The harmonic balancer coming apart seems to be a problem with the 4.0 in Ford engines also.
The ones I've seen come apart where the rubber meets the outside ring on the balancer.
The ring begins to slip and gets looser and looser until it finally spins into the timing cover or smokes the rubber and alarms the owner. Once the belt is off the ring simply falls off the main part of the balancer.
I have seen it blamed on the amount of HP needed to supply the needs of the belt driven appliances. I appears to be rare, indeed, but is happening.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" /> <img src="/forums/images/graemlins/pfft.gif" alt="" />


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A common source of belt noise on the 4.0l is a separating crankshaft damper pulley. Check the spacing on the rear of the crank pulley to the timing cover. Normal space is about 1/16" to 1/8". Any more or less indicates a damper that is coming loose, the resulting misalignment causes noise. Sometimes the rubber is seen to be squeezing out between the inner hub and outer ring.

Hope this helps!

professional bovine relocation specialist