reduced the friction that is the basis of the torque spec !) This did not occur to me until after what happened next...

Everything was going normally at first, like when doing the axle bolts. It took a huge amount of force to keep it turning past the 148 ft-lbs, just like the axle bolts had. But before it even reached an additional 90 deg, all of the sudden the turning force got EASIER. It felt exactly as if something had "given way". The feeling was very much like when you torque something too much and strip the threads. So I tried turning a bit further, to bring it to the 90 degrees mark. During this last effort, there did not seem to be any change in the force needed. It was definitely less than the peak force (before something yielded) but was still more resistance than the original 148 ft lbs. Out of curiosity, I put the torque wrench back on that was set to 148 ft lbs, and the bolt did not move at all at 148 lt-lbs.

A few things could have happened here (listed below from best-case to worse-case). Please let me know your thoughts, and your opinon on how to proceed:

1) The best case scenario is that this is actually what was supposed to happen. I read in another post that stretch bolts are intended to exceed their yield strengh and to actually deform. (This is presuambly why they can only be used once.) This would explain the reduction in the torque needed to turn it, as the yield strength was exceeded.

2) The second scenario is that this is not supposed to happen. (That it is not supposed to get easier to turn, or to actually exceed the yield strength of the bolt.) In this case, the bolt's integrity has been compromised (it might be about to snap, or the threads have been partially stripped.)

3) The worst case scenario is that the threads inside the Crankshaft itself have been partially stripped. This would be disasterous, for obvious reasons.

If scenario #1 is the case, then all is good. But if someone could tell me for sure that this is definietly not the case, then we are looking at scenarios #2 or 3.

If the bolt is damaged, but we are reasonably sure that the shaft threads are fine, then I would just replace the bolt. (And not put any grease between the bolt head and washer this time ! )

But if there is any chance that the shaft threads may have been partially stripped, then it's probably best to just leave the bolt in there as it is. It may still be holding good enough, because it definitely would take more than 148 ft-lbs to loosen it (at least at the moment). As long as nothing changes over time, then this should be enough to hold the toothed-sprocket in place.

I'm only going to keep the car for another 85K or so (currently at 415K -- going for the big 500. lol) So, that crankshaft bolt will never have to be removed again. I'm leaning towards just leaving it as is, unless someone can assure me that the shaft threads are much stronger than the bolt's threads, and that the bolt would definitely strip/break before the shaft threads were damaged.

Any thoughts would be greatly appreciated !
 

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As you don't list the engine Etc in your profile. It would help if you mentioned here that it is an ATQ.
Did you use a new bolt ?
I can't say with any certainty but I think a damaged bolt would be more likely than damage to the thread in the shaft.

It is also possible that the key in the pulley could be damaged, are you sure the key was properly seated in the shaft ?
See this link: Audi Crankshaft Pulley - Audi Crankshaft Pulley Technical Tip

I wouldn't run it as it is.
 

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Discussion Starter · #3 ·
As you don't list the engine Etc in your profile. It would help if you mentioned here that it is an ATQ.
Did you use a new bolt ?
I can't say with any certainty but I think a damaged bolt would be more likely than damage to the thread in the shaft.

It is also possible that the key in the pulley could be damaged, are you sure the key was properly seated in the shaft ?
See this link: Audi Crankshaft Pulley - Audi Crankshaft Pulley Technical Tip

I wouldn't run it as it is.

Thank you, as always ! Yes, I forgot to mention it is indeed an ATQ (V6 30V 2.8L). Also, I did use a new bolt, from the VW dealer.


Excellent point you raised regarding the key in the pulley. I'm 100% certain that there was no damage to that key. I previously read that service bulletin, and it was for that reason that I decided to put some grease between the bolt head and the washer. This lubrication reduced the amount of torque transmitted from the bolt head to that pulley key while the bolt is being tightened. It actually was a good idea for that purpose. But I forgot to consider that this same lack of friction could result in too much tightening (stretching/stripping) of the bolt !

Before tightening, I was certain that the pulley was fully seated and the keyway was fully engaged. I also marked the position of the pulley, and it did not move at all during the torqueing procedure. The crankshaft was locked in place with the proper locking pin tool. (At first I thought that I had sheared off that locking pin tool, but then when I saw that the pulley had not moved from it's original position, I knew that the locking pin was still holding the crankshaft in place. Phew ! )

I'm afraid to remove the crankshaft bolt now, because if the inner threads of the crankshaft are already damaged, then the current bolt installation is going to be the best shot I have at it being usable. If I were to remove and reinstall a new bolt, then that would compromise the inner threads even more if they already are weakened.

