Vortec knocks when cold

[rotnhoe]

96 hoe does it.119,00 miles. 15-20 sec. Colder it is longer it takes to go away. I asked GM service manager if i would have the same issue with a new crate motor.His reply was I wouldnt till carbon built back up on the piston and causes piston to tilt to side causing the "slap". Guess I'll live with it for now. I've heard V-8 toyotas have similar issue. Still take a noisy Chevy over a quiet ford.

 

[RHamill]

Truckin'Ain'4sissies said it perfectly.

 

[jeff's88silverado]

From the sound of it some of you have piston slap. Which is pretty much one of your cylinder wall is a little wore and you have excessive clearance. and the tap noise is the piston skirt hitting the cylinder wall, then as the engine warms up and the piston expands it takes up some of the clearance and the noise stops. It just happens with normal wear and tear and the engine is perfectly fine driving it.

Going to school for automotive and thats out of my book

 

[Tony_SS]

Mine does it too.... and if its cold and I start up and take right off.. it'll knock for a few mins.

I always let it run for a min before driving when its cold. I have 172k on the clock.

 

[96-1500]

My '96 5.7l vortec occasionally knocks for around 5-10 seconds when cold started. I've had the truck almost a year, it has 355'000 kms.
I never take right off, especially if it knocks. Usually I let it idle for a few minutes. If it's below -20 I go out and start it up 5-10 minutes before I plan on going anywhere. Maybe I'm being too cautious.

 

[RHamill]

I always warm it up, I don't want someone waking me up and expecting me to work right away without letting me gather my bearings first (no pun intended).

 

[TruckinAin4Sissies]

My '96 5.7l vortec occasionally knocks for around 5-10 seconds when cold started. I've had the truck almost a year, it has 355'000 kms.
I never take right off, especially if it knocks. Usually I let it idle for a few minutes. If it's below -20 I go out and start it up 5-10 minutes before I plan on going anywhere. Maybe I'm being too cautious.

You do that at -20? Hell, I do that at +50 LMAO

 

[96-1500]

You do that at -20? Hell, I do that at +50 LMAO

LOL, well, better safe than sorry right?

 

[adambru]

I actually timed how long my vortec knocked this morning, 1 min 10 seconds lol. That's about - 5 degrees also. On the edge of selling her or doing a total rebuild in spring

 

[stroker52]

The 5.3 an 6.0 are bad about having a lifter tap.. they don't have the old gen 1 piston slap.. I had a 2004 truck come in under extended warranty with a tapping sound and I tried to tell the customer that it was a normal deal. He said fix it. Ok then so I put a set of lifters in the motor the noise was still there. It has to do with the way the block from the factory was done. I mean you show me one block that don't have core shift and I will buy it lol..

I've sonic tested well over a few blocks for my dirt car and you will be floored at how thin some of the blocks I've checked.. I've had 3 zero bore 3970010 4bolt main blocks that was junk.. to much core shift.. one side of the bore would be paper thin and the other would be real thick..

So that's why some motors have the piston slap an others don't..