Thread: Help Identify Knock Sound Chevy 350

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Denny W. A few days ago we did pull each wire to see if it would isolate the noise but it stayed the same on all cylinders.

Regarding transmission, I will get up under there and flex it more on the converter and see if it sounds like it can be an issue or just a normal movement. When I put the engine in I did not notice any apparent cracks and driving was limited to our long driveway. It is possible it got cracked somehow. If it turns out to be an issue I do have one sitting on another engine.

Is the flexplate likely to crack if the starter gear does not engage properly? I know this has happened a few times where the starter hits the plate not engaging gears as I think we need a shim in it.

Denny W

So you think lifters? My confusion still is though that the rockers appear not to clack and I would assume if it is a lifter that it would also clack the rockers and never adjust.

Regarding oil, I live in a cold state, IDAHO and it gets very cold in the winter. I have always been suggested 30w is the best all around. You feel 10/40 is best?

Also I have added Lucus oil stabilizer which is very thick so would imagine it is much thicker than 30w now. If a person is going to use additives such as Lucus, is it best to use thinner weight being you will be thickening it?

Denny,

I appreciate your assistance. A real hair pulling ordeal. Other than the time it takes to pull the intake, replacing the lifters and gaskets is fairly inexpensive.

I did order the clips as mentioned. One more clarification to make sure I am correct. The screw driver test is to put the screw driver pressed down on the rock at the push rod side, correct? I assume the theory is to keep pressure on the lifter to take up any clearance.

Thanks again all.

Have you checked the rockers to make sure none of them are cracked below the center? I've had a couple crack and cause a clacking noise that was a bear to locate. I'd definitely do Denny's test though because it sure sounds like a bad lifter from what I can hear too. Hopefully you don't pull the lifters to find crowns starting. That's usually the luck I've had.

I am going to pull the cover off again today and check.

I'm going to be the negative Nelly here and say it sounds like a rod to me. Seems to be too deep for a lifter. I had a 3800 with a spun rod that did the same thing.

Hey Seth, my sons name is Seth.

How did you end up fixing your rod issue?

Thats pretty cool that he has the same name.. I replaced the motor because the crank and rod were ruined because of the spun bearing. I need to back up on it though, the more I think of it, it was quiet at an idle and knocked when I reved it up. I listened to your video again and it's hard to tell, but if it's a fast knock it can be something like a spun rod or wrist pin loose. That's because it will knock once going up and knock again when it changes direction going down. My brother in law had a motor that was knocking like a spun rod once and it ended up being the distributor cap was cracked causing pre detonation on one cyl. I had a knock on a motor that ended up being carbon build up on too of a piston smacking the head, just a few more ideas for you. I think if I were you I would drain the oil in a clean pan and let it sit overnight and check if any glitter settled to the bottom, also cut your filter apart and loo for any metal in it.

Charles,
I went back and re-read all of your posts. It seems to me that you've ruled out lifters, since you stated that as you adjust them you bring them to individual clatter, then silence them by adjustment, but the knock remains consistent as you adjust lifters.
One thing that you have not shared is the history of this engine. You mention "...when I put it in (the truck)..." that initial driving was limited to your long driveway, but you haven't said anything about the engine being new, old or in between. Can you share some details about when it was last apart, who put it together and if a machine shop or rebuilder was involved?

I hope not rod knock. However, I can say it does sound more prominent up higher than towards pan.

Rspear, the engine was slightly used. The only history I have on it is that it ran strong and was rebuilt. When I got it, it looked pretty fresh.

When I listen to the the engine run I believe the noise sounds like it's about half crank speed. If I am right that means it's in the valve train cam, lifter, push rod, rocker arm, valve spring, or valve, of course I could be wrong just my 2¢ worth

No I have flat hydraulic tappet.

I also tried the screw driver approach for noise but did not seem to make a difference. However, if I was to lean towards any speculation I would say it is first a lifter or two, second thought would be piston rattle.

Originally Posted by fan4chevys

No I have flat hydraulic tappet.

So you have flat tappet lifters, and from post #19 you said:

Originally Posted by fan4chevys

After working with the rockers 4x in the near past few days, the rockers can be adjusted to clacking, then zero and 3/4 turn with no clacking on the rocker. However, the knocking sound still persist.

If by saying "...the knocking persists" you mean that it continued to knock while you were adjusting the rockers, how can it be a lifter? You've also told us:

Originally Posted by fan4chevys

....the engine was slightly used. The only history I have on it is that it ran strong and was rebuilt. When I got it, it looked pretty fresh.

No disrespect, but you bought a pig in a poke. You have no idea what "rebuilt" meant to the guy who sold this engine to you. To some, a set of plugs, some gaskets to stop the oil leaks and some rattle can paint means "rebuilt" (rattle can rebuild). To others, it's throwing in a set of new rings on old pistons without measuring anything, or having any machine work done. I hope that's not what you've got.

I been on the road for the past week-----------few comments that haven t been mentioned----rebuilt????? wrong size bearings, loose pistons in old tapered bores----piston in backwards so pin offset is extreme and will be noisy and/or maybe fracture skirt on thrust side of cylinder

Do all the lifters/pushrods spin same as motor idles????????

Passenger side rear ----------thats where the distributor sets in its boss/hole-------IFFFFFFFFFFFFF------the dist is not exactly straight into hole( manifold at top not centered perfectly in line) lower housing undersized, oil rings ( if used) bulged out into oil galley, and/or a few other conditions-----------at low speeds the flow of oil to those rear tappet bosses can bleed off enough that there isn't flow/pressure enough to do the job of filling the tappets at a rate that beats the collapse rate of the lifter-at higher speeds the lifter isn't in that part of the cycle for it to collapse far enough to bottom out. but-------at some certain rpm rates the lifters spend more time ( mili,mili micro sec) that total collapse is possible where at high rpm ( compare 600 rpm to 6000 rpm---10x ???) and add in the oil flow rates etc etc---------

Also-being an used older block that has been rebuilt-which brings up countless areas where clearances have been disregarded ( mainly $$$$$$$$$) in the rebuild process-------if a flat top piston is put on the rod backwrds, it will double the amount of normal wrist pin offset-----some HP forged pistons won't have any offset and are noisy at some idle range rpm with lighter oils---I'll try to look up the specs on the wrist pin offset so I can maybe explain this line of thought----