Please let me know if your advice has changed considering this new information. Thank you again, as always.
 

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Discussion Starter · #4 ·
If the shaft's inner threads are of a stronger metal than the bolt, then the bolt would be damaged before the shaft threads. This would be good engineering design. So I agree with your presumption.

It could also be that the engineers designed them to be the same strength. Which means they would fail more or less simultaneously.

Can anyone answer whether the shaft thread metal is stronger than the bolt's metal ?
 

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When the bolt is tightened correctly, it ends up at about 300 ft lbs. So from what you say I doubt you have damaged the bolt or shaft, but I'm not there.
I expect that bolt would break before there was any serious damage to the thread.
I suggest you check the pulley alignment with the crank lock pin fitted.
Fit the bottom TB cover (leave bolts loose) and the harmonic balancer, and check the timing marks. You should be able to move the cover back and
forth so that the mark moves a little each side of the mark on the harmonic balancer.

If the TB pulley is correctly aligned I would try to complete the tightening. When a bolt has been tightened to 145 ft lbs and then sits,
when you start to turn it again it will take more force to start it moving than it takes to keep it moving. maybe that is what you experienced.


I reiterate "I wouldn't run it as it is"
 

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Discussion Starter · #6 · (Edited)
When the bolt is tightened correctly, it ends up at about 300 ft lbs. So from what you say I doubt you have damaged the bolt or shaft, but I'm not there.
I expect that bolt would break before there was any serious damage to the thread.
I suggest you check the pulley alignment with the crank lock pin fitted.
Fit the bottom TB cover (leave bolts loose) and the harmonic balancer, and check the timing marks. You should be able to move the cover back and
forth so that the mark moves a little each side of the mark on the harmonic balancer.

If the TB pulley is correctly aligned I would try to complete the tightening. When a bolt has been tightened to 145 ft lbs and then sits,
when you start to turn it again it will take more force to start it moving than it takes to keep it moving. maybe that is what you experienced.


I reiterate "I wouldn't run it as it is"


Okay, thank you. You convinced me.

I'll check it out when I get home tonight.

btw, I noticed that the bolt has a "9.8" designation stamped on the head. This means Class 9.8. There is a good chance that the shaft metal is as strong or stronger than this. So I don't think the inner shaft threads were damaged.

I already ordered a new bolt and will have it tomorrow. I plan on just reinstalling the new bolt, making sure that everything is seated properly, although I'm 95% certain that it was seated properly the first time. I was VERY careful of this, since already being aware of the TSB on the key damage issue.

I'll probably take the advice of others on this board and will just torque it to the initial (148 ft-lbs) but will not do any additional tightening after that. But will use red ("permanent") thread-locker. I might still put grease the area between the bolt head and the washer (to prevent torque being transmitted to the toothed pulley, thus preventing damage to the key), but will not torque it past the 148 ft-lbs this time.

Please let me know if you have any further comments, especially on the red thread-locker concept ! Thank you !
 

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NOOO! Do not use Loctite red...unless you want to be using a torch (thus ruining the front crank seal) the next time that bolt needs to come out.

All VW torque specs are for dry bolts! (Enough with the over-thinking of things, sheesh) Application of any fluid to a TTY bolt will result in a :crazy: (improperly torqued bolt).

Good catch on the original post though. :thumbup:
 

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Discussion Starter · #8 · (Edited)
NOOO! Do not use Loctite red...unless you want to be using a torch (thus ruining the front crank seal) the next time that bolt needs to come out.

All VW torque specs are for dry bolts! (Enough with the over-thinking of things, sheesh) Application of any fluid to a TTY bolt will result in a :crazy: (improperly torqued bolt).

Good catch on the original post though. :thumbup:

Thanks for the reply. I agree not to use thread-locker. Even the blue thread-locker might not be a good idea. It's probably wise to stick with the original design and VW recommendations. But there seems to be some lack of clarity in what is supposed to be done, regarding the oiling (pre-lube) of the bolt:

I checked Bentley and Haynes again very carefully. For the Axle stretch bolts, they both say to torque to 140 ft-lbs and then an additional 180 deg (half turn). Bentley and Haynes both do not mention anything about oiling/lubing the Axle bolts beforehand.

