Snapped crankshaft bolt

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I had the crankshaft bolt on my NL fail on the drive home from a trip to central NSW. That was fixed but it appears the CAS was damaged which wasn't picked up by the local mechanics. Short story, couldn't be fixed quickly so now on a truck to get it home to my mechanic.

If there is anyone there able to look at the attached photos and give a metallurgical explanation/opinion as to what has occurred I would be grateful.

As an aside, we were covered by RACV's Total Care package and their assistance was top order. Several nights accommodation, taxi ($200) from the NSW country town to the nearest hire car depot and the rental car from there back to home. Plus flat top tow truck recovery of the vehicle and trailer back to Melbourne. VERY glad we'd taken it out some time ago, well worth the money and highly recommended

[old Jack]

A clear close up photo of the threaded part that was left in the crankshaft would also be helpful.
I need a good look at the thread form from the side.

OJ.

[erad]

Looking at the photos, it seems as if the bolt has sheared off close to the end of the threaded section. It is probably that the bolts has been overtightened at some stage. The second photo shows some undercut in the thread at about 5 o'clock position. This is rusty and it could be a crack which was there in the first place or maybe started because of fatigue. The area I am talking about is discoloured - almost rusty and well below the minimum diameter of the thread. This would certainly be a stress concentrator.

I recall a long time ago reviewing the tightening torque specified for this bolt. From memory, it is rather close to what would cause yield in the bolt (depending on the steel used in the bolt). I vaguely recall that the bolt is made from a medium strength steel (probably a 450+ MPa yield). Seeing what I think is a failure point (end of thread), this is consistent with the bolt being over-tightened at some stage.

Normally, with a fatigue failure, the initiation point of the failure is some little stress concentrator eg a scratch or a flaw in the steel. Looking at the first photo, there is an angled face with what looks to me to be a crack at about 4 O'clock position. This apparently has nothing to do with the thread failure point referred to above. However, if there was a flaw in the bolt material at manufacture, eg dirty section of thread above, then anything is possible.

With fatigue failure, there is normally a series of lines emanating out from a point (the stress concentration point). The lines leave a traditional pattern which resembles clam shell markings and thus they are called that - clamshell lines. I cannot see any real evidence of these markings in the photos, so I assume that the bolt has been tightened to beyond its yield point, and that the failure was relatively quick. Was the bolt removed and replaced recently? If so, I think this has been the problem. Mind you, if you don't tighten the bolt sufficiently, the bolt will definitely fail in fatigue at some stage down the line.

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Here's a few more photos which hopefully give you the detail you're after.

For the photos the main part of the bolt was aligned with the stub and, as the main part was rotated to get a different angle, the stub was also rotated to keep the alignment the same.
The final photo is of the fracture end of the stub. The had to drill out the centre and then use "ezi-outs" to remove it.

Of interest to me as a layman is that, in the last photo, there is a "circumference" of metal remaining in all but the two-o'clock position and that appears to line up with thread damage/disfigurement on the main bolt in the fifth photo. But, as I said, this is to the unqualified eye.

Yes, there was a timing belt change around 35,000 km ago. In talking to my mechanic today I overlooked asking if he used a new bolt. I did ask about Loctite and he said that it was normal for them to use it.

The mechanic who did the harmonic balance repair said that there was no obvious damage to the end of the crankshaft nor around the "sprocket" (my term) behind the harmonic balancer and suggested the failure was pretty rapid as a result. Certainly whilst driving everything was normal until, coincidentally, the a/c stopped, the volts dropped and the steering became heavy - all as a result of the belts throwing when the pulleys were no longer aligned. There were no unusual symptoms until then.

[erad]

Looking at the second batch of photos reinforces my opinion that the bolt had not fatigued in service, but had been over-tightened. Looking closely at the point of failure you can see the start of 'necking' where the bolt yields and stretches to the point where the diameter is reduced slightly. The threads seems to have been stretched slightly as well, so I assume that the failure point has been right at the end of the crankshaft thread, not somewhere inside the crank thread.

Over-tightening is probably the cause of the failure. Probably the bolt was done up with a rattle gun. My recollections of earlier discussions about bolt tensions tell me that the bolt torque specified results in the tension of the bolt very close to the yield point of the bolt (depending on the steel used in the bolt). This leads also to the question "Do you lubricate the thread or not when torquing the bolt?" My preference is to use light machine oil on the threads unless otherwise specified. But I used to calculate the torques for the fitters for each individual critical application. I used to start with the fluctuating loads applied to the bolts, then calculate a load which exceeded these fluctuations so that the bolts had one steady load on them all the time and they never felt the in-service fluctuations. Thus fatigue was not a problem with those bolts. This is standard engineering stuff. You calculate the tension you want and this give you the bolt extension you need. Then knowing the pitch of the thread, calculate the torque required to achieve that tension. this is where friction comes into the calculations. Lubricating the thread surface gives you a more assured bolt tension. Other ways of getting the required tension include heating the bolt and measuring the extension, and then nipping the nut up before the bolt cools and shrinks. This is commonly used on large (100 mm dia plus) bolts. As long as you know for sure that the thread surfaces are clean, you can get reliable bolt extensions by torquing with the bolt surface dry as well, but there is less control over the tensioning.

