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1996 Chevrolet Silverado C3500. 2018 Silverado 2500HD LT
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Discussion Starter · #1 ·
Still chasing a now louder knock (knock thread: Question: - Engine noise - looking for opinions) in my 6.5 after new injectors and a lot of other stuff. It goes away when cylinder #8 injector is turned off, so I pulled the oil pan to check the bottom end.

Questions on rod bearing fit:

1. Service procedure says "When using selective fit rod bearings, always use the standard bearing in the connecting rod and the undersize bearing in the rod cap." That means the rod side bearing is always standard?

2. See the picture of the rods. The paint mark looks faintly green, especially compared to the yellow on the crank. That should mean the cap end bearing is .026mm undersize... also see the picture of the back of the cap end bearing. Do the markings have anything to do with sizing?

3. I've plastigauged at the cap end on rod #8 with the original bearing, it seems to be within spec (another picture). Is it worthwhile to plastigauge at the connecting rod side too? Wear on both halves looks pretty good for 226k miles.

My plan is to replace just this bearing and see if the knock goes away (I also found 3 main web cracks). Is it best to buy both standard and .026 undersize and use the bearings that provide the most optimal clearance?

Thanks for any input.
 

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It goes away when cylinder #8 injector is turned off, so I pulled the oil pan to check the bottom end.
If you turn the cylinder off and the noise goes away it would indicate an Injector issue...
 

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Discussion Starter · #3 ·
Injectors are new "GM Genuine" replacements. Before I replaced them, the same conditions were observed. The knock was either gone or much quieter when Cyl #8 was turned off.
 

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Injectors are new "GM Genuine" replacements. Before I replaced them, the same conditions were observed. The knock was either gone or much quieter when Cyl #8 was turned off.
Regardless of "New" you probably have a bad injector.
A few years back a member went through 4 sets of AC/Delco injectors from Rock Auto before he got a working set of injectors...

Again, if you turn off the cylinder and the noise is not there it is indicative of the Injector as being the issue.

I would at a minimum pull the #8 injector and pop test it
 

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Discussion Starter · #5 ·
Thanks for the input. I will either pop test it or swap it with another cylinder if the noise remains when the rod bearing is refreshed, since the pan is off anyway. It would be an unfair coincidence to have two noisy injectors in cyl #8, unless the IP can cause that symptom.
 
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I’ve never seen a std bearing on top and a undersize on the bottom within the connecting rod on those engines.I wouldn’t attempt to mix the std bearing and the undersize on the same rod.That doesn’t seem right to me.Someone can correct me if I’m wrong.

I do know that GM sometimes used different variations of bearings within an engine,but I feel you either should use two new std bearings on that connecting rod or use two undersize.Then check your clearance.You should have at least .002” and that would be ideal.What type of bearings are on the cylinder 7 rod?Same type of situation?

If your acdelco injectors have gold lettering/numbers on the body you can be sure they are China made.
 

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Discussion Starter · #7 ·
Thanks dieselolds. I haven't taken apart the #7 rod, kind of didn't want to touch anything else after I saw the main web cracks. It has the same almost green paint mark, which I think means .026mm undersize when I read the service manual. The quote in question 1 is directly from the service procedure.

The new injectors didn't have gold lettering, but they do say Bosch India on the nozzles. Added a pic.
 

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Thanks dieselolds. I haven't taken apart the #7 rod, kind of didn't want to touch anything else after I saw the main web cracks. It has the same almost green paint mark, which I think means .026mm undersize when I read the service manual. The quote in question 1 is directly from the service procedure.

The new injectors didn't have gold lettering, but they do say Bosch India on the nozzles. Added a pic.
No problem.Glad to help anyway possible.Your injector looks to be genuine Bosch and India made.That’s where the Bosch injectors are made nowadays.It used to be Germany years ago but now it’s India for the most part.I’ve always had good luck with the India injectors in the past.

You could probably try rockauto and pick up some bearings in the sizes that you need.That’s the first time I heard of bearing sizes being different on the same rod journal.I’ve only used std on both halves for on rods and mains and .010 bearings on a 5.7 diesel for all its rod journals but std for the mains.
 
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Discussion Starter · #9 ·
I have both standard and .026mm oversize bearings on the way. I'll start with std in rod and undersize in the cap and check clearances.

I hadn't heard of mixing bearing sizes on the same rod until this engine. I've attached a picture of a section of the 6.2L product service training manual that shows std bearing in rod side and .026mm oversize for the cap on rods marked with green paint. Old reference, but from what I have read the process was the same for the 6.5.
 

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It's normal for this motor and many others... plastigauge is not the most accurate way but if that's all you have use it... why not replace each shell with what comes out ?
 

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Discussion Starter · #11 ·
I don't know how to identify the size of the original bearings, except for the service information that says green paint mark is std upper and undersize lower. The markings on the back of the top and bottom shells are identical. My plan is to start with what I think is original sizes, and verify as best as possible with plastigauge. I would be more concerned about inaccuracy of plastigauge if the block didn't have main web cracks that make it not rebuild-able.

