What does a rod knock sound like? [Archive] - Chevelle Tech

: What does a rod knock sound like?

Mar 30th, 05, 1:43 AM
SInce my rebuild of my 402 BB i have had a ''soft'' tapping sound at idle that speed up with acceleration. I micd the crank and all seemed good. I couldnt seem to figure out what was making the noise. Ive heard rod knocking is only heard when engine is under load?? Ive noiticed that the tapping is in synch with the flashing of my timing light and was wondering is it is valvetrain? Does certain exhaust leaks cause this noise?? My oil pressure is about 65psi at idle and 80psi at cruising. If bearing clearences were way out shouldnt i be getting lower oil pressure and wouldnt the tapping be twice as fast?
Posted about this before but didnt really get anywhere. Ive just been driving it like i normally would. Drives excellent but this noise HAS TO GO.
Anyone have an idea?

Mar 30th, 05, 8:14 AM
Find out what cylinder is making the noise. If it is a rod knock, the sound should go away if you pull the spark plug to that cylinder. If not, then it's likely a lifter/cam problem.

If you believe that it's a cam/lifter problem you can find it easily by running the engine with no valve cover and pressing down on the pushrod side of each rocker arm. You'll feel the difference when you come to one that is loose.

An exhaust leak will tend to be a ticking sound. Look for black residue where it is leaking.

The other possibilities are wrist pin, and piston slap.


Mar 30th, 05, 9:07 AM
I pulled each spark plug and the noise didnt go away. It doesnt have the same sound as a loose rocker. Is there any other noises that come from cam/lifters?
I see no black soot, most likely not rod knock, don;t think its lifters, so its narrowing down.
During rebuild i found a piece of metal that was broken off a piston skirt (#7 cyl.) I looked at app the other pistons and they were all good. I replaced that piston with a new one.
Its probably hard to diagnose, where could i take it for someone to listen to the sound? I dont want to go to one of these franchise rip-off places... maybe a machine shop?

Mar 30th, 05, 9:44 PM
Replace your fuel pump. It may go away.
70 ragtop

Mar 30th, 05, 10:13 PM
Broke or bent flexplate? . Or torque converter bad had one do this b4.

Mar 30th, 05, 11:27 PM
I dont run a mech fuel pump and i have a manual trans. Is there anythign on flywheel or clutch that can be making a (tsst tsst tsst) noise?

Mar 31st, 05, 7:47 AM
One of the things that I had mentioned earlier is piston slap. What was your piston to bore clearance on all of the cylinders when you assembled the engine?

I've also heard of people getting the pistons mixed up and since there is an offset, it will make noise if a piston is on the wrong side.

If you take off all the belts and run the engine for a few minutes it will help you figure out if it's an accessory noise, or an engine noise. If it's an engine noise besides the cam/lifters you'll have to pull the engine whatever the final diagnosis is.

At this point it might be a good idea to let a local machine shop with a good reputation give it a listen.


Mar 31st, 05, 9:19 AM
I believe the piston to bore clearence is .010, (or would it be .005 if the piston is centered?) this is a .030 piston in a .040 bore and .040 rings.
I made sure all the pistons had the arrow pointing foward and the rods even had numbers on them that i matched to the appropriate cylinder just like i had done on the first rebuild (and had no problems).
I beleieve i was getting piston slap before rebuild when i had a broken piston skirt but i replaced that piston.
Is there anyone around Portland, Or. area that can give it a listen? or maybe recomend a good shop?

Mar 31st, 05, 10:04 AM
I believe the piston to bore clearence is .010, (or would it be .005 if the piston is centered?) this is a .030 piston in a .040 bore and .040 rings.

.010" is way too much clearance. Are you confident that is what it was? I had the old style forged pistons which require a lot of clearance and they where either .003" or .005".

The piston isn't centered when making the measurement. You just go with the total clearance.

Are you saying that you are running a .030" overbore piston in a cylinder that is bored out .040"????? If so, I think we found the problem.


Mar 31st, 05, 10:18 AM
when i first rebuilt the engine, it ran great without any noises at all. It had the .030 piston in a .040 bore 402 with .040 rings. ran great for 3months before i wiped a cam lobe. fixed that and it still ran great, then my bearings gor shot from metal flakes from the wiped lobe. Then i rebuilt it again and when i fired it up it had the noise.
are these cleareneces really that bad?

Mar 31st, 05, 11:13 AM
when i first rebuilt the engine, it ran great without any noises at all. It had the .030 piston in a .040 bore 402 with .040 rings. ran great for 3months before i wiped a cam lobe. fixed that and it still ran great, then my bearings gor shot from metal flakes from the wiped lobe. Then i rebuilt it again and when i fired it up it had the noise.
are these cleareneces really that bad?

Without being flip, that's why they make .030" overbore pistons and .040" overbore pistons.

When in doubt, take a look at what the factory shop manual says is the max piston to bore clearance.


