The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Any idea on what this knocking is 72 C20 350

**AussieinNC** · 09-12-2017, 04:56 PM

Remove the passenger side rocker cover.

You need to manually turn the engine so that each passenger side cylinder comes up on compression stroke...you can watch the inlet rocker close off, then turn the engine 1/4 turn...

On that cylinder, check the rocker arm clearance to the valve stem...it should be just touching the stem.

Try and push down on the pushrod end of the rocker arm,,,if it goes down at all, you have a bad lifter.

If the rocker arm has any clearance over 1/16" the adjustment nut on that rocker arm has either backed off...or, worst case, the stud has pulled slightly out of the head. Either way...its not a simple fix...

What is the oil pressure with the engine running?

I have also heard a noise like this when the torque convertor bolts come loose...you can check by removing the flywheel cover plate if it fitted and check each of the three bolts and nuts for tightness...

While you are under there, grab the flew plate and try to move it rotationally back and forth...listen for a clunking sound...

If it clunks it could be a cracked flex plate...

09-12-2017, 04:56 PM	#11
AussieinNC Moderator Join Date: Jun 2017 Location: Cherryville, NC Posts: 2,162	Re: Any idea on what this knocking is 72 C20 350 Remove the passenger side rocker cover. You need to manually turn the engine so that each passenger side cylinder comes up on compression stroke...you can watch the inlet rocker close off, then turn the engine 1/4 turn... On that cylinder, check the rocker arm clearance to the valve stem...it should be just touching the stem. Try and push down on the pushrod end of the rocker arm,,,if it goes down at all, you have a bad lifter. If the rocker arm has any clearance over 1/16" the adjustment nut on that rocker arm has either backed off...or, worst case, the stud has pulled slightly out of the head. Either way...its not a simple fix... What is the oil pressure with the engine running? I have also heard a noise like this when the torque convertor bolts come loose...you can check by removing the flywheel cover plate if it fitted and check each of the three bolts and nuts for tightness... While you are under there, grab the flew plate and try to move it rotationally back and forth...listen for a clunking sound... If it clunks it could be a cracked flex plate...