Harmonic balance wheel problems

[motorazr]

Well after only 5 months, the old girl snapped a second harmonic wheel balance bolt. At least this time I wasn’t 600klm from home and stranded. The first time was just out of Bacchus Marsh heading to Adelaide. This time at least I wasn’t doing 100 kph, and have the wheel fly off into the never never. I was only doing 60 kph and heard a loud clunk, lost power steering and battery light came on, I was able to recover the wheel which was resting against the bottom of the radiator.
New bolt clean snapped, again leaving a good part of it in the crank. Might there be any particular reason that this keeps happening? Talking to the towy on the way to the repairer the towy mentioned that perhaps two locating lugs could be placed on the end of the crank shaft similar to what are on high performance vehicles. Any thoughts on this?

[disco stu]

I guess the question is-what is actually causing the bolt to snap? I feel that 2 pins certainly wouldn't hurt, but I don't think it is your problem, as I don't think that in itself should snap the bolt

[motorazr]

That was also part of the discussion in that why would the bolt snap? Possibly a pully out of alignment?

[brw0513]

Could it also be that the blind threaded hole in the end of the crankshaft is full of crud? You might get to the desired tightening torque but not fully seat the balancer.

Another possibility is the thread form in the crank is damaged. Maybe run a tap into by hand to check?

Two broken bolts is a pattern that must have a cause.

Are you using genuine MM bolts with the right part number?

[motorazr]

Yep, all genuine parts and on both occasions the new style was used.

[Kaldek]

For what it's worth when I'm tightening my crank bolt on my 3.0 NH Pajero, I use one of those large "cam sprocket holders" which come with pins that fit into the balancer. I torque the bolt down to 160Nm, dry. I make sure the threads are clean but I don't use lube and I don't use loctite.

[brw0513]

For what it's worth when I'm tightening my crank bolt on my 3.0 NH Pajero, I use one of those large "cam sprocket holders" which come with pins that fit into the balancer. I torque the bolt down to 160Nm, dry. I make sure the threads are clean but I don't use lube and I don't use loctite.

This is a very good point.

[erad]

Any bolt which is subject to vibration or cyclic loading requires a certain tension in order to overcome the fluctuating loads. By tightening the bolt to above the fluctuating load, you eliminate fatigue in the, bolt, even though the tension in the bolt is higher than the peak fluctuations. I would say that in your case, the tension was not enough to overcome the fluctuating loads and the bolts failed in fatigue. If you look at the failed surface, you may even see evidence of fatigue failure - typical is clamshell lines emanating from the final point of failure.

Why did the bolts fail? The tension in the bolt was not high enough. Maybe the torque was as specified, but as suggested above, maybe there was some crud in the thread, or maybe the bolt was bottoming out in the crankshaft. Did you have a washer under the bolt head? Any of the above may cause the bolt to bottom out before achieving the correct tension. I would thoroughly clean the whole assembly and lubricate the threads with light machine oil and tighten the bolt to the specified torque. In particular, check the seating face of the pulley against the crankshaft. If that is not clean, the pulley may settle at a later time, leaving the system with inadequate tension. Run the engine for say 1000 km and then go back and check the tension. Slacken the bolt off and then re-tighten it - do not simply try to tighten it up from its static position because that will not give you the correct tension.

[motorazr]

Thanks for the input, it is appreciated. Will have a chat with my mechanic about the points you have raised.