However, for the Crankshaft bolt, Bentley says "Oil threads", while Haynes says "with bolt oiled". (both specifiy148 ft-lbs and then an additional 180 deg) Neither Bentley or Haynes says anything about lubing between the bolt head and the bearing washer; but I applied grease to that area to help prevent too much torque being transmitted to the key on the back of the toothed-pulley, which has been known to shear off occasionally during the tightening procedure. I'm afraid this measure may have caused the "side effect" of the bolt being over-tightened.

I read a GREAT deal on the internet today regarding stretch bolts and "Torque-To-Yield" (TTY) bolt usage. One article said that when these torque values are established in most test labs, they typically use 30 weight (30W) oil on the threads AND on the area between the bolt head and the bearing washer. They cautioned not to use anything else but 30W oil. But my using grease instead of oil between the head/washer, that still may have caused the bolt to be over-torqued. (I did use only 30W oil on the bolt threads, because I did not want to reduce the friction too much on the threads.)

I will write more in reply to Tomvw's latest post, which is also related to this.

Thank you again.
 

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Discussion Starter · #9 · (Edited)
When the bolt is tightened correctly, it ends up at about 300 ft lbs. So from what you say I doubt you have damaged the bolt or shaft, but I'm not there.
I expect that bolt would break before there was any serious damage to the thread.
I suggest you check the pulley alignment with the crank lock pin fitted.
Fit the bottom TB cover (leave bolts loose) and the harmonic balancer, and check the timing marks. You should be able to move the cover back and
forth so that the mark moves a little each side of the mark on the harmonic balancer.

If the TB pulley is correctly aligned I would try to complete the tightening. When a bolt has been tightened to 145 ft lbs and then sits,
when you start to turn it again it will take more force to start it moving than it takes to keep it moving. maybe that is what you experienced.

I reiterate "I wouldn't run it as it is"
Thanks again. Your advice is right on, as always. (Please also see my recent reply to Electron Man, which is related to this.)

So I checked the timing marks as you suggested, and they are dead on. When I wiggle the play in the cover, the mark straddles each side by equal amounts.

I totally understand your point about the "stick-tion" of getting it moving. Now that it's been over 24 hours since it happened, I'm having a hard time remembering exactly what happened. But I think I would have realized if that's all that was happening (the sticktion thing). I did several Axle bolts previously, and I never got that "oh shit" feeling like I did yesterday during the Crankshaft bolt tightening. For the final turning, I used a 3/4" breaker bar with a 4 foot pipe on the end ! 300 ft-lbs would mean only 75 lbs pushing down on the end of that pipe. I think I was applying about 50 lbs (200 ft lbs) when that thing started to get easier to turn, right in the middle of a turning motion. With the greased contact area, that could have been equivalent bolt tension of 300 ft-lbs if it were dry.

I read a lot online today, and it said that when stretch bolts reach their yield point, the turning can feel like it gets easier. So maybe this is all it was, and if so, would be confirmation that it has been properly "torqued-to-yield" ?

Now I'm starting to lean towards just leaving it as is. But I'm going to keep reading online.

I also just sent an email to VW in Germany. I'll post their reply here.
 

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According to the TSB damage to the key usually occurs during removal of the bolt.
It might be OK as is, but it is an unknown quantity. From what you say it should be close.

Note: Thread lubrication or cadmium plating reduces the torque requirement by about 30%.
 

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Discussion Starter · #11 · (Edited)
According to the TSB damage to the key usually occurs during removal of the bolt.
It might be OK as is, but it is an unknown quantity. From what you say it should be close.
Note: Thread lubrication or cadmium plating reduces the torque requirement by about 30%.
This is excellent information. Excellent points. Thank you !

Please see the attached diagram and table. I'm convinced that I'm in one of the regions indicated by the blue arrows with question marks.



Text Line Diagram Font Design



Text Line Font Number Parallel



I have not heard back from VW yet, but using your 30% factor, and the graph and table attached, here is my engineering analysis:

I pre-torqued the bolt to 145 ft-lbs. Since I used oil on the threads, and grease under the head, this could have been the equivalent of 190 ft-lbs (when compared to a dry bolt, using your 30% factor). From the attached table, the maximum conventional torque of this bolt is 225 ft-lbs. So, everything was still fine after the pre-torqueing.