Pat Kelley
Mar 31st, 05, 11:42 AM
If you have .030 pistons in a .040" bore you have more than .010" clearance since the pistons have the clearance built into them. If a .030" piston needs, say, .004" clearance, it will be .026" smaller in diameter than the nominal size. Put this in a .040" bore and you have .014" clearance. That is huge.

Mar 31st, 05, 12:27 PM
Not to mention that the rings will not seal at all due to the piston movement. Are you sure about the parts you used?

Mar 31st, 05, 12:45 PM
This was my first engine rebuild and i followed the book (How to Rebuild a Big Block Chevy). I am sure about all the parts. Just WISH i could find the source of this noise.
What would go wrong with using the current pistons?

Regarding crank journal size, what clearences are tolerable?

Mar 31st, 05, 5:14 PM
Once you've heard a rod go you will recognize the sound. It is not subtle. As the others are saying if you have a .030 piston in a .040 cylinder the sound you hear is another piston skirt getting ready to hand grenade. You can't get by just putting larger rings on the smaller piston.

Probably not the answer you wanted but if what you said is right you need to pull the motor and get the right pistons in there. If you don't something very bad is going to happen that may break at best the lower end, worst the block too.

Pull your oil filter and cut it open and check for metallic bits in the oil. Drain the oil and do the same. If you find metal then it's a good bet more than one piston is dragging on the cylinder walls. At this point you can probably save the crank, rods and block without an overbore. If not you might be into taking it out to .060 and getting new pistons. Good luck, there's good advice here.

Mar 31st, 05, 10:25 PM
that sounds like what i have to do. Pull it again and get correct pistons. Where can i get a .040" piston for the 402? Can i reuses the rings? Is there anything else i have to change?

I found these pistons for a 4.165" bore. are they any good?

Apr 1st, 05, 7:50 AM
Those look okay. With that type you have to run a large top ring gap.

My personal preference in the hypereutectic is the Speed Pro Hypereutectic piston with the coated skirt. I believe that you can run a standard top ring gap with these. Any local auto part store can get them for you. I'm sure that Summit or Jegs could also.

I would get new rings. The piston manufacturer will specify exactly what ring to use with the piston that you buy. I'd rather spend the extra money rather than have to pull the engine again.


Apr 1st, 05, 8:58 AM
So does everyone seem to agree that its piston to bore size thats causing this noise? When i had the pistons out last time i noticed microscopic vertical scratches in the cylinder walls where the skirts ride against (is this normal?)

Apr 1st, 05, 10:50 AM
Try changing your left rear shock.....

April Fools!

Apr 1st, 05, 1:35 PM

I did not read this entire post, but did skim each reply, so please excuse me if I missed a few items.
First you need to know if it is in the upper or lower end of things, and it appears that you are not sure. An easy way to tell if it's in the upper or lower end, is to put a timing light on it. If the noise is the same as the timing light, it's in the upper area, Cam, rockers, valves, etc. If the sound is twice as fast as the light flash, then it's in the lower crank area. My first question is, what is your compression on the cylinders? It it is where it should be, I would wonder if you have piston problem at all. Second question is, how much does it smoke out of the exhaust? Does it really smoke when you get on it, does it smoke when it warm, etc? If not, again I would question if you have a piston problem. Also, is the vacuum where it should be, has it dropped lately?
As far as the very light scratches, I have seen this before on several engines that have run fine. I'm not sure what causes these, may be came break in, etc, but if they are very fine, I would not be concerned. I would not go out and purchase pistons and pull the engine until you have taken a good look at everything else.


Apr 1st, 05, 8:30 PM
The engine does not smoke at all. The sound DOES match the timing light. Ill run compression check but havent noticed any performance loss. It actually runs better than i can remember... just this darn noise!

Apr 2nd, 05, 8:23 AM
I would think that i just put something back together incorrectly. Is there anything in the valvetrain that could be rubbing? WHat else should i look for?

Apr 2nd, 05, 10:36 AM

"The engine does not smoke at all. The sound DOES match the timing light."

Since it does not smoke and I expect the compression will be good, at this point I would look at the top end. Especially, as you noted above, the sound
does match the timinig light. What happens when you put a vacuum gauge on it?

"Is there anything in the valvetrain that could be rubbing? WHat else should i look for?"

Good question, and I'm no expert in this area by any means. I've never really had any problems with my engines, so very little trouble shooting experience. At this point, I would suggest using a Mech Noise scope, or a long screwdriver, long pipe, etc. Just to see if you can narrow down where the noise is, or even just what side it is coming from. At that point, I would warm up the car just a little bit (not so that it is hot), then pull the valve cover and see if you can tell what's going on. You may want to readjust each rocker, to see if it changes the noise. Are you just running a Hydr Flat tappet Cam, solid, roller, etc? What condition are the head of the valves in, have they been reworked lately? Are they stock type heads, or have you had studs put in, are the valve springs stock, what condition are the valve guides in? Are the Rocker arms roller tipped or just stock? Answering some of these questions, may give you some suggestions for you.

