'90s Chevy truck elusive engine knock. Rod/Main bearings, or what?

[Just Dropped In]

The belt thing is interesting. As long as they are all the right PN for the application, the tensioner should keep the same amount of constant tension on them. You say the Gates is stock size, are you saying the Goodyear and Continental aren't stock lengths?

I never paid much attention about these belt sizes, until recently when I was trying to sort out this knock issue. As to the belt tensioner, it is new. I also have the original GM tensioner, and I think the original GM belt. Of the 4 belts I have, the original GM is shortest of all. It seems every time I got a new belt, it would be slightly longer than the previous belt/belts. Basically these trucks had a A/C equipped, non-Airpump equipped belt size. While the longer belts seem to alleviate the knock issue somewhat, they make the A/C comp noisy as heck. I can't explain why the belts have inconsistent lengths, as they were supposedly correct for the vehicle.

 

[kschachn]

I never paid much attention about these belt sizes, until recently when I was trying to sort out this knock issue. As to the belt tensioner, it is new. I also have the original GM tensioner, and I think the original GM belt. Of the 4 belts I have, the original GM is shortest of all. It seems every time I got a new belt, it would be slightly longer than the previous belt/belts. Basically these trucks had a A/C equipped, non-Airpump equipped belt size. While the longer belts seem to alleviate the knock issue somewhat, they make the A/C comp noisy as heck. I can't explain why the belts have inconsistent lengths, as they were supposedly correct for the vehicle.

Is that so?

 

[Just Dropped In]

Is that so?

Yes, the R4 compressor rattles like crazy with the 96-1/2' to 97" belts, at least on this engine. And with the slightly shorter belts, is quieter. Not that R4s are all that quiet anyway. Of course, while I've been having this knock problem, I've quit using the AC altogether.

 

[Just Dropped In]

Just checked all 4 belts. The old GM Goodyear I have does not fit the truck (might have come off my old Firebird). So forget that one.

Here's the three I've purchased over the past few years (2012, 2022, 2024 respectively):

1. Gates K060956 (6PK2465) @ 96-1/16"
2. Goodyear 1060968 (6PK2459) @ 96.8"
3. Continental 4060970 @ 97"

The Goodyear seems to be the best fit. Unless its super hot outside, it works ok with the AC, and the knock it very faint or not at all.

Here's what I'm going to do. I'm going to drive the truck on basic short trips for a while, and see how things go. The slightly noisy valve/rocker cleared up today, so that doesn't need addressing. In a few days I might go back to T5 15w-40, or some other brand/weight if you guys have a better suggestion?

 

[kschachn]

Sounds like a plan.

Hey, you don’t suppose the belt was causing the rocker arm noise, do you?

 

[Just Dropped In]

Sounds like a plan.

Hey, you don’t suppose the belt was causing the rocker arm noise, do you?

I guess one of the valves/lifters just settled in. It was a light tick, fortunately, it didn't require pulling the VC.

 

[kschachn]

Fortunately.

 

[Just Dropped In]

I think I've sorted things out. First, the correct belt size must be 96.8" (the Goodyear posted above), and the AC comp is working fine. The other thing, I dumped the Valvoline 10w-30 HM I put in after the repair, and went back to Shell T5 15w-40, which I've been using for a number of years. Not getting any knock at all, and it's getting good oil pressure both highway and idle (45ish and 25+ respectively). So, I'm going to go back to lightly using the truck as before, and see how things go. Thanks again for the replies and advice everyone.