Now, looking at the TTY diagram, we can see that as the bolt is turned further, we continue to travel up the steep slope, and then reach the yield point. This most likely accounts for my perception of a “decreased resistance”, although this diagram would suggest that the resistance should level off, not decrease. So I’m still a bit leery.

But, since I only turned an additional 90 deg, and not the full 180 deg, it would seem unlikely that I exceeded the peak “ultimate tensile stress”. I base this on the fact that, looking at the x-axis (bolt elongation), the amount of elongation (bolt turning) to get from zero stress to the yield point is about the same amount of turning to get from the yield point to the ultimate tensile strength. We know for sure that we were not at the yield point yet when I pre-torqued to 145 (190). And I only went 1/4 turn past that point. This small amount of additional elongation could not have been enough to push it past the peak ultimate tensile strength.

This analysis assumes that the graph is “to scale” and is an accurate representation.

Right now I’m inclined to leave it as is, but I’m still waiting to hear back from VW. I won’t be reassembling the car until this weekend, so I have a few more days to make a final decision.
 

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This is a great conversation on a subject I have always been curious about. Thanks for starting the conversation. While not versed specifically on the technical details of stretch bolts or torque values, I would still caution you on making assumptions of that example graph being to scale. I would assume the size of the areas labeled 'Elastic Range' & 'Plastic Range' would be highly dependent on the material composition of the bolt itself. I don't think this is an example of 'one size fits all'.

If you know the makeup of the bolt itself, and spend enough time on the internets you might be able to pull up the stress v. strain for your specific material.


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Discussion Starter · #13 · (Edited)
This is a great conversation on a subject I have always been curious about. Thanks for starting the conversation. While not versed specifically on the technical details of stretch bolts or torque values, I would still caution you on making assumptions of that example graph being to scale. I would assume the size of the areas labeled 'Elastic Range' & 'Plastic Range' would be highly dependent on the material composition of the bolt itself. I don't think this is an example of 'one size fits all'.

If you know the makeup of the bolt itself, and spend enough time on the internets you might be able to pull up the stress v. strain for your specific material.
Yes, great point. The bolt is M18, Grade 9.8. I will try to find the stress vs. strain curve for this bolt on the internet.
 

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Discussion Starter · #14 · (Edited)
I could not find the stress vs. strain curve for this particular bolt. But I did find that the harder steels tend to not have a significant flattening out of the curve when exceeding the yield point. See the several attached diagrams below.

I also found out that a significant portion of the resistance is from the bolt head contact area, and not just from the thread friction. Typically, half of the friction occurs at the head contact area. See picture of actual bolt below. So, whether or not that area is lubricated will have a significant effect on the torque vs. tension relationship.

Also, it turns out that anti-seize has the greatest lubricating properties when used for bolt tensioning. It's about twice as effective as oil. Some sources say it can even be twice as effective as moly grease. See attached graphs and tables on that as well (in the next post directly below).

Since there may not have been a flattening of the curve after the yield point, and because I used anti-seize under the bolt head, it is quite possible that I tightened right past the curve peak and into the zone of possible failure.

Additional attachments (regarding the effect of lubricants) are attached in the next post below.
 

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Discussion Starter · #19 · (Edited)
Interesting finds! What do you think you will do?

I'm going to check what torque it's currently at. Tonight I bought a 3/4" drive torque wrench from Harbor Freight that goes up to 300 ft-lbs. ($64 after the 20% coupon) I already know that the bolt is torqued to more than 145, which is what my 1/2" drive torque wrench went up to. If the current torque is between 200 and 250, then I may just leave it as is. But I have to think about it some more. I might just replace it with the new bolt I already bought; but this time just oiling the threads like the Bentley manual says. I won't put anything in between the bolt head and the bearing washer. I noticed there is already a waxy substance there, as it came from the VW dealer. So I'll just leave that on there but will not put on the anti-seize, which has the potential to reduce the friction too much.

But I'm also thinking that I may have started to strip the inner threads of the shaft, and by taking the existing bolt out it might just make things worse. From what I read online, it seems that the threads of the bolt might be stronger and harder than the threads of the shaft.
 

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Hmm. Interesting. I'd probably just back the bolt out and start over. I'd also hope that the crank threads would be okie doke if you end up being able to back the bolt out. Don't know if I've ever seen a case of the bore threads giving before the bolt, but i haven't unbolted a crank pulley before.

I feel ya though, these things can be nerve wracking.


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