I bought my first Jeep, 1999 Wrangler, a couple of months back. It has a 3" lift with 33" Mickey Thompson Baja's. On its first highway drive, it vibrated a lot. Vibrations were felt through the steering wheel and seat. I heard jeeps ride rough. I have nothing to compare it to, so I figured it was simply the Jeep riding rough. On that drive home, my driver-side engine bolts broke. Was it from the vibration??? I replaced the bolts. A month later, I put street tires on and drove 7 hours to work. Again, crazy vibrations through the steering wheel and seat. Turned out, none of my street tires were balanced! I got new tires and drove an hour today. I still have the same vibration at 60+ mph. And now my engine mounting bolts are broken again! I will have these new tires balanced one more time. Do you guys think it's the unbalanced tires causing enough force to break those bolts? I am starting to wonder if it may be something else. The driveshaft, maybe some engine issue ... or the flux capacitor. Any suggestions are greatly appreciated.
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It’s hard to tell by this photo alone, but if you look closely at that bolt fracture, it looks more like a fatigue cracking issue rather than a clean break.
Note how the break has a large number of small lines. This is a classic example of a pattern generated by bolt fatigue cracking. Essentially, a small defect or crack in the bolt starts to expand, initially very slowly, and eventually propagating most of the way through the bolt. In your case, over 80% of the bolt area was cracked at one point, and eventually the bolt failed entirely, leaving the small fast fracture zone you see on top.
This is important to know because bolt fatigue is caused by different issues than a bolt simply being overloaded and snapping off. There could be a number of different causes, all of which could lead to this type of failure. Vibration would certainly exacerbate almost any of these issues, but solving the vibration may not fix the root cause.
1st, if the bolt were to become loose, it could allow some slop in the bolted joint which can cause severe repeated reverse loads on the bolt, especially in bending. This is my theory for this one. Basically, in a properly pretensioned slip-critical joint, the force is transmitted directly between the two metal pieces by friction. The bolt doesn’t bear any appreciable load, but instead serves to clamp the two pieces together to create enough friction. If the bolt becomes loose, it will act as a pin instead, and all of the force will be transmitted through the bolt rather than the part matin in interface. This can create a host of issues, including failed bolts.
2nd, it is possible that the differential thermal expansion between the block and the bolt would cause the bolt to stretch and relax every time the engine is started. I don’t think this is likely an issue here, since it would probably affect most, if not all, TJs.
3rd, it is possible that the other bolts being under-torqued could lead to this bolt failing. If the other bolts are allowing slip, this joint could inadvertently become a pinned joint, thereby placing the bolt in unintended loading. If this happened repeatedly, it could certainly cause fatigue cracking.
4th, it is possible the bolt was substandard or counterfeit. If it had a decent surface defect on it already, that may have been enough to start a crack that could propagate through the bolt. This is also especially common with reused bolts. Realistically, in a properly clamped slip-critical joint, you can put in any fastener and torque it any amount up to the point that it yields and you’ll still be fine, as long as the bolt develops enough preload to prevent the joint from slipping. However, if it does not generate enough preload at any time, it can result in any of the aforementioned unplanned loading scenarios, which would quickly fatigue a substandard bolt.
