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Cold rod knock. EDIT:Now my motor work thread. Back up and running!

y5mgisi

y5mgisi

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EDIT: Turning this into my "motor work" thread so as not to do as i usually do and start 100 new threads for each project.

So on my blazer, when i start it in the morning it sounds good and normal and quiet. But, when i take off down the road i can hear (what i think is) a rod knock. It lasts about 2 blocks then goes away. Thats the only time i hear it. So, I pulled the oil pan off casue im doing all sorts of other stuff too, and theres a couple rods that seem looser then the others. So, i pulled one of the caps off. The bearings look worn but pretty good. I just figured i would replace them but it turns out they are .0005 bearings. Now, im not sure which direction to go. Should i just put it all back together as if nothing ever happened and just run a heavier oil? Should i get a couple new .0005 bearings and then put it back together? Should i get a .000 bearing and a .001 bearing and put them in and plastigauge them and see which one comes up best?

Which brings me to part to of my question. What clearance should i have on my rod bearings using the plastigauge? googling made me very confused on the topic.
 
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The first thing to do is make sure you aren't using a FRAM oil filter. Beyond that, I have nothing.
 
i would plastigauge your current bearings and see what they read that way you know which way to go with new bearings maybe you can get away without having your crank machined i can't remeber the exact spec on tolerance but i think it like .0045 get a chiltons manual it will tell you the spec
 
That's what I'm thinking. I know that what I'm doing isn't exactly a great idea but if it buys me another rattle free 10k miles I will be happy. I'm thinking I'm going to plastigauge them all and probably end up with a mix of old an new bearings. Starting to really wish I just would have pulled the motor!
 
Most GM engines run .001 -.003 on rod bearing clearances...with .003 being the wear limit...its not uncommon to find GM "mixed" bearings to get the proper clearances,for example they might put one half of a rod bearing thats standard size with the other half being undesized..I've seen rngines that were .010 under come right from the factory,and if you didn't see where they stamped the .010 on the oil pan rails you might well put standard bearings in it and cause a real headache!..

I am surprised you only heard the noise after it warms up--usually loose rod or main bearings clatter like a woodpecker upon a cold start and then quiet down as oil pressure builds up...pistons usually are noisier when they are cold too...oil viscosity going south after the engine warms up can cause bearing knocks especially if clearances are on the loose side..

I've replaced bearings with the engine in the chassis a few tmes--used a bent cotter pin to "roll" them out..that job is a 10 on the "suck-o-meter"scale!--nothing worse than having to lie on your back and have oil dripping in your eyes while your trying like hell not to get any dirt in the bearings...
 
Small block Chevys are amazing things. I would just moniter you fluid levels and keep running it the way it is untill either you are ready for a full rebuild or it blows up all together. I got mine hot, twice, and now it smokes really bad, but still runs fine. Keep in mind tho, if you loose a rod it might damage your block. To me its no big deal cus I have a new one ready to be rebuilt.
 
I suppose I should clarify the knock. On cold start up I can't hear the knock until I put it in gear. And even then, sometimes I don't hear it until actually hit the gas to back out of the driveway. It only lasts a couple blocks or so at the most. I'm just going to go ahead and plastigauge each bearing and put the appropriate bearing on the appropriate journal. .0005 bearings are looking like they are hard to find but it looks like .001 bearings are somewhat common. Debating on whether or not I wanna put a new oil pump in right now or not also. What do you dudes think?
 
Well, you are already into the engine, new stock type oil pump would not hurt.
I would plasitgage and put on what gets you the correct clearance values.

I have been thinking about this whole thing, I am beginning to wonder if you are on the right track.
It sounds like the pump is not picking up good at startup.
Don't guess you have an oil pressure gauge. But with a gauge or light,
you should see a warning if the pressure is low enough to make the bearings knock.

Its just weird that it only knocks when first started. Makes me wonder if its a ping.
FWIW, my old truck got partially submerged when a culvert collasped as I was driving across it.
I was able to winch myself out, but when I turned off the engine on the other side, the starter had mud between the points and would not engage.
So, I got towed home, replaced the starter the next day, and parked the truck for about 5 days.
Checked the oil just before cranking it, and found rusty crud halfway up the dipstick.
I had not realized that water had gotten in the engine.

