Main bearings, when they go bad

What are the signs that the main bearings are going bad? Do they make a sound under load?

 

When engine is cold, they pound until oil pressure comes up, then quiet down

 

Cut open the oil filter, and look for flakes of babit. No flakes you may still have time, to save it.
Warn piston skirts will nock on cold start up, a little heat and they expand and get quite !

 

as you put load on the engine the oil pressure drops and comes back up as you ease up

i must not have made myself clear on this. you can pickup main brg's going bad this way long before they start makeing noise by paying close attention to how oil pressure responds to load.

 

my experience with the old chevy 216's was mains thumped under load and rods hammered on freewind or compression...but that's been over 50 years ago...what was the question, again??

 

If it IS in fact a bearing noise, pull your plug wires one at a time and listen. If the knock decreases with a wire off, the rod bearing for that cylinder is bad. If no change, main bearings more likely.

Like the guy above said, low oil pressure (especially cold) is a sign of excessive main bearing clearance.

Either way..... the engine comes apart.

 

Bad bearings will show low oil pressure, whether cam , rod or main. Lippy

 

That's how I remember it too.............


Ray

 

This is in a 350 SBC oil pressure is good. When car has run for a long period you hear anoise when you rev the motor standing still. When you pull off and it's going though the gears you hear this sound if you let up at 2,200 rpm it goes away.

 

I disagree. Check a diagram of oil flow through an engine and I bet you'll disagree too.



http://www.chevytalk.org/fusionbb/showtopic.php?tid/192284/

see post #6 by Grumpyvette

 

Without hearing the noise, noone can tell you what it is....we can only guess.

Mine would be piston pin slap.

 

Kinda like a girlfriend I had once. Seemed to be going bad and knocked a little untill she was warmed up, but then yikes! Sorry smart ass in every crowd. Good luck with your delima. ~sololobo~

 

Nothing wrong with some comic relief

 

I had a Chevy 396 that suddenly developed a knock, it had GREAT oil pressure, 60 PSI at idle. It turned out to be one spun main bearing. A set of worn bearings across the board can show a pressure drop.

Drove an OT Chevy 305 powered car to Florida one July and the worst thing I could have done was to install an oil gauge.. it was 20 PSI at 60 MPH.

Bob

 

I've used this for years. Mains don't respond to having the load taken off them.

Bob

 

That is more characteristic of rods than mains............typically, rods rattle under light load............mains under heavy load.

That said, your noise could be something else altogether......

Ray

 

I agree this is the way I checked them for years. The oil needs to be warm, not cold.

 

Had a bad fuel pump make that kind of noise start with the easiest things to check first....................

 

running the old chev 235 splashers and full pressure , the crank would rattle in the middle when torking in high gear at about 30 MPH, solution was to pull the shims under the mains

most of you youngins are never going to experience the problems we old farts had with these crappy cars like re ring jobs every 10K miles unless you were a Ford guy

 

Newer people to the hobby think cars routinely went 100k plus between rebuilds. Hah!

Just throwing it out there, loose torque converter bolts can cause an intermittant knock that sounds just like a bearing.

Make a recording of it and up load it for us.

Well by 'us' I mean y'all, that was my best tip right there

 

So we don't loose sight of your Original question ! What condition's do you hear a troubling sound.
Or just being curious ?

Did it just start, or has it been constant for a while.

Bob

 

Benn constant for a while. I talked to an engine builder today and he said to change the fuel pump. I will try that tomorrow. I have a new one that I was going to put on anyway.

 

Had a friend years ago that rebuilt an engine because of bearing noise, turned out it was in the distributor!! Fuel pumps i have heard, also water pumps and v-belts can make some noises..

 

Well, I just finished changing the fuel pump. The noise is less however it is still there. i guess the wrist pins or mains are the culprit. I now have to get my dines together and get myself a short block. The mechanic I know told me that I could ride like that for years. I can't that being said the motor could go at anytime. I like riding to much for that so I got to find me a short block. The wrist pins can only be repaired with the motor out right. If they are all worn thenis that why the sound is so slight and only atload?

 

Is this possibly a pre-ignition/ detonation issue?
Have you checked the timing and the advance curve.

 

Or a busted flexplate. they knock and rattle pretty good...

 

I took the trans cover off and looked at the plate however I did not see any cracks. When you say pre-ignition do you mean pinging? It does not ping up hills and I use high test gas. It is not hard starting usually when the timing is advanced to much you get that ping. I will try to record the sound and post it.

 

a ?...if a bearing is spun, does it always score the crank?

 

I have been hunting this sound and asking the question on here for a month, well yesterday i found out what the sound was....it's the lifters. I have two on the passenger side that have failed. They are adjusted to the limit. they are just dead. when you rev the motor they are clattering and at high speed they are chattering. Thanks everyone for all the suggestions. llok on the bright side a new short block with additions 2,700.00 I have decided to replace the cam,lifters, water pump harmonic balancer. 484.95 Finding the problem...priceless

 

On the engines I saw with a spun bearing the crank was scored and usually the rod or block bearing bore was junked up.