I notice that you originally thought that you had .010 piston clearance, I would expect that you really meant around .001 clearance, if you are using Coated Hypers. I know that the last 383 that I built only had a little over .001 piston clearence. If you really had .010, any machine shop would have caught this when the fitted the pistons.


Bob West
Apr 2nd, 05, 12:45 PM
a double knock, sounds like its coming from under the intake manifold.

Apr 2nd, 05, 8:56 PM
I just had the heads reworked with ground seats, replaced guides, stock springs, stock rockers, hyd. flat tappet cam.
I DID build the motor with .030" pistons in a .040" bore.. I really wasnt aware they honed the cylinders out .01" ( i was just planning on reusing the pistons that were in it which was a .030" bore). Once i realized this, the motor was already running and sounded great. It ran great form months before i had cam trouble from running to high of lift. THen i changed the cam and everything ran great again...very quiet engine. It was only after the last rebuil to change the bearings that i wiped out that the noise started happening (immediately on fire up). The noise Does seem to quiet down after warm up.
What does piston slap sound like??I just want to narrow that out

Apr 4th, 05, 1:56 PM

Ok, so basically you have a engine that already has ~.01 wear in the cylinders, I don't think that the extra .008 is going to cause enough piston slap to hear. If I remember correctly, the last 350 we tore apart had over 120K on it, and around .017 clearance between the old piston and the wall. You could hear a little slap (or tapping noise) at start up, but after a few minutes it went away. So I would still wonder if you should be concerned about piston slap (at this point). If you had a rod knocking, you can hear it real well under the car and on top. It's a loud noise (normally), that seems to be everywhere, not a tapping type noise at all. It almost sounds like a small ball-peen hammer hitting inside of the engine real quick. Since the timming light and the noise are a match, I would concentrate on the upper half of the engine. Maybe a bent pushrod, a lifter probelm, something along these lines. If you decide to pull the valve covers and watch, make sure to have carboard wall, cut open valve cover, or something like this made up so the oil will not get all over the place. Sometimes just pushing down on the rocker arm with a block of rubber/wood will allow you to pinpoint the problem.


Apr 4th, 05, 7:16 PM
i will try that.

Apr 5th, 05, 12:10 PM
Am I the only wierdo that uses a length of washer hose to exactly pinpoint a noise?

Apr 5th, 05, 1:28 PM
ive tried using hose. i havent found a spot that is louder than anywhere else

Apr 5th, 05, 5:35 PM
Am I the only wierdo that uses a length of washer hose to exactly pinpoint a noise?
Are we still talkin' cars here??????????????

Apr 6th, 05, 10:48 AM
I don't know if I missed something along the way, but I'd have to point to your excessive piston clearance as the culprit. Keep it simple and fix the major problem there first. I think your knocking sound will go away. Good luck. Make sure you measure your cylinder bores to see if you did any damage to them running clearances like that.

Apr 6th, 05, 4:07 PM
If the noise happens everytime the timing light flashes doesnt that ppoint to valvetrain? If it were a piston rattling in its bore wouldnt that go twice as fast since the piston has to move up and down twice before the cylinder fires (and the piston would make a noise each time it went up and down)?
My Gramps is thinking valvetrain too after he listened to it.. What would a bent pushrod or any other common valvetrain noise sound like... Is there any common things to check for? When i rebuil the motor one of the pushrods was a little bent (just barely wobbled when rolled across the table). I replaced that. Should i look for anything? Valves??

Apr 6th, 05, 4:46 PM
You might want to recheck the flexplate, my son had a noise like that and the flexplate appeared tight, make sure you torque them to specs, you have to tighten them up really good, retightened his up and that took care of it.


Apr 6th, 05, 7:45 PM
You have tried Gene's hose trick, to narrow it down you will need to pull the valve cover (one at a time) and use the hose or broom handle method on each rocker as the engine is running. Use a piece of cardboard to make a dam on the bottom and ends and plan on cleaning up some oil quirts. If you have an old valve cover you can cut the top off, leaving the sides and bolt that on too. If you find a suspect, loosen and readjust the rocker. Look for obvious broken valve springs, bent push rods etc. In another thread someonewas having problems with rockers banging on the valve covers, check out yours for dimples. The search continues......... Just had a thought any chance its coming from the fuel pump rod? Sometimes they go south.

Apr 6th, 05, 11:14 PM
Ill definately look into the valve/rocker area. I don;t have a flexplate because i have a 4 speed. And i am running a electric fuel pump. I dont think its the piston bore clearence. Dont cylinder bores wear over time anyway? Isnt that why you have to remove a 'ridge' when taking pistons out? How much do they usually wear out because if i have .005"clearence on each side of the piston that is not very much...you can barely see it and i would think that is easily what a piston wears out in the bore over time... jsut a thought, am i wrong?
Id have to say it doesnt sound like a rod knock from what people have described to me.