Knocking noise in 1970 Chevelle [Archive] - Chevelle Tech

: Knocking noise in 1970 Chevelle


sftika
Jun 7th, 09, 10:27 AM
Have a 1970 Chevelle Original 396 (402) with a knocking noise after installing a complete roller assembly with new square bore heads and intake. After about an hour of running time it started knocking like a rod bearing or wrist pin. Still have good oil pressure, checked all springs, rockers, etc..No noise at oil pan, used a stethoscope and the noise was mostly at top of block area and only knocks when accelerated. Having a problem determining what the problem is and was looking for ideas before tearing the motor out of the car. Please help. Thanks.

oman
Jun 7th, 09, 10:38 AM
Have a 1970 Chevelle Original 396 (402) with a knocking noise after installing a complete roller assembly with new square bore heads and intake. After about an hour of running time it started knocking like a rod bearing or wrist pin. Still have good oil pressure, checked all springs, rockers, etc..No noise at oil pan, used a stethoscope and the noise was mostly at top of block area and only knocks when accelerated. Having a problem determining what the problem is and was looking for ideas before tearing the motor out of the car. Please help. Thanks.

Always look at the last place you made a modification / change.

Do you think the pistons might be hitting the new heads? If you went to closed chanmber heads with the smaller 396 / 402 bore perhaps there are issues there. By square bore heads I assume you mean rectangle port heads. You need to explain what heads you added. Are they open or closed chamber? What pistons are in the engine and what heads were on the engine before this all started?

This also might be the cam. If you have the cam installed improperly you can get the engine to run but there will be a lot of valve to piston clearance issues. As me how I know. You say you installed a roller. Roller profiles allow a lot of lift to be brought in VERY quickly after the valve leaves the seat. It is all about "area under the curve" and the ability of rollers to accomodate radical lobe shape.

Anyway you can get into trouble because with a roller the piston cannot "get out of the way" fast enough considering the amount of lift that can be RAPIDLY generated with a roller cam profile. Last but not least. Absolute lift is NOT ususally the cause of piston to valve clearance issues. The issue comes in because the the rapidity of the lift input. Issues with large domes also come into play in terms of piston to dome clearance issues so you need to think about what pistons are in that motor.

I would remove a head right now before something inside the motor gets killed.

sftika
Jun 7th, 09, 10:58 AM
I put in period correct service replacement 990 rectanglular port heads which were totally gone over at machine shop, they are closed chamber heads but work with open or closed pistons. Motor was rebuild approximatel 5000 miles ago to stock specs. Purchased complete roller assembler through Lunati, I gave them all my specs for motor they gave me their new retro fit roller assembly for this motor with the rectangular port heads. You could get these heads on my motor in 1970 as an option the 375 horse 396. The motor runs and sounds great, the knock comes in at about 2500rpm and only under acceleration. I don't think this is a clearance issure, do you have other ideas what could make that noise other than a wrist pin or rod?

oman
Jun 7th, 09, 11:58 AM
"The motor runs and sounds great, the knock comes in at about 2500rpm and only under acceleration."

This would have been helpful from the get go!

"......do you have other ideas what could make that noise other than a wrist pin or rod"

I stand by my original statement. Something you changed resulted in this nosie appearing. Why would you conclude that a rod or a pin suddenly and coincidently went away when you swapped the cam and heads?

"You could get these heads on my motor in 1970 as an option the 375 horse 396." & "Motor was rebuild approximatel 5000 miles ago to stock specs"

You are not clear here. The motor was rebuilt to stock specifications...what specifications? Was it rebuilt to 375 HP specs with 375 HP pistons or was it redone to 350 HP stock specifications and then you added the 375 HP heads??? There is a subtile but important difference here.

Was your motor a 375 before the rebuild? If it was and it was rebuilt to stock 375 HP specs then we can assume you have domed pistions. If it was the 350 HP motor and it was rebuilt to the 350 HP specs then you have different pistons than the 375 HP engine came with. The heads you now have were NOT a stand alone option they came AS PART of an option PACKAGE that contained different pistons so we have to know what pistons are in that motor now. "Rebuilt to stock specs" is not sufficient information.

Reading your post I could conclude two different things here.

1. It is a 350 HP bottom end with 350 HP pistons still in it after the rebuild and 375 HP heads on the top. You added the 375 HP heads from the optional 375 HP engine on top of the engine.

2. It is a rebuild to stock 375 HP specifications (regardless of what it started out as) WITH stock 375 HP pistons and you then bolted 375 HP heads on it.

1 and 2 are different! Which is it?

