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Discussion Starter · #1 ·
I replaced the crankshaft seal behind the harmonic balancer, replaced the bolt, torqued it to 74ftlbs and then added 90 degrees. Put it all together and fire it up. Within a few seconds the harmonic balancer bolt has backed itself off.
It is on a 2015 Sierra 3500hd with the 6.6L if that changes anything. What am I missing here?
 

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2007 GMC Sierra LBZ
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You must put a new bolt in. Them bolts are torque to yield bolt just like head bolts. Try that first then if it still does it you have more problems such as a broke crank. Installing an ARP replacement bolt would be better as you can reuse them
 

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I’d try ARP. Harmonic balancer bolt is important enough to spend the money IMO. If that backs out you might have to Tap/helicoil.

I did this on a SBC a couple years ago. I was pretty nervous drilling into the crank but it held up well.
 

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That bolt should not back out unless something is wrong. Another thing i thought of is make sure the little crank pin is in place. Does the engine run fine?
 

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Discussion Starter · #6 ·
Engine runs great, no codes, no issues other than the bolt backing out. No issues when work began; replaced the seal and it just will not stay in. (Thank you all by the way for taking the time to read this.)
 

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Get the ARP one and install per the instructions that come along and see what happens.
 

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This the instructions for my 2008 LMM:
  1. Lubricate the crankshaft balancer internal hub with clean engine oil.
  2. Install the crankshaft balancer on the crankshaft.
  3. Lubricate the crankshaft balancer bolt with clean engine oil. Notice: Refer to Fastener Notice .
  4. Install the crankshaft balancer bolt and washer.
Tighten the bolt to 353 N.m (260 lb ft).
 

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Discussion Starter · #9 ·
What are your thoughts on the possibility of the seal not being set deep enough? Perhaps the rubber seal sticking out too much is catching the harmonic balancer just enough to break the bolt free...
 

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I doubt it. The seal rides on the nose of the oil pump gear. Like i said before make sure the pin is still in place that keeps the balancer from spinning on the crank. It’s just a tiny little pin and perhaps it was loose and fell out.
 
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