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Old 06-17-2006, 02:13 PM   #1 (permalink)
Tahoe Joe
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Broken crank? Why?

So I was on a trip with my truck last week. Everything was going good, good MPG, oil pressure at about 50 psi, crusing along 75-80 mph. I started up a grade, and I started to lose speed, so I shifted out of 5th into 4th, and the truck started to pick up speed again. All of a sudden, BOOM!!!! The engine died, so I pulled over. Tried to start it, and CLUNK! I looked under the hood, and noticed that all of the belts were wrapped up in the fan, and that I also had a bad oil leak coming from the front seal. Tried to start it again, it cranked, but had a horrible clunk. I'm assuming that the crankshaft broke...I also say that because you can move the balancer ever so slightly too. So my question is, what would make the crankshaft break? Are these engines prone to crankshaft breakage. It should also be noted that I am running a Banks turbo, which maxed out at 11 psi boost, and I never let the EGT's go over 1100* absolute max! Any thoughts.......?
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1980 Chevy K-20, 6" lift, 10 bolt front, 14 bolt rear, NV4500 trans, NP 208 t-case, BFG 35x12.50x16.5 mud terrains, Banks turbo, and a blown up 6.2 Changing over to a '98 506 casting 6.5 with the Banks turbo and an '86 front clip.

1983 GMC Suburban K-20 Sierra Classic. Banks turbo, rebuilt heads, Autometer pyrometer and boost gauges. 6" lift, BFG Mud Terrains, 35x12.50x16.5. TH 700R4 Trans and NP 208 t-case. Shows 155,000 on the odometer.
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Old 06-17-2006, 04:55 PM   #2 (permalink)
Rio_Grande
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Not sure they are prone to failure, but it is a failure I have seen a bout 5 times in diffrent engines in the last few years. I used to know the cause but cannot remember it right now.
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Old 06-17-2006, 05:06 PM   #3 (permalink)
High Sierra 2500
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Bummer!

These engines are perhaps a bit prone to crank failure. Although it isn't common, it is more common than a crank failure in, say, a 350 chevy. It seems to be more common in turbocharged engines (I have heard of several incidents of crankshaft breakage, and I think only one of them was in a non-turbo engine).

Not very common, but it does happen. Was the engine rebuilt? If so, was the crank magnafluxed before installation? If it wasn't magnafluxed, it might have had a crack. If the engine was not rebuilt, who knows...

Any history of bearing failure? A bearing failure could cause scoring or heat problems.

Now do keep in mind that many engines will continue to run after a crankshaft failure. If it broke in the front journal it could continue to run (usually the break is at an angle and the two pieces lock together). If it quit, you may have other issues even if you fix the crank... Cam could get out of time and bend the valves, among other things...

Are you sure it was a crankshaft? You didn't bust a timing chain or something stupid like that, right?
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Old 06-17-2006, 07:08 PM   #4 (permalink)
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r u gonna fix it? if not i might be interested in the turbo kit.
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Old 06-17-2006, 09:17 PM   #5 (permalink)
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A friend of mine had one break on a NA 6.2, but it had over 300k on it at the time.
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Old 06-17-2006, 11:27 PM   #6 (permalink)
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I have talked many times with our mechanic at work and he has delt with these 6.2 a lot.
Forgive my ingnorance on the terms I use but he said that the cranks on the 6.2's were built sort of out of spec if that makes sence. You could take the stock cranks and have them spun to a true clearance and this would help them spin better.
With the turbo 400's at those speeds you speak of the engine is spinning up there, still in the power range but up there, and if crank is not balanced correctly it will fail. But you also mention of going into 4th gear so it sounds like you don't have the 400 in there.
Like High Sierra said, there have been those that have failed and the engine will still be running but only on 2 or four cylinders depending on where the brake is.
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Old 06-18-2006, 08:13 AM   #7 (permalink)
High Sierra 2500
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Quote:
Like High Sierra said, there have been those that have failed and the engine will still be running but only on 2 or four cylinders depending on where the brake is.
I've seen trucks that ran and drove for miles with broken cranks. They will run on all eight under certain circumstances, but 4 or 6 of the cylinders will run rough or misfire in most cases.
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Old 06-18-2006, 12:42 PM   #8 (permalink)
Tahoe Joe
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Nope, she won't even begin to run, and after the nice "noise" I heard, that was it...260 miles from home too! What a time for a failure! Anyway, I did get another engine, a runner, and it looks good too! It just needs a rear main seal, but while I'm at it, I'm gonna take a look at the bearings, replace the oil pump and timing set. Should be in the truck by next weekend, and by then, I should have my old motor torn down to see what went wrong...I'm really curious now. As for the old motor, I never rebuilt it, I got it from a friend of a friend who said it was a good running 6.2. It was a good running 6.2, I put almost 15,000 miles on it in 6 months time, and made a couple of trips to L.A. and back...about 500 miles each way. I've hauled many cars with it, towed a friend's 1953 Dodge M37 with it, as well as just commuted back and forth to work...most of it N/A, but I did do a lot with it turbo'ed as well. I was really disappointed when I was stuck on the side of the road with it, but then again, I guess things happen for a reason! I'll keep you guys posted on what I find! Thanks for the input...
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1980 Chevy K-20, 6" lift, 10 bolt front, 14 bolt rear, NV4500 trans, NP 208 t-case, BFG 35x12.50x16.5 mud terrains, Banks turbo, and a blown up 6.2 Changing over to a '98 506 casting 6.5 with the Banks turbo and an '86 front clip.

1983 GMC Suburban K-20 Sierra Classic. Banks turbo, rebuilt heads, Autometer pyrometer and boost gauges. 6" lift, BFG Mud Terrains, 35x12.50x16.5. TH 700R4 Trans and NP 208 t-case. Shows 155,000 on the odometer.
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Old 06-18-2006, 03:04 PM   #9 (permalink)
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good to hear you got another engine. if bearings look good, might change tiimg chain while you're there, so you know what you got up front.
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Old 06-18-2006, 05:49 PM   #10 (permalink)
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i have a 6.2 with a broken crank out of my brothers pu it had a four speed behind it and had 225k and not the best treatment in the world , sitting on a pallet in my shed.
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