There is another angle to this. Some blocks had notches on the cyl walls for valve clearance, some did not. The notches become particularly important with small bore engines like yours. Adding the heads at the same time you added the cam could have introduced larger valves than you had before. Go check what size valves you had before the cam / head swap and after the cam / head swap. You might have shot yourself in the foot with the larger valves. It is possible.

Again this goes back to what your engine was earlier. I am not sure if the 350 HP blocks had the notches. Possibly they did possibly they did not. Not knowing what you started out with as far as the engine performance level from the factory leaves a lot of guessing as to this problem and its cause.

"Purchased complete roller assembler through Lunati, I gave them all my specs for motor they gave me their new retro fit roller assembly for this motor with the rectangular port heads."

So???? Lunati can't make a mistake with a cam recommendation? Based on the completeness of the info you present in your earlier post I could see that potentially Lunati was not given all they needed. Even if they were given what they needed they can make mistakes. Not saying they did I am saying they COULD.

Forget the "rods and pins" guesses. This is related to something that changed when you messed with the cam and heads.

sftika
Jun 7th, 09, 1:44 PM
[1. It is a 350 HP bottom end with 350 HP pistons still in it after the rebuild and 375 HP heads on the top. You added the 375 HP heads from the optional 375 HP engine on top of the engine.]

It would be #1 and yes the valves are bigger.

Old Intake 2.06 and exhaust 1.72
New Intake 2.19 and exhaust 1.88

plain 69
Jun 7th, 09, 6:07 PM
Correct me if I am wrong but aren't 990's the open chamber square port head with 2.19 and 1.88 valves that came on the over the counter LS-6 engine or LS-7 454? I don't believe an engine that has valves or piston interference problems will run more than a couple minutes at the most. Valves will bend and pistons will break, I have seen it. You would have definate idling problems. I can see you having a valve train knocking, fuel pump rod, even a bearing but valves smacking pitons and valves hitting cylinder walls are going to be making major noises. A wrist pin will make a loud knocking noise at any speed. Just my .02

sftika
Jun 7th, 09, 6:20 PM
Correct me if I am wrong but aren't 990's the open chamber square port head with 2.19 and 1.88 valves that came on the over the counter LS-6 engine or LS-7 454? I don't believe an engine that has valves or piston interference problems will run more than a couple minutes at the most. Valves will bend and pistons will break, I have seen it. You would have definate idling problems. I can see you having a valve train knocking, fuel pump rod, even a bearing but valves smacking pitons and valves hitting cylinder walls are going to be making major noises. A wrist pin will make a loud knocking noise at any speed. Just my .02

I already changed fuel pump and rod, the noise if it's coming from the valve train is coming from the lifter and I already replaced that lifter where the noise is heard the most. Cam bearing I would lose oil pressure, correct? A rod should make noise on de-acceleration and acceleration, any other suggestions?

TDW
Jun 7th, 09, 6:45 PM
Rocker arm clearance on the rocker cover?

oman
Jun 7th, 09, 6:52 PM
Rocker arm clearance on the rocker cover?

Not if it starts at 2500 RPM

sftika
Jun 7th, 09, 7:02 PM
Have already put valve cover risers on it and the noise isn't coming from the top.

oman
Jun 7th, 09, 7:04 PM
Correct me if I am wrong but aren't 990's the open chamber square port head with 2.19 and 1.88 valves that came on the over the counter LS-6 engine or LS-7 454? I don't believe an engine that has valves or piston interference problems will run more than a couple minutes at the most. Valves will bend and pistons will break, I have seen it. You would have definate idling problems. I can see you having a valve train knocking, fuel pump rod, even a bearing but valves smacking pitons and valves hitting cylinder walls are going to be making major noises. A wrist pin will make a loud knocking noise at any speed. Just my .02

Yes there would be huge noises if there were issues with the pistons / valves coliding. All I was trying to do with my set of questions was figure out what is in this engine.

If this problem started after the head change and cam install then it is related to some kind of mismatch / assembly problem that occured when that work was done.

All this other stuff is just speculation. Bearings and rods (regardless of what kind of noise they make ) don't coincidently fail at the same time that a cam and head swap is done. I would agree that the result of serious collision of the pistons and valves would result in destruction of parts and quick failure of the engine to run.

The only other thing I can say is "Are we sure this nosie was NOT there before the cam and head swap"? If somehow this motor is quieter now than it was before then perhaps the nosie was always there! I am thinking that possibly the noise does not start at 2500, perhaps it is there even at lower RPM numbers but it is just not audiable until the motor reaches a higher RPM number.

