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Smile if you know Jesus
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I've got a 350 in my '76 K5 Blazer that has a dull knock (not loud, but noticable if you know how an engine is supposed to sound) comming from the rear of the engine on the passanger side. I've read a lot on engines and rebuilding V8s but I have never got into one. My guess is that it is from piston slap (piston rocking in the cylinder). There is also a noticable pop (at idle) from the exhaust on the same side of the engine. This engine was supposed to have been rebuilt about 2-3 years ago. The engine runs fine, I just am wondering if this is cause for concern.
 

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Smile if you know Jesus
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timing is good. I time my cars to max vacuum. It's around 6 or 7 degrees on the Blazer.
 

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Knock

Doc here:pimp:

Header leaks?
Maybe a Distributor Gear going south?
Flat lobe on the cam?
Loose Bolts on the flex plate?

I would think, If you had that much Slap, you'd have Some other indications too...Oil consumption, Smoke, Gas in the oil, low compression, High crankcase pressure, Misfire..just to mention a few...fouled plug..

Look at The easy stuff first...

Doc :pimp:
 

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if your timing is good and you've tried higher octane gas(at the pump)maybe you need to check the plugs and do a compression test.it sounds like a rod bearing or rod knock or as you say,piston or valve slap.
 

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I'm with GoneNova if your timing is good try high oct gas or try a tune up.. If that's not it post again, and see what I can help you with
 

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Consider its a rear main bearing. However sometimes the kind of oil will cause this.... go to a 20/40 standard oil no synthetics. Piston slap is a bit between a noisy lifter and a rod noise. Remove spark plugs one at a time with it running see if you can tie it to one cylinder.
 

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Plug

Doc here :pimp:

Check for a loose or Broken plug (separated at the glass from the metal) At that cylinder.

Leakey or broken header / Collector can make the same noise...look for "Black marks " around mating surfaces.

Doc :pimp:
 

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Piston slap is usually only noticed when the throttle is snapped opened, and mostly during the warm up period. If it's a broken piston, then accelerating the motor like I said will accentuate this. Killing the ignition on the suspect cylinder will stop most piston noise - use this method to isolate the cylinder. A big end bearing becomes noisier as heat builds up. It produces the noise when the throttle is being closed, with the RPM floating on and off between roughly 2,500 to 3,500 RPM. Main bearings are the least often heard components in an engine. They usually create a deep rumble, that's felt as much as heard on low speed acceleration. The motor takes on a rough feel in its operation as the crankshaft wobbles in its location - the opposite of a really smooth engine that has a super rigid mains set up.
If the noise is constant then there's a more than likely chance that the noise is in the valve train.
 
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