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Old 04-04-2006, 06:13 PM   #1 (permalink)
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How to tell the difference between piston slap and bearing knock?

Further to this thread /forum/chevy/459791-rings-bearings-85-305-a.html
Is there a way, prior to pulling the engine out and apart, to tell whether the knock is piston slap or bearing knock or both? Read the last post on the above thread for the history of this beast. It's an '85 1500 Suburban 2x4 with a carburated 305 and auto tranny.
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Old 04-04-2006, 09:02 PM   #2 (permalink)
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Need more info on what its doing. Need to know if its cold or hot. What rpm is it "knocking" at. Details are key here...cant diagnose a engine noise in new england from guatamala with "its knocking" on the internet.

Or just pull it apart anyway and figure it out. I had a broken flexplate in my old k5 and everyone swore the bottom end or valvetrain was going to fly out of my 400sbc (now living happily in my 84 k2500) while i knew it was tranny related (tranny was abused and smoked along with several other key parts of the k5). Since you dont really know what your listening for just pull the fucker apart and youll get it. Only way to learn is to do it.

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Old 04-04-2006, 09:24 PM   #3 (permalink)
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if you have piston slap.

you will have low comprsion and possibly an oil fouled plug in the hole.


bad bearings tend to show low oil pressure.


but you never really know until you tear her apart.


if you're trying to save money, by fixing just what is wrong, start by pulling the oil pan and inspecting the rod bearings. oil pan gaskets don't cost as much as a head gaskets.
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Old 04-04-2006, 09:31 PM   #4 (permalink)
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dont bearings in the crank usually get "spun" and the knock your refering to usually end up in the valvetrain? seems to me when my rod bearings let loose, it just made a bad grinding sound, no power, whereas the bearing "knock" sounds more like a lifter tap?? but im with the others, tear it down, find out for sure. maybe im just talking out of my ass who knows for sure
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Old 04-04-2006, 10:34 PM   #5 (permalink)
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I'm going to do a compression test tomorrow. It knocks hot or cold at all speeds (more rapidly at higher speeds of course) and it's NOT quiet.

I thought these engines had hydraulic lifters. Do hydraylic lifters still "tap"? I thought they were always in contact with the pushrods and rockers.

We only want to do what is necessary. The truck simply isn't worth much so to put much $$ into it is silly.

I'll post up the compression numbers tomorrow.
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Old 04-05-2006, 04:56 PM   #6 (permalink)
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Hydralic lifters can and do tap if they are not pumping up all of the way or if the push rod has bent to less then normal length.

Rod knock is normaly goes hand in hand with low oil pressure all across the RPM range and gets louder the warmer the engine is and piston slap tends to be a cold engine thing and oil pressure will not be off what it was before the noise started.

Pistons also sound a little more like a rattle... they sound almosy diesel like when bad.
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Old 04-05-2006, 05:10 PM   #7 (permalink)
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Compression on the Suburban is as follows:

1 - 120, 2 - 145, 3 - 135, 4 - 125, 5 - 130, 6 - 120, 7 - 130, 8 - 135

Since there is significant variation from a low of 120 to a high of 145 and specification is 150 it is likely not worth doing anything to this engine.

I listened for the knock on startup and I couldn't hear it right away but it was there within 3-4 minutes of simply idling after startup. All plugs are black at the ends.

I'm going to suggest simply getting a new/wrecker engine instead of messing with this one. I DO need to know if there is a 305 or 350 that is still carburated (preferably only 2 barrel for fuel economy's sake). I know a TBI would be better but we don't have the computer or anything else and there are not too many wreckers here that have any domestic vehicles. Remember 350's might be on every street corner stateside but here Toyota's are everywhere so I'm dealing with a less common engine with the 305 or even the 350.
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Old 04-05-2006, 06:04 PM   #8 (permalink)
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Quote:
Originally Posted by cruiser guy
I'm going to suggest simply getting a new/wrecker engine instead of messing with this one. I DO need to know if there is a 305 or 350 that is still carburated (preferably only 2 barrel for fuel economy's sake). I know a TBI would be better but we don't have the computer or anything else and there are not too many wreckers here that have any domestic vehicles. Remember 350's might be on every street corner stateside but here Toyota's are everywhere so I'm dealing with a less common engine with the 305 or even the 350.

