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1999 C2500 8600GVWR. I've had the truck since about 225K miles. It now has 440k miles on it. I'm self employed and this is my work truck. My dad bought the truck from the state of Kansas when it had about 150K miles on it. I've talked to my dad about it, and he never did anything to the truck except the FSD heat sink, replaced a brake hard line, and replaced the trans right before I bought it. I've never done the chain either and I'm assuming the state didn't do it either based on how many miles it had on it when dad bought it. I was recently chasing down an oil leak and it ended up being the pan was rusted through so I replaced it. While I was in there, I noticed the chain was really loose, but not so loose I thought it could jump time or anything like that, so I put everything together and went on with life. While I was researching something else on here, I noticed a post from someone who's chain broke at like 200K. So then I got worried and started digging and of course I now see there's a ton of talk about the chain on these engines.

The vast majority of the 440k miles on the truck have been in town. It gets driven 5-6 days a week, every week, for the last 11 years I've had it. I only average around 12Kmi/yr., but they're all city miles. I'm diligent about fluid changes and making repairs when they need made. The engine itself has never been opened up except for the oil pan replacement I just did. If any of the main bearing webbings are cracked, it's so small you can't see it with your eyes. No major oil coking. There was even still some crosshatching left on the cylinders that I could see from underneath. Given it's age and mileage, I think it's still in good condition.

So, how worried should I be about that chain? I know it may seem crazy, but I have no intention of getting rid of the truck any time soon. It has served me very well. It's very easy to work on, parts aren't all that expensive, it gets reasonable fuel economy, taxes and insurance are super cheap, etc.. The cab/bed will probably rust off the frame and then I'll have to get rid of it, but I plan on running it until that happens. Plus, what are the chances of running another vehicle to a half a million, mostly in-town, miles???

I know these engine's have a bad reputation, but my personal experience hasn't been that case. But I want to keep it going, so if I need to pull the front off the engine and do the chain, I'll do it.... but I don't like wasting my time either. If I continue to be as diligent about oil changes as I usually am (3K miles, Rotella T4 15w-40 and a Purolator oil filter) will the chain keep wearing, or just stretch? Does it continue to stretch until it fails, or stretch initially, and then just wears after that? If the oil changes are regular, how much wear does it really see?

I know I don't post on here, but I've learned A TON about my truck over the last decade, and a lot of that came from here, so thanks to everyone who's still posting on here and thanks to whoever needs to hear it for keeping the site alive.
 

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Welcome to DieselPlace
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At almost 1/2 million miles? I would replace the timing chain. They wear out over time. Replacing the timing set is part of normal maint in a high mileage engine.
If it suddenly breaks or slips. You are looking at major engine damage. If you depend on your truck. IMO you should plan on replacing it.
 

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Welcome onboard🍻

Good advice from tommy above.If you do go ahead with the timing chain and sprockets I’d replace the water pump followed by the harmonic balancer and outer crankshaft pulley if it’s never been done before.I realize it’ll cost a few extra bucks but at least then you won’t need to mess around with the front section of the engine for years to come.
 

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The interval for the timing chain is 30k miles per the manual.
 
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I'm not questioning the validity of 30K chain replacement, is this due to exorbitant stress diesel engines in general?
My engine building is more with vintage V8 gas engines. At 30K, they are hardly broke-n! LOL
 

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I’ve never seen one of these chains break yet as it is a double roller but they certainly do stretch as the miles rack up on them.
 
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Chains wear, they will make noise long before they will jump. Pins wear, appearing as "stretch" but it is only wear.
Low speed idle places the most wear on chain pins, cams and harmonic damper. This is due to high load and low speed which causes breakdown of oil film.
 

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I’ve never seen one of these chains break yet as it is a double roller but they certainly do stretch as the miles rack up on them.
Thread from way back: Broken timing chain

And a more recent pic off the web of another broken chain:
Automotive tire Motor vehicle Gear Alloy wheel Rim


Yes they have been documented to break.
What's better is when another one breaks less than a week after the anti-gear drive crowd claims chains don't fail. While the factory spec allows for quite a bit of stretch and still be "ok" it gives quite a bit of timing variation.
Wished I would have bought a few sets of timing gears (Phazer) 20 years ago for future projects.
 

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Thread from way back: Broken timing chain

And a more recent pic off the web of another broken chain: View attachment 653771

Yes they have been documented to break.
What's better is when another one breaks less than a week after the anti-gear drive crowd claims chains don't fail. While the factory spec allows for quite a bit of stretch and still be "ok" it gives quite a bit of timing variation.
Wished I would have bought a few sets of timing gears (Phazer) 20 years ago for future projects.
Yes I always thought about using the phazer set but never did bother with it.I always just stuck with the typical chain setup.
 
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I'm not questioning the validity of 30K chain replacement, is this due to exorbitant stress diesel engines in general?
My engine building is more with vintage V8 gas engines. At 30K, they are hardly broke-n! LOL
Diesel hardly broken in at 200K.
I think GM just trying to protect their b**t because in general, they have been producing vehicle with questionable inconsistent quality over the years.
 
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It needs changing, but you know that.. Truck will run better. Thing to know most is the chain keeps the IP in time as well. The IP really performs poorly when the chain is loose as the slack causes chatter. Search how I replace the chain with the older chain setup.
 
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