Since the recommended torque takes the bolt tension to near yield, it is important to renew the bolt every time you remove it. Using loctite or similar to lock the bolt in service is a good idea except that it makes it hard to get it out when you want to. Indeed, this may even have been the cause of this failure - someone may have used a rattle gun on the bolt to get it out and stretched it in the process. Then it was used again having already yielded. This is not a problem if a new bolt is used on assembly What could be a problem is if a rattle gun is used. You have no real idea as to the tension which is applied to the bolt with a rattle gun. A torque wrench is the only way to go here.

Having said this, I renewed timing belts on my NL Pajero and my wife's TF Magna for years. I never renewed any bolts and my torque wrench didn't go up to the torque required either, so I guessed how much extra to apply. I guess I was lucky that neither vehicle suffered this type of failure. I know for sure when I do any more what I will do next time though.

[disco stu]

Would that corrosion in pic 5 indicate that as well Erad? Its either corrosion or threadlock, and either way I thought it indicated a crack that had been opened for a fair while

[Just Cruzin]

The point is mute if its an air bag model.

[erad]

Would that corrosion in pic 5 indicate that as well Erad? Its either corrosion or threadlock, and either way I thought it indicated a crack that had been opened for a fair while

Stu: I think it was probably threadlock. Certainly there was some form of damage there - a bit like painting over a cracked weld. I noted this but I couldn't positively identify what it meant from the photos. Suffice to say that I think that the bolt was damaged before it was inserted, and eventually it did fail. New bolt every time from now on....

All I could find about the bolt is that the Mitsubishi manual (I have a CD copy) states that you should lubricate the thread and washer face with oil. It doesn't say to renew the bolt, so I assume it is not meant to be torqued to the point of yield.

If you use oil on the bolt:
(a) you can achieve the required bolt tension with a lot less pulling on the torque wrench and
(b) the fact that you have oil on the thread means that they don't want you to use threadlock compounds.

All of the above points strongly to why you should not use a rattle gun to tighten this bolt - torque it up manually. Mitsubishi have a special tool to hold the pulley - it is like a large C spanner with dowel pins to engage presumably with holes in the balancer hub. That would be better than the way I did it - my wife sitting with her foot on the brake and the car in 5th gear (another good reason for manual transmissions).

[disco stu]

There was the Mitsubishi service bulletin thing, can't remember if they said new bolt every time though. I thankfully did mine with engine in stand so could just fix the back of the crankshaft

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In picture 5 there is a red mark on the wider shank of the bolt to the right.

Initially I thought this was corrosion but I was able to pick it off with my finger nail. Loctite or similar? My mechanic here said they use it!!

I couldn't shift the discolouration in the thread of the snapped off part with my nail but I'll try tomorrow in daylight with a needle or some-such.

Thanks for the replies to date.

[erad]

This all adds up to the bolt being loctited in at some stage, it was then over-stretched (the steel yielded) during removal and then the bolt was re-used. Otherwise, it was over-tightened during installation. Ask the mechanic if they use a rattle gun to tighten the bolt. I bet they do (or someone did) because to overtighten this bolt with a torque wrench requires a lot of weeties in the morning.

Further to this saga, in heavy industry, the tightening of large bolts often uses a relatively short flogging spanner and they hit it with a sledge hammer until the bolt and spanner ring or the guy cannot hit it any harder. Not very scientific at all. the proper way often is to use a hydraulic torque wrench which anchors of adjacent studs and pushes the spanner hydraulically. That is great for a head cover or whatever where there is a ring of bolts but for a single bolt, it is often the rattle gun which comes into play. We can see what happens...

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A postcript;

Car was trucked to Melbourne and given to my mechanic. He stripped the new harmonic balancer off and found that the "tone wheel" was damaged and misaligned and that there were contact marks on the crankshaft angle sensor.

He replaced the crankshaft sprocket, the crank angle sensor and fitted a new "tone wheel blade" and it is now running well again.Overall quite an expensive exercise and when MMA compulsorily acquire the vehicle at their market valuation, I might just break even

[disco stu]

Seems like a weird situation having to get your car fixed so that you can get it purchased back by mm, so they can go ahead and destroy it

Glad it's sorted for you at least

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Yes, had to have the car, simple as that but will be delaying the theft, sorry - compulsory acquisition - for as long as possible. Even fewer $$$'s now to get a decent replacement when the time comes

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Well, having had the harmonic balance replaced (with a new bolt) after the previous dramas, the issue has recurred. A week or so ago there was a rattling sound (turned out to be a shredding alternator belt) and investigation of which determined that the harmonic balance was loose. The mechanic tells me he was able to undo the bolt by hand.

It's all put together again, new harmonic balance and new bolt, but the mechanic is concerned that I have a bent crankshaft which is causing the issues due to vibrations beyond what the harmonic balance is designed to accommodate.

So:-
(a) what would cause the bolt to loosen - excessive vibration as suggested or not having been torqued up properly last time?
(b) what can bend a crankshaft in normal service? The car has never been damaged nor driven harshly.
(c) are there any tests that can determine what the vibration levels are?

The first event happened at 337,000 km and this latest one at 360,000 km. Time interval between events (probably not relevant) was 28 months.

Vehicle is a '98 Automatic GLS.

Interested in opinions.