Topend "Knocking" sound

[beater69]

So I have a pretty noticeable knocking sound coming from my engine. It is only really noticeable above 2000 rpm and under load. I'll start with a little background, I'll try and keep it short .
I don't really know too much of the history of the engine, it came from the disc brake donor truck I got for cheap (82 c10). It ran better than my old one. So I cleaned it up, new gaskets, fresh coat of paint.
Now just recently after eliminating other noises, exhaust leaks, u-joints, etc... I got on the freeway and noticed this sound. It's not pinging, I've played with the timing, disconnected vacuum advance, etc...
I started to suspect maybe rod bearing, wrist pin, or piston slap but it doesn't change at all when I pull plug wires one at a time.
With a mechanics stethoscope I've laid under the truck and listened all over. Not much of any noise. Get up to the top and it comes mostly from the pass side valve cover. Pull the cover and I can isolate it to #8 intake rocker arm stud. This is after I already replaced all the lifters. No bent pushrods, I swapped the rocker arm with the one next to it. Adjusted and readjusted valves. Now i'm looking at pulling the head and seeing if there is something embedded in the top of the piston hitting the valve?!
Any suggestions?
See....I said I'd keep it short!

[68gmsee]

First thing that comes to mind is it's more pronounced under load. Are you saying that you can hear it better at that time or is it louder? If it was something hitting the piston I would think that it would pretty noticeable at idle also.

Another think is to try to determine whether it's a metallic tinny sound or a deep thunk type of sound. Crank main bearings sound deeper and more like a thud sound and more noticeable under load.

If it were me, I'd check the crank main bearings first.

[beater69]

It is pretty much there all the time you can just hear it better under load (light acceleration). You can hear it at idle too. The part that steers me away from bearings is that it has not got any worse. Its been a couple hundred miles too. And with my stethoscope it is whisper quiet all over the bottom end and becomes more and more pronounced as you get closer the the pass side valve cover and then "bulls-eye" on the #8 int rocker stem.

[beater69]

Another thing I keep thinking about is, it doesn't sound like a hammer hitting the side of the block, as I've heard a bearing knock described. It sounds closer to a marble in a coffee.

[dwcsr]

Valve seat may be loose and needs to be replaced. Rods get louder under acceleration wirst pins get louder when you let off the gas. If if it was mains it would be loud under the car less noisey on top. I would pull the head and see whats up, remove the valves on number 8 and check the seats. Also check the rocker stud to see if its being hit by the rocker. You may have an early rocker on it that has a short slot.

[beater69]

Originally Posted by dwcsr

Valve seat may be loose and needs to be replaced. Rods get louder under acceleration wirst pins get louder when you let off the gas. If if it was mains it would be loud under the car less noisey on top. I would pull the head and see whats up, remove the valves on number 8 and check the seats. Also check the rocker stud to see if its being hit by the rocker. You may have an early rocker on it that has a short slot.

Thats pretty much where I'm at. I've pulled the rocker arm and it doesn't look like its been hitting. Then I swapped it with the one next door and the noise stayed with the same rocker stud. I guess I'll be pulling a head tomorrow

[spinning wheels]

At my old job, the old man that owned the company is a major car freak. Ok he owns a racetrack too.

He had a '90 SS truck, when they installed a turbo, the truck ran great. When they removed the turbo and drove it for a while, one of his son's noticed a strange sound coming from the motor. They changed lifters, pushrods, checked the rockers. Nothing seemed to make a difference. Come to find out, one of the pistons developed a hairline crack, right by the wrist pin. After they changed out the piston and checking all of the other pistons, they got the motor running and no more unusual noises. The truck was sold before anything other problems developed.

[68gmsee]

From the way you desicribe it, I agree with the recommendations. I've seen a worn wrist pin give a similar sound. A slapping piston pretty much sounds like it's slapping on the side of the wall. At this point all you can do is like your planning -remove and inspect.

On the wrist pin, we couldn't detect the problem when we removed the piston. We actually had to take it to a machine shop to have them inspect the piston and remove the wrist pin. That's when we could tell there was a flat spot on it.

[beater69]

Thanks everyone, I'm gonna pull the head this morning. I'll let you know what I find.

[dave2953]

I'm interested to hear about your findings. I've had the same problem with my motor for three years. I never tore the motor apart, though. Does your noise come and go, or is it consistent? Mine goes away sometimes and comes back later, usually when it's cold out.

[Sammy]

We pulled my son's (firedog)out for that reason. About 1800 rpm a slight knock profound at 2000. Smooth n silent at idel. Compression good, no slop at the rods or crank or scored walls from the bottom. We suspect a wrist pin on #6 slot.

[beater69]

Well, I pulled the heads and found nothing. Everything looked good so I directed my attention toward the bottom end. Jacked up the motor pulled the pan and discovered both #8 and #7 had cracked bearings. Pretty weird I wasn't able to hear it with my stethoscope at the bottom end (oil pan, oil pan bolts, etc...) No serious scoring so I'm gonna just throw some new bearings in and see what happens. I know some of you guys will probably tell me to not waste my time but I think I go ahead and waste it anyway . Thanks again for all the help.
-Ben