I think I said this earlier...pull a head off and see what there is to see. While the head is off take out a valve or two and be sure nothing is bent. Removal of the head will also clarify if this block has reliefs in the cyl walls.

oman
Jun 7th, 09, 7:18 PM
I already changed fuel pump and rod, the noise if it's coming from the valve train is coming from the lifter and I already replaced that lifter where the noise is heard the most. Cam bearing I would lose oil pressure, correct? A rod should make noise on de-acceleration and acceleration, any other suggestions?

This is also new news...the lifter replacement. Did you determine why that lifter needed replacement? You should be considering why that lifter failed because whatever failed it might be causing others to fail and that might be your noise.

I don't know for sure what the pistons in the 350 HP 396 look like, never owned a 396. Perhaps the reliefs in those pistons are not capable of handling the larger dia valves in the current heads. Possibly the reliefs in the 350 HP pistons are not deep enough for the lift and profile of the roller that you have.

Are you sure the cam sprockets are correctly installed? I had a 454 with one of those POS cam gear sets that allow several advance retard settings using different slots in the gears. I installed it wrong...my fault no passing blame there and the valves hit the pistons. This was essentially the same geometry issue that can occures with a roller cam profile. The piston was not able to get out of the way fast enough because of the cam cahin mistake. The motor DID run but it made a lot of nosie at low speed.

Deep six the cam bearing idea. Same reason as mentioned about the rods and or engine bearings. If this was a cam bearing issue it was an issue before the heads were swapped. If the noise really started after the head swap then the issue is being caused by parts that were swapped in that head cam change.

I am wondering if this noise really starts at 2500 RPM are you SURE about that?

sftika
Jun 7th, 09, 7:30 PM
Here is some more new news for you, motor ran fine through new motor cam break in, drove it a couple of times ran fine. Put on a new carburator and was adjusting timing and vacuum on carburator and got that set best as could took for drive after about ten minutes at about 3000 rpms shifting from 3rd to 4th the knock started and a puff of smoke came out of driver's side which is the same side the noise is on. The lifter didn't fail I just changed it because that seemed to be where the noise was coming from. I also changed that push rod, rocker and valve spring. I visually checked all other rockers,rods and springs. Everything seems fine. With the stethoscope the noise may come in a little before 2500 rpms but not there at idle. You can hear the noise the best holding the stethoscope just below the head on the side of the block at number 7 cylinder which is the one I changed everything. You can also feel the knock with your finger when you rev the motor from the carburator slightly. I ran the car with valve cover off the noise is definately not on top. Sorry I didn't list all the info to begin with it just seems like a lot.

oman
Jun 7th, 09, 8:26 PM
You can hear the noise the best holding the stethoscope just below the head on the side of the block at number 7 cylinder which is the one I changed everything. You can also feel the knock with your finger when you rev the motor from the carburator slightly. I ran the car with valve cover off the noise is definately not on top. Sorry I didn't list all the info to begin with it just seems like a lot.

The puff of smoke, the bang and the knock all started at the same time and after the cam install right? Does this thing shake like it is outta balance?

Pulling the head on that side might be your next step! The sound coming from just below the head might be in the area of those valve reliefs. If it is not an issue there then it is deeper in the motor. If you can feel it up near the carb something "is rotten in Denmark" as they said in some Shakespear Play.

You seem to have isolated the issue to #7 cylinder. What does the plug look like on that cyl? I am afraid that looking at the plug is the only "non invasive" diagnostic test you can do. Wish there was a better answer. If you pull the head and see nothing I guess I would have to move to removing #7 piston.

Can you get the pan off that motor without pulling the motor? Does this motor have blow by? What is the compression in #7 cyl? I am just now starting to think a cracked piston. Perhaps a skirt cracked??? Are you sure the pistons were quality pistons? Some of the offshore junk that is out there today is attrocious. Is this noise there when the motor is cold or only after it warms up???

One last thought here. Noises can have a funny way of traveling. You say that the noise is near (most prevalent) around the base of the head at #7. Just for the hell of it check the opposit side of the block at the same crank throw that has #7. Does the noise appear at #8 near the base of the head or anywhere near #8 ? If it does it might be indicative of something common to the 7 / 8 crank throw. Just thinkin out loud here. If it is not audiable at the same relative location on #8 then it almost has to be something associated with #7.

I guess I would start thinking of your engine as a 1 cylinder engine composed of just #7 cylinder if the noise is really restricted to that location as you say it is when you use the steth. You need to look at everything associated with that #7. The more I think on this I think that removal of the head has to be the next step after you look at the plug and check the compression. Hopefully that is not followed by removal of the pan.