Looking at the condition of the original bearings, it seems unlikely to be the source of the knock, but there are enough defects/wear I want to replace with new. In the other thread, I mentioned the stethoscope on the oil pan close to the rear was where the noise was loudest, which is how I ended up with the pan off after new injectors had no effect and it was still very much cylinder #8 making noise. I'll upload the latest video (I wanted to capture the first start with 4" exhaust) so others can listen in a bit.
 

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Discussion Starter · #12 ·
Here is the latest video:

Yes the tailpipe rotates... the clamps weren't fully tightened at that point, one of the stock hangers didn't work. If you're impatient, the knock is loudest in the last 10 seconds. And yes, I hear the miss in the middle of the video, likely air in the system from replacing all rubber fuel lines at the same time as the injectors.
 

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In all honesty it doesn’t sound bad at all.Just typical combustion noise.Doesn’t seem like a rod knock.I’d probably add some performance formula from stanadyne and give it another go.
 

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Discussion Starter · #14 ·
Thanks again for your comments. I thought it was not likely a rod either, but the new injectors didn't fix it, the only place I could locate the knock was in the oil pan, it goes away with #8 injector turned off... so, I'll replace the #8 rod bearings and drive it a little. I'd rather not grenade the engine since it is the original; I like the idea of keeping it as original as possible (except for the exhaust upgrade... and wicked wheel... and glow plug harness :D). I was pretty sad to see some cracks in the main webs. Maybe I'll just replace it with an optimizer and the 141 will become my coffee table base. I've moved past denial and into acceptance. I'll report back with results of the bearing replacement, but in the mean time I'm still listening if anyone has input.
 
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As my self rebuilt motor is still running great but has a definite knock... which is more easily heard at temp... I still have the suspicion on my case to be a precup. I decked the heads with thick plate glass and sandpaper and some of the precups varied as to what was hitting first. My guess is that since they were hitting first at all they were not bottomed out and now have material shaved off of them creating room to "hop around" when loose (at temp).

This is just another potential that will be at least potentially solved when and if I put my low mileage spare set on which ran great. I have also not gone into fussing with injectors, the thing runs like a top...

Figure another potential culprit wouldn't hurt. Good luck!
 
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Thanks again for your comments. I thought it was not likely a rod either, but the new injectors didn't fix it, the only place I could locate the knock was in the oil pan, it goes away with #8 injector turned off... so, I'll replace the #8 rod bearings and drive it a little. I'd rather not grenade the engine since it is the original; I like the idea of keeping it as original as possible (except for the exhaust upgrade... and wicked wheel... and glow plug harness :D). I was pretty sad to see some cracks in the main webs. Maybe I'll just replace it with an optimizer and the 141 will become my coffee table base. I've moved past denial and into acceptance. I'll report back with results of the bearing replacement, but in the mean time I'm still listening if anyone has input.
Looks like a real clean truck you have there👍
 

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Discussion Starter · #17 ·
Thank you. It is in remarkably good condition for being a NJ then a WI truck. Now as a MI truck it lives mostly retired inside and won't see the winter and salt, just like my other truck :D.

I try to clean stuff up when I take it apart too, like in the before/after pictures attached.
 

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93 C3500, Crew, LB, Duals, F code with 141 Block. Replaced 90% under the hood this year.
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That is the same noise I have in my 93 6.5. Replaced rod bearings, cam, all lifters, pushrods, rockers, injectors.....Still has the same noise. I doubt it is a rod, that noise is at valvetrain speed, not crank speed. Runs strong, but that noise drives me crazy. On mine the noise is loud where-ever you listen. Running with the intake planum off, that noise just screams out the top of the intake. I've put a couple thousand miles on it since all the new parts and no difference. I'm going to do one more check with valve covers off and try to at least nail it down to one cylinder. Please keep us posted on your progress. I think there are a number of folks here with that same noise. Good liuck! Jim P
 
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Discussion Starter · #19 ·
That is the same noise I have in my 93 6.5. Replaced rod bearings, cam, all lifters, pushrods, rockers, injectors.....Still has the same noise. I doubt it is a rod, that noise is at valvetrain speed, not crank speed. Runs strong, but that noise drives me crazy. On mine the noise is loud where-ever you listen. Running with the intake planum off, that noise just screams out the top of the intake. I've put a couple thousand miles on it since all the new parts and no difference. I'm going to do one more check with valve covers off and try to at least nail it down to one cylinder. Please keep us posted on your progress. I think there are a number of folks here with that same noise. Good liuck! Jim P
Thanks, Jim. I read your thread and watched the videos at least five times in the past few months. New (well unused at least, one set is date coded 2011) bearings arrived today. Update coming soon.
 

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I definitely hear the noise,but if it were rod it would be knocking from the start in my opinion. Strange how the sound comes in like a lifter would losing prime or with a weak spring. I'm not saying its a lifter just using it in the example. Do you hear any noise around the injection pump?
 
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