What does a rod knock sound like? - Chevelle Tech
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post #1 of 35 (permalink) Old Mar 30th, 05, 12:43 AM Thread Starter
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Chris White
 
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What does a rod knock sound like?

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?

Chris White
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post #2 of 35 (permalink) Old Mar 30th, 05, 7:14 AM
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Re: What does a rod knock sound like?

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.

Mike

Last edited by Unclepennybags; Mar 30th, 05 at 7:18 AM. Reason: additional trouble shooting info
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post #3 of 35 (permalink) Old Mar 30th, 05, 8:07 AM Thread Starter
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Chris White
 
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Re: What does a rod knock sound like?

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?

Chris White
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post #4 of 35 (permalink) Old Mar 30th, 05, 8:44 PM
 
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Re: What does a rod knock sound like?

Replace your fuel pump. It may go away.
Paul
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post #5 of 35 (permalink) Old Mar 30th, 05, 9:13 PM
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Re: What does a rod knock sound like?

Broke or bent flexplate? . Or torque converter bad had one do this b4.
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post #6 of 35 (permalink) Old Mar 30th, 05, 10:27 PM Thread Starter
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Re: What does a rod knock sound like?

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?

Chris White
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post #7 of 35 (permalink) Old Mar 31st, 05, 6:47 AM
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Re: What does a rod knock sound like?

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.

Mike

Last edited by Unclepennybags; Mar 31st, 05 at 6:51 AM. Reason: additional trouble shooting idea
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post #8 of 35 (permalink) Old Mar 31st, 05, 8:19 AM Thread Starter
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Re: What does a rod knock sound like?

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?

Chris White
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post #9 of 35 (permalink) Old Mar 31st, 05, 9:04 AM
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Re: What does a rod knock sound like?

Quote:
Originally Posted by bubba68ss
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.

Mike
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post #10 of 35 (permalink) Old Mar 31st, 05, 9:18 AM Thread Starter
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Re: What does a rod knock sound like?

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?

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post #11 of 35 (permalink) Old Mar 31st, 05, 10:13 AM
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Re: What does a rod knock sound like?

Quote:
Originally Posted by bubba68ss
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.

Mike
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post #12 of 35 (permalink) Old Mar 31st, 05, 10:42 AM
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Re: What does a rod knock sound like?

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.

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post #13 of 35 (permalink) Old Mar 31st, 05, 11:27 AM
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Re: What does a rod knock sound like?

Not to mention that the rings will not seal at all due to the piston movement. Are you sure about the parts you used?
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post #14 of 35 (permalink) Old Mar 31st, 05, 11:45 AM Thread Starter
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Re: What does a rod knock sound like?

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?

Chris White
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post #15 of 35 (permalink) Old Mar 31st, 05, 4:14 PM
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Re: What does a rod knock sound like?

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.

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