I would be really concerned by the fact that you can feel the knock up by the carb. Whatever you do don't rev this engine up ...you already know there are issues at 2500 RPM. If someting is getting ready to quit you don't want an "inspection hole" in the side of the motor.

I am afraid that you have to get aggressive by taking this apart to see what is cooking in there. Check the plug first then the compression. IF everthing passes muster then I think you have to head for the tool box.

Wish I had something simpler as an answer!

plain 69
Jun 7th, 09, 8:40 PM
I know knocks like that are hard to find. It only knocks on acceleration in gear? It won't knock in nuetral or park when revving?? I had a motor that had too much piston clearance when we put cast pistons back in. It sounded like a diesel on acceleration.

sftika
Jun 7th, 09, 9:18 PM
The puff of smoke, the bang and the knock all started at the same time and after the cam install right? Does this thing shake like it is outta balance?

Is this noise there when the motor is cold or only after it warms up???



It does not shake like it is out of balance. The motor does not knock bad at cold temp but when the motor is at running temp you can really hear it knocking at about the 2500rpms. The spark plugs are all a little dark, running a little rich.

oman
Jun 7th, 09, 10:45 PM
It does not shake like it is out of balance. The motor does not knock bad at cold temp but when the motor is at running temp you can really hear it knocking at about the 2500rpms. The spark plugs are all a little dark, running a little rich.

More noise at warm temp...lets ask what happens at warm temps versus cold start up. I dunno where to go with those two observations but perhaps someone else following this will get a clue from what I am saying.

#1 Oil thins out..this points to a bearing
#2 Clearances tighten up as things warm up. Cylinder wall clearances tighten up. Particularly if you have forged pistons. Do you have forged pistons? This points to a wrist pin or a piston problem. More noise when hot...especially if there are forged pistons in there could mean that the piston / bore clearance when hot is such that whatever is knocking knocks louder because it is tighter???

Spark plugs....The issue is #7 not how the set looks. Is #7 different from the others? Do you see where I am going? If there was a crack in #7 piston..the plug might look different. If #7 piston is at issue the compression might be different in that hole. You don't say anything about blowby ... I was going at the point that if there is a crack in #7 it might cause blowby. I still think we need the compression on #7 versus its brothers.

Have you tried to run the motor with no spark to #7? If this reduces or eliminates the noise then we are homing in on a piston or a wrist pin issue. It seems to me that wrist pin issues first manifest themselves at an RPM level more or less around a given RPM. As things get worse the noise is perceptable at a wider and wider range of RPM. Not sure of that..again possibly someone will chime in. This is a test to run at cold start up and at warm operating temps.

Since it does not shake like it is outta balance I doubt the skirt shed a piece. It could happen that the piston shed a section of the skirt... that is why I asked about the shaking. Parts like aluminum can fall off and land in the pan where they do no immediate catastrophic damage. I was thinking along those lines but if it is not shaking like it is outta balance. I guess we move on past that idea now.

You did not answer the question about the bang, the puff of smoke, the appearance of the noise and the timing of the cam head swap. Did the bang and the puff and the appearance of the noise all occur before or after the cam head swap? Was there just one puff of smoke and now the tail pipe "emissions" look normal as in no oil getting past the rings / piston?

I am running outta ideas here. I still think it is related to #7 but you don't say if you can hear the nosie on the block near #8. Try the spark test outlined above..do it both cold and hot. Check the compression in #7 and in some of its neighbors. It might be valuable to check the compression with the motor both cold and hot...I am grasping at straws here however.

sftika
Jun 8th, 09, 5:09 PM
I want to thank all of you for all of your help. You have given me enough information to keep me busy for awhile and probably enough to help me figure out the problem. Thanks again.

YenkoChevelle69
Jun 8th, 09, 6:42 PM
Do you guys think it could be a cracked flexplate?

70ChevelleRagtop
Jun 9th, 09, 12:56 PM
Do you guys think it could be a cracked flexplate?


I got burned by a loose torque converter bolt that I seriously thought was a rod bearing going out last year. The bolt had backed out just enough that it was hitting the back of the block in one spot every revolution.

sftika
Jun 9th, 09, 5:51 PM
It is a 4 speed car

rkd
Jun 9th, 09, 11:05 PM
The country boys list of strange noises heard and chased over the years...

A loose sparkplug, makes scary sounds.

A loose header, collector or similar exhaust gasket will show up at awkward times and sound like everything else in the world.

Is there any chance at that rpm the engine shifts and hits the steering, or something similar.

I bent two pushrods once because I pulled the valvecover, found the obvious bent pushrod and quickly fixed it. Imagine my surprise when the stuck valve bent the replacement....

Beyond that, it seems time for compression/leakdown test, and internal investigation.