SBC mystery knock

if only the pontiac was a little more complete i would just dump it in the chev... oh well.. gotta go pick up the wife and kid... we'll see how she does on that trip.. will report back shortly.

 

tada!!!! fuckin chevy... so after the hour long treck in traffic to down town portland and back... listening to that god awful tick.. when we got home i just left it in drive butted up to the curb so the knock would presist.. started pulling the plug wires one by one down the passenger side since they are easyest to get to.. low and behold, get to the last one TADAAA... cylinder 8.. the knocking ceases when spark is cut to cylinder 8..

thoughts???? its very tinny sounding so can we assume possibly a rod bearing is spun or close to it? it doesnt have the trademark "double knock" i have read the wrist pin will make it seems to me to be one singular light knock..

 

You take away that spark, and it is not shoving the piston and rod down against the journal.
If it is a rod, it should get louder, at higher rpm...
and maybe louder on the return to idle, from my understanding/experience ( but mah 'sperience IS limited...)
I would say there is no sense in guessing now, though...it IS in a cylinder you identified...that means you're goin' in, anyway...you will find out!
Good luck...

 

i had one that wouldnt knock until after i ran it pretty hard, then after it cooled down again it wouldnt knock , had a very small crack ..i was about to pull the motor when an old guy told me to pull the convertor loose and give the flywheel a GOOD look.sure enough very small crack...doesnt make sence ,,but this is how it was acting....

 

i guess tomorrow we will see, givin i have enough daylight after work.

would a main bearing around cylinders 7 & 8 cease to knock if the load was reduced by one cylinder?

 

we'll see today possibly..

 

I know you have sort of isolated this to the number 8 cylinder but if that doesn't pan out you may want to consider the tranny pump. I had a TH350 that started knocking inside once. I didn't know what or where the noise was at first but it was a free beater '73 1/2 ton so I decided to drive it till whatever it was came apart. One day about, 3 weeks in, the tranny started slipping and then just gave out. Flywheel was fine. Something internal. traded the whole heap for an SKS.

 

If you have an ABS plastic flex plate cover, it might be distorted when cold and rub on the flywheel. Happened to me on a new Chevelle I bought years ago. On cold start up it would knock like a SOB. When it warmed up, the noise went away. Pulled the cover saw where it was rubbin', changed cover....no more noise.

 

I had a spun rod bearing on a 350. Pulled the pan and pieces were laying in the bottom. Noise was constant and very loud (idle or not).

Try pulling the pan and attempt to wiggle the rods, especially #8, to see if there is any movement (forward/backward or side-to-side).

 

In my experience a main bearing knock is more of a deep dull thud than a crisp knock like a rod is. I wouldnt think that pulling the spark from #8 would make a main bearing knock go away since you still have cylinder #7 pounding away right next to #8.
If you are going thru the trouble of dropping the pan and have the space and a torque wrench, pull #8 rod cap while you are there. Might tell you what you want (or dont want) to know!

 

What?

 

Had a similar "knock" on a 350SBC but on cylinder one.

It was the MSD distributor advance springs that was to weak, allowing a to step advance curve, Causing a "ping" that sounded as a bearing knock.

Put in a stiffer spring and it disapeared

 

we will see tonight. im having an experienced ear come over a little later to see what he thinks. on the highway it is virtually non existant till the RPMs come down.

 

ok, so my pals diagnosis was exhaust manifold.. a crack/tear in the gasket.. does this make sense? you would think that would be apparent all the time, not only under load.. and would be MOST apparent while the engine is COLD and would quite as the engine heats and expands.. opinions?

 

guess its a possibility..

I had a ford engine that had a cracked exhaust manifold..it sounded like shit..until it heated up than it slowly went away..

but to me it was obvious it was an exhaust sound..

so with out hearing your sound,,... like i said earlier in this thread..its hard for us to get a feel for it..

you tube is your friend..my friend..
make a video...refer back to post #53

 

I have had exhaust leaks that made quite a noise. Get the drop in gaskets from Fel-pro. They are 3 peices per side. Fix the exhaust leak first, then see if it solves your problem.

 

i was planning on doing the exhaust leak possibility first as it is the cheapest. and see if it solves anything (eventhough i am doubting it).. i will drive the car around for a bit and see if i cant get a decent vid.

 

here is a vid... sounds a little shitty but you can hear the audible clacking..

 

DEF. didnt know that was gonna show up like that.. sound doesnt show as crisp as in real life.. that vid may be a waste of time.. tried a vid from engine compartment but it does not appear in that one.

 

yeah i agree..sometimes its best to hear it with your own 2 ears in real time..

my guess is..its an exhaust leak..
but , if not..you got your work cut out for ya

good luck with that, and let us knw if it was an easy one or not

 

Not metalic enough in the video. From that I would agree that it is an exhaust leak.

 

Sounded like a exhaust leak to me to. Install some gaskets, you will have to anyway if it isnt.

 

And an exhaust leak would also hold true for your earlier test (below). Use a rubber hose, hold one end close to your ear and move the other around near the #8 exhaust port. You may well hear it. You can also dribble some oil or other fogger down the carb while it's running to see if you get any smoke puffs near #8. It can be hard to see the smoke puffs in daylight, but if you're in a darker place and use a flashlight the smoke puffs are easier to see.

"started pulling the plug wires one by one down the passenger side since they are easyest to get to.. low and behold, get to the last one TADAAA... cylinder 8.. the knocking ceases when spark is cut to cylinder 8.."

 

Back on post #33 someone suggested that you check the exhaust manifold or header bolts....take the five minutes to throw a socket on them and give 'em a little twist, it might save five more pages of posts....

 

i had tightened them already as i believe i said.. maybe i didnt say.. but yes i had already tightened them.. but if the gasket is cracked or torn then it really wouldnt matter if they were tight.

 

Just my 2 cents, but I had a knocking problem from a streched timing chain. It was slapping the cover and sounded like a knock. It didnt even do it all the time about like this problem. Wouldn't a spun main bearing cause a drop in oil pressure?

 

Hey Junior!!!

I think it might be a bent rod! You are a Rod-Bender aren't you??

 

oh the irony.. if that IS infact true.

 

Any updates on what your problem was? I'm still doing various things to hunt mine down. I've figured out that I had two things going on. One was loose rocker nuts, which I think I've pretty much solved. The other is TBD. It sounds deeper in the engine, and when I listened to your video, it sounds a lot like mine from what I can tell. Like another poster said, it's not a metal slap that ends in a *clack* metal on metal noise. Yours, from the video, sounds like a *thop* type noise. Does that make sense?

I'm running these "reusable" aluminum gaskets, and I've been wondering how much problem they might be causing opposed to the tried and true FelPro ones. Let us know how it came out!

 

what he said.