Chinese diesel question - head gasket

I have a chinese truck. It has a diesel engine. It has oil in the radiator due to a bad cylinder head gasket. I had replaced it and fixed the over heating issues which was causing the head gasket to fail. After a couple of months and less than 10k kilometers my gasket failed again. Air is blowing out of my radiator, no water getting mixed in the oil, just like before.
Why did it fail so early? I did not clean the radiator thoroughly, and it pretty much had a lot of oil sludge, would the oil in the radiator damage the gasket?

Apologies for the incorrect entries, Foton is not listed below.

It is possible the head cracked, but if is a head gasket, I can think of a couple things.

Maybe the head bolts were not tightened and torqued in the proper sequence. Some companies recommend torquing the bolts again after a few hundred miles. The gasket could have been poorly made or not an exact replacement. The block or head may not be warped (not perfectly flat) especially after being overheated. If any overheating occurred after the repair, it could have repeated the earlier problem.

Those are my guesses.

Our second ever story categorized under “Diesel-repair.”

I will open it up this weekend. I am hoping it is not a cracked head. It did not overheat like it used to that kept killing the gasket so my suspect was the oil sludge in the radiator that killed it prematurely. Or maybe the head IS warped… what’s acceptable tolerance?

I don’t know what is the torque required nor the bolt sequence due to no documentation available (ive tried looking but no luck. Its an N485 engine from quanchai china). I gave it 80 ft lbs and started with the middle bolts circling outwards.

Does the engine have an aluminum head?

It’s possible the head gasket replacement procedure was done incorrectly. the two mating surfaces must be nearly perfectly flat for the replacement head gasket to be able to effectively handle the high combustion pressures. That “flattening” job usually requires taking the head to a machine shop, and is a step often skipped to make the job faster and reduce the initial expense. It’s not unusual to also have to take the block to the machine shop for the same reason.

Not aluminum.

That is possible. I will be more diligent in this next re install

You can use a steel ruler, if you have one long enough. Place it on the head and block at different angles. This will let you see how much warp (if any) there is in the block and head.

I do not know the torque pattern for your vehicle. On some, you start in the center and tighten down opposite bolts, one front, one rear, crisscrossing across the head. Tighten the bolt firmly in that pattern for all bolts, then repeat with the proper torque (using a torque wrench). When done, go back over the bolts a 3rd time with the torque wrench to make sure each is properly seated. If you can find a manual, it will tell you how tight the bolts should be.

If a torque wrench is not available, try to give each bolt the same pressure. Some times that is the best we can do.

Given the repeated failures I’d have the head trued up at a machine shop, I’d use a torque wrench, and I’d check if new head bolts are needed. Some are not reusable.

+1 and, to me, 80 ft lbs sounds quite low, given it’s a cast block and head. On the other hand, I’ve never heard of a Quanshai engine before.
You are sure that it ain’t - what’s the english word, torque to yield? - bolts? You know, those which stretch and are torqued to an angle spec.

texases, regarding 80 ft lbs being low, what would be a good torque value for a 16mm cylinder bolt head? I dont know if they are torque to yeild bolts but the machine shop said they are still usable. (That’s on the first time i brought it to the machine shop).

All points mentioned above by the responders are not new to me and i understand the concepts totally.

The only thing that came to mind why it failed is the oil sludge in the radiator that i was not able to remove thoroughly. I saw a video on gasket replacement pointing out not to contaminate the gasket with oil as it will degrade it’s performance.

Maybe the oil seeped through the low torqued bolts… Or maybe the head cracked… Will know once i get to work on it…

Forgot to mention, i torqued it 80 ft lbs wet (ie. bolt covered in oil, but not dripping) so the torque values would be a bit higher…

A 16 mm bolt HEAD says nothing about the diameter of the bolt iself, but I imagine it would be a 10 mm bolt. My brain say +/- 110 ft lbs. Problem is that my brain does not count. Only the factory specs count and we still don’t know what type of bolts it is. I would dismiss the theory about the sludge, never heard of a case where that was the cause of such a problem as You describe.

Part of the problem is we don’t know if he’s near a machine shop, or the tools available. This is a Chinese Foton truck, so could be in NZ with the Cummins engine… but who knows what they ship to Russia, Bangladesh, Philipenes, or???

How do you folks determine that your straight edge is actually straight? The only way I can think of is to use it to draw a line on a sheet of paper, then turn it 180 degrees and compare it (the same edge) to the line. If it matches up to the line the whole way, doesn’t that mean it is straight?

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I compared mine to my T-square and level, sitting atop a white surface. All 3 agreed.

It seems like comparing it to a steel edge which is known to be straight is better than the drawing the line method. It’s difficult to accurately draw a thin line on paper that consistently follows the straight-edge.

Yep, lots of 3rd world machine shops around. I doubt them myself. I have tools, not really a mechanic but like a regular motorhead… hehe. I am based in the Philippines.

Or use a plumb bob. Hang it and compare your straight edge to the taught line :)… would that work?