Changed the oil, cranked it, ran it until warm then changed the oil again. Filter both times.
Ran fine, no knocks, good oil pressure. But I just could not get that dipstick off my mind.
Finally about 2-300 miles later, I got a friend to drop the pan and check the bearings.
He said the crank looked good, but the bearings were toast. He replaced them all. I never asked what sizes he put in, I suspect the gaged them like you are going to do.

I put well over 50K on those bearings with no problems. Stumbled upon a deal for a blueprinted engine for the price of the parts, and swapped it.
Gave the old engine to a friend, he wore out two trucks with it.

So, at least once swapping the bearings worked.






Of course, it was a Ford engine, so I suppose its to be expected.........:D
 
make sure the torque converter to flywheel bolts are tight also. they will give a little knocking.

I changed rod bearings (and oil pump) once with two rods knocking and it lasted until the rings slap wore out. Changed them with what I pulled out which was std size. If I had cared about the engine I would not have done that but it was a 305 and I was going to trash it anyway. The pump was high press and high vol.
 
I don't believe that's rod knock at all as you describe it.

Rod knock would be pronounced on decelerarion. And will not go away while it heats up.

Sounds much more like piston slap, as described.


Sent from my Desire HD using Tapatalk
 
Ok, from what you are describing you have piston slap. What you have discovered as far as bearing sizes are the factory selective fit bearings. You CANNOT buy those bearings anywhere and it IS NOT adviseable to try and use a standard bearing or even a .001" undersize to try and correct any clearance issue(s). If you want to replace the bearings you need to pull the crank and have it machined to the next undersize that it will clean at and then buy the appropriate bearings to install. Also the bearing clearance is usually anywhere between .0007"-.0025" with .003" being the MAX wear limit. If the engine is stock you should be looking for clearnaces on the tighter side and if it's an engine that you run hard then the clearances should be on the higher end of the spectrum (.0025").
 
Interesting! Piston slap then maybe. Crazy. Im going to just plastigauge them and see what i come up with. Probably just going to put the bearings back in that come out cause so far they look about perfect. Some just SEEM tighter then others when you try and give them a wiggle. The other thing tho is that my oil pressure is fairly low. Like around 15psi hot at idle. Cold and going down the road tho its almost 30.
 
Took this video to show how loose the chain was. It was pretty loose! Original nylon teeth set. The video turned out rather "entertaining" but i assure you that i was just trying to show how loose the dang thing was! Not trying to "get it off" as it may appear...:haha:

[YOUTUBE]2-bY9crs_k4[/YOUTUBE]


Tomorrow i gotta run to the parts house and get the plastigage and the hardware store for some longer bolts to pull the lower timing chain sprocket.
 
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Come on now, you got to give us a fair shot.

You very plainly said it was a cold rod knock. You can't expect us to diagnose the problem if the description is off.
If you had said it was a rattling knocking sound from the front of the engine, someone would have suggested the timing chain........

And for some reason I can't put my finger on.................Yeah, I said that.

That video is the funniest thing I have seen in a long time.

As I watched, I kept thinking well, he is poking it. Now he is poking it again.
He is still poking it.
Oh God, he is going to poke it again.
Wait, wait, yes, he poked it yet again.....

I was reminded of the Friends episode where they thought that the naked guy in the next building was dead.
So they built a long arm out of things in the apartment and reached across and poked him with it.
He woke up, and one of them said oh look, he's alive. He woke up when we poked him.
And another said yes, he is awake now and looking around.
And another one said and yet we are still poking him........
 
LOL! I was really just making sure I had a good angle to show how loose it was! I really don't think that's the source of my noise. I have heard and had rod knocks before on other vehicles and my noise sounds just like a light rod knock that only lasts a couple blocks. In fact, now that I think of it, one of my YouTube videos has the blazer in it and you can kinda hear it in it.
 
This one,

You can here it a little bit when im backing in the driveway.

 
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