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350 small block engine knock

  
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350 small block engine knock

 
1975_k20 1975_k20
New User | Posts: 15 | Joined: 03/10
Posted: 10/18/10
12:23 PM

Hey all,
I've got a 1st gen 350 small block, been bored .030 over, miled cam, 9.0:1 pistons, eagle steel crank, edlebrock preformer carb and intake. recently started knocking while under load. Thought it was a rod bearing knock, but it never lost oil pressure. Changed them anyway, nothing looked bad. Timing is set properly, and rockers are adjusted to 1/2 turn past 0 lash. Been told that I could have a bent push-rod? Does this sound possiable? Getting tired of just throwing parts at it, with no results.  

Pennsy86MonteSSguy Pennsy86MonteSSguy
User | Posts: 50 | Joined: 07/10
Posted: 10/18/10
03:28 PM

Did you look at the main bearings?  
'86 Monte SS...rebuilt Goodwrench 350, Vortec heads, Headers with true duals, mild street cam....this mouse does roar!

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 10/18/10
04:40 PM

a few things... its only takes a few minutes to check the push rods with the valve covers off..

spin them in place.. do they spin.. or do they wobble...


a agree on checking the main bearings...

there are a LOT of things that can also make a knocking noise..

do you have the 5 and 7 wires on the drivers side separated...

how is the pick up coil.... no broken magnets inside the pick up coil????  that will cause random sparks out of time to be generated...

separate the spark plug wires..  use your timing light on each one... do you get a single flash????  if you have cross firing problems.. you will get 2 flashes then a space...

have  you inspected the cap... rotor.. in cap coil????

what happens when you back off the timing.???

do you have a big breaker bar... and a 5/8 six point deep socket???  so you can stick it on the crank center bolt... work the crank back and forth when it is quiet and listen for knocking noises... rotate the crank and eighth of a turn and try again...


how is the oil pressure?????  

do you still have the oil filter that was on the motor when it started hammering????  cut it apart carefully.. make sure that you did not loose any filter media...  there have been problems where the filter have broken up.. but it usually results in  blocked passages... burning the rod and main bearings... but if the flow is only restricted... it may only knock at higher rpms.. where there is not enough oil flow to support the main and rod bearings...  

Dave632 Dave632
Addict | Posts: 2217 | Joined: 07/08
Posted: 10/18/10
05:32 PM

A rod bearing knock will usually occur under no or light load conditions like when you let off your throttle. A main bearing knock will usually occur under load.
Valve noise or clatter will usually be constant thruout the rpm range, just changing frequency.  
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skyeking skyeking
Addict | Posts: 2738 | Joined: 08/09
Posted: 10/18/10
06:45 PM

Hi there.
 I have found that main bearing knocks will come straight in on starting up then fade away when the excess gap in the shell is refilled with oil and pressure.This is when the bearing is fairly well worn.In heavy Diesels the sounds seems worse.  
skyeking

chevelle83 chevelle83
Addict | Posts: 2280 | Joined: 11/09
Posted: 10/18/10
08:24 PM

I wonder if it could possibly be a wrist pin??? or cam bearings??    I had a 305 engine a few years ago that ran great, but it had a slight knock that you could just barely hear at idle, in fact it took a trained ear to even notice it.

However,,once you put the load of the car on it then you could tell there was a knock.  It was timed with the crankshaft so I knew it was a bottom end knock, upon tearing the engine down it was obviously a wrist pin that had maby .020-.030 play in it.  

 Hope this helps even though it may not be the right answer.  

redneckjoe69 redneckjoe69
Addict | Posts: 2337 | Joined: 03/10
Posted: 10/19/10
07:04 AM

i had a crank turned a few years ago, & the machine shop gave me the wrong size bearings. my fault. i should have checked first. they gave me another set and said," oops, our fault".     lesson learned. check everything twice.  

1975_k20 1975_k20
New User | Posts: 15 | Joined: 03/10
Posted: 10/21/10
07:57 AM

Well, I'm still lost. Main bearings tight, makes 50 psi of oil pressure at operating temp., no bent push rods, re-adjusted rockers while engine was hot and running, tried backing off the timing. Still no change. It runs smooth and quiet at idle and thru-out the rpm range with no load on it. It starts to knock at about 1800rpm when under load. Been told by a few mechinics that it sounds like a top end rap, but I cant find anything wrong. Guess next step is to pull out of the truck and start taking it apart till I find something, unless anybody has a better idea.  

Dave632 Dave632
Addict | Posts: 2217 | Joined: 07/08
Posted: 10/21/10
08:05 AM

You say main bearings "tight".
How were they checked?
This still sounds like a main bearing problem to me. Classic main bearing noise.  
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redneckjoe69 redneckjoe69
Addict | Posts: 2337 | Joined: 03/10
Posted: 10/21/10
10:10 AM

i had a buddy who thought he had a rod knock on a 305. turned out to be a stretched timing chain that was rattling on the front cover.  

Pontiacman2 Pontiacman2
Moderator | Posts: 8956 | Joined: 09/08
Posted: 10/21/10
03:31 PM

+1 dave  
Pontiacman2
Pontiacman2

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Budnicks Budnicks
New User | Posts: 38 | Joined: 10/10
Posted: 10/21/10
05:04 PM

1975k20 what about the flywheel/flexplate I have had that happen or maybe a piston skirt cracked use to happen with cast pistons usualy or maybe timing chain slap but you can usualy see that in timing or maybee water/power steering pump, try putting a long screw diver or long rod of some kind to your ear against the motor check all over the place with some one in car w/ebrake on and in gear on laod, just a couple of ideas, OLD SCHOOL ha ha ha Budnicks  
"Fill Your Library Before You Fill Your Garage" Good Luck  "Budnicks"

Pennsy86MonteSSguy Pennsy86MonteSSguy
User | Posts: 50 | Joined: 07/10
Posted: 10/21/10
07:57 PM

Still sounds like a main bearing to me....I had the identical problem and eventually ended up with 2 spun main bearings.  
'86 Monte SS...rebuilt Goodwrench 350, Vortec heads, Headers with true duals, mild street cam....this mouse does roar!

1975_k20 1975_k20
New User | Posts: 15 | Joined: 03/10
Posted: 10/22/10
06:50 AM

After doing some more tests, I discovered that I'm only getting 30psi of compression in the #3 and #5 cylinders, all others are making 160psi. So i'm thinking blown head gasket. The heads are comming off tonite to check. And for all those asking, the rod and main bearings are new as of last weekend.  

Dave632 Dave632
Addict | Posts: 2217 | Joined: 07/08
Posted: 10/22/10
07:54 AM

Changing the bearings without checking the crank will not help if that is your problem.  
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