What you need to look for is a late 70s early 80s chevy car, i believe that they would still be carburated or a 70s through 86 chevy/gmc truck. It would be easy to tell if its carb or not all you have to do is look at it. Also a 2 barrel wouldnt help fuel economy very much, plus the engine would be a slug. Fuel effeicency comes from an engine that breaths well and is tuned up right.
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Old 04-05-2006, 09:19 PM   #9 (permalink)
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Actually I don't think the compresion variation is that bad for an old motor... but a unker is an easier swap.

If I where in yoru shoes I would look for an older truck are car with a pre emissions Q-jet 350. For one it will easier to swap and more trouble free inthel ong run but it will also be more efficent inthelong run. The 305 breifs well on paper but in practice they suck on fuel cost because you have to work it so hard unless you drive like a granny. The Q-jet can be a suprisingly efficient carb if you stay out of the back to barrels because they are smaller then the duo-jet GM puton everything back then... why do you think that when smog and polution laws got tuff they put Q-jets on everythign and not 2 barrels?

HTH
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Old 04-06-2006, 10:52 PM   #10 (permalink)
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Originally Posted by rebelchevy02
dont bearings in the crank usually get "spun" and the knock your refering to usually end up in the valvetrain? seems to me when my rod bearings let loose, it just made a bad grinding sound, no power, whereas the bearing "knock" sounds more like a lifter tap?? but im with the others, tear it down, find out for sure. maybe im just talking out of my ass who knows for sure
your not talking out of your ass. when a rod bearing spins the piston postition changes raiseing the location of it and usually starts to hit the cylinder head. making a loud knocking sound up top.






cruiser guy.

from what you said i'm thinking a bad lifter. after the motor warms up the oil gets thinner, causeing the oil pressure to go down. making the hydrolic lifter start to calapse, (if it's bad.)

what oil pressure does it have, after warm up?

i don't know your situation, but if you just need a good running motor and your wrecker pickings are slim, this may not be too bad of a motor.

try hot lashing the valves. while the knocking is going on. if the knock goes away while you are cranking down on the rocker then you may just need to change out the cam and lifters, (and timing chain) which are notorious for going out somewhere around 140,000 miles on SB chevies.

hope it helps.

mike
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Old 04-07-2006, 01:04 AM   #11 (permalink)
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I think I might have a collapsed lifter on my 454, cause lately when I start it up in the morning, I have a valve that rattles for 1/2 a second and then shuts up, but after the engine is warm I hear a slight clatter in the left cylinder bank, I'm thinking #2 or #4.
Thinking of hot lashing to see if it helps, what do you guys recommend for big blocks on hot lash? I never done a BBC. Only small blocks.
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Old 04-07-2006, 03:57 PM   #12 (permalink)
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Quote:
Originally Posted by Hawaii500_1999
your not talking out of your ass. when a rod bearing spins the piston postition changes raiseing the location of it and usually starts to hit the cylinder head. making a loud knocking sound up top.

I would really like to have this explained to me... The bearing is the same thickness all the way around and it is circular so it can spin all it wants and the piston/deck hieght will remain the same no matter how it is oriented.

I have seen more then one motor spin a rod bearing and never contact the head.
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Old 04-09-2006, 08:45 PM   #13 (permalink)
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Quote:
Originally Posted by DUG
I would really like to have this explained to me... The bearing is the same thickness all the way around and it is circular so it can spin all it wants and the piston/deck hieght will remain the same no matter how it is oriented.

I have seen more then one motor spin a rod bearing and never contact the head.
sure i will try my best to explain it to you.



the rod bearings are two piece.

after a motor gets a ton of use. the bearings start to wear out. they get thinner and the rod and cap bore gets out of round.

one bearing half spins between the rod cap and the other bearing half. that is what changes the height of the rod and piston assembley. bringing the piston upwards in the cylinder.


now. if the lower bearing half spins between the upper half and the rod. the piston will ride lower in the cylinder.

no matter which one happens you will definatley know something is wrong. usually the oil pressure will go down and up and down and up or stay down. also the motor will lose a ton of power cause the rod is dragging down the motor significantley and grinding down the crank.


i myself hand a rod bearing spind on an old 307 that was tired like you can't believe. it spun the upper half, (rod side) between the lower half and the rod cap. it made the most hurrendoss knocking sound. and then i did the wrong thing. i drove it home. the crank was completley wasted and so were my valves. i was 17 and that was a hard earned lessen. i should have had it towed.


hope it made sense.
mike
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