I discovered a failed harmonic balancer on a friend's '96 Mustang 3.8 V6 auto (no performance mods; bone stock) that seems to be pretty unusual.

Take a look at the pictures here:

http://community.webshots.com/album/552022226RktMDS

Yes, the balancer is broken into two pieces. And look at that keyway: It's more than *twice* as wide as it should be.

I suspect that the balancer cracked some time ago - but was held in place by the crankshaft bolt. As long as everything stayed in place, it still worked OK. But the vibration started abrading the keyway and causing movement along the crack line. The '96 gets its ignition timing from the crank sensor and the star gear on the balancer, so timing quick got out of whack.

Questions:

- Any idea what really caused a failure like this? Has anyone seen a similar failure?

- I plan to simply replace the balancer, put it all back together and hope for the best. Are there issues I should be aware of, or other problems I may have missed?

Thanks for any insights.

I have only seen a harmonic balancer do that one time it was on an 89 Buick Park Avenue. Funny thing was it 3.8 liter V-6. The actual center of it broke free of the outer ring. All that we did was replace the sensor behind it, set it TDC and replaced it with a new one. The car ran beautifully after that. So that is all that you really have to do. I am not quite sure what caused it, but the car sounded as though a rod was knocking constantly, but after we found the problem it ran smooth.

Thanks for the info.

After reviewing what I discovered while pulling it apart (sealant in places the factory wouldn't have put it), I've come to the conclusion that the front cover has been removed at some point in the car's past (probably 4+ years ago, before the current owner). I'm guessing the harmonic balancer wasn't properly torqued at that time - and years of vibration caused the keyway damage, and ultimately fractured the balancer.

It's still in my garage - haven't had time to track down parts and get it back together.

OK, here is my tale of woe, hopeully someone else will learn from my trials.
Last summer, the engine of my SC Riv 3.8 with 140k miles started making a terrible noise from the general area of the timing gear. So off came the belt tensioning assembly, water pump, and harmonic balancer. Whoa! One DISINTEGRATED harmonic balancer! The rubber that held the two pieces together had shredded and there was about 30 degrees of play between the timing tabs and the hub. I replaced it with a DORMAN (made-in-China) balancer for about $100. Replaced the timing chain, gear, and water pump, as long as I was there. Not a difficult job, just a bit annoying because of the stuff you have to move to get to the bare front cover.
Car ran great, for exactly 405 miles.
Limped home one night and as I creeped up the driveway, it started spewing oil out the front seal all over the place. "This ain't good" I thought. Three months of miserable cold weather and the Riv had to sit on the driveway until three weeks ago when I put together enough will to see what happened.
The answer was pretty quick in coming. When I took off the balancer (remember, made in China for $100) it came out in two chunks. Seems that the hub had fractured at the keyway and this was the cause, I figured, for the "running like crap". The splintered hub must've taken the front seal with it, causing the oil hemorrage. Off came the front cover (in order to replace the front seal) and just for the hell of it I took a real good look at the oil pump, which on the 3800 is driven by the crank, just behind where the balancer sits. Good thing I did. The housing was cracked from the balancer failure and this is where I was losing oil.
I took the balancer back to Lee Auto in Arlington Heights IL and told them my tale. They took the old balancer and gave me a new one, with no questions. Just for the hell of it, I took the new balancer to my machine shop of choice and this is the purpose for my rant.

The NEW BALANCER WAS DEFECTIVE!!!!!!!!!

Seems my machine shop guy Joe teaches at a local jr college and mentioned that the Dorman guy told him these things are failing left and right. So we checked the new one very closely. Not only was the keyway cut too wide, it was cut too shallow on one end; torquing this one on the crankshaft was sure to fracture the hub. Most likely this is what happened to my first replacement balancer - it started to fail as soon as I installed it.

One might be quick to label all stuff from China as crap and I'm not sure I would argue that. Certainly I will never go the cheap route on a mission-critical part again.

Perhaps the better lesson is to very carefully inspect (maybe a better word would be MEASURE) mission-critical parts.

ralfwho -

Your entry reminded me that I never followed up with the rest of the story on this problem.

I went with a junkyard balancer pulled from a Windstar 3.8 (same as a Mustang 3.8 for this year). It was a time-consuming reassembly, but all went together well - and it has been running fine ever since. I've seen no further problems. The owner has commented that it now seems to have more power than when he first bought the car - which reaffirms my suspicion that the root of the problem began quite some time ago.

Your story is certainly a worst-case scenario of what can go wrong when important parts have difficult-to-find flaws.