My new ZZ4 engine has developed a “knock”. I have the GM 1.6 rockers, GM hot cam with hydraulic roller lifters, fast burn aluminum heads, MSD ignition and a Demon 750 carb. I believe the noise is coming from the valve train but it sounds very much like piston slap. I use a full turn after contact when adjusting the valves. I can’t hear the knock in the “block area” which is why I think it is in the heads. The knock subsides when the engine is warmed up. I only have about 500 miles on the engine.

Has anyone heard of this problem? Any help would be appreciated – thanks

|UPDATED|1/3/2005 10:37:34 AM (AZT)|/UPDATED|

Take a valve cover off and use a mechanics stethoscope to determin the origin of the sound, if you dont want to buy one use a long scewdriver and hold it to where you think its coming from and put your ear to the back end. warm up only slap could be cold pistons not expanded to the cylinder bore. As it warms up the piston expands and closes the freeplay. Even freshly rebuilt engines could slap at warm up if they got forged pistons. How hard has its life been? Was it broken in correctly? Too hard on the fresh engine? Maybe its a warrenty covered issue?

|UPDATED|1/1/2005 4:50:53 PM (AZT)|/UPDATED|

Sometimes exhaust leaks could fool people into thinking its a internal engine sound. Check for tightness on the header flanges, the cool headers warm up and expand to close the leak and quiet the sound. just some more two cents.

A member of my local Vette club had a ZZ4 motor in his '67 and had two pistons break with about 9k miles on the motor. I don't have any idea if noises he experienced were similar to yours, but he was told it looked like detonation damage. He did drive the car hard, I know that for a fact. 90 mph was about his normal cruising speed.

Before you go tearing alot of stuff apart, get a stethescope and check your engine that way. If you just put the motor in and used parts from your old one or bought an after market fuel pump you might check the pump for the knock. I used a crate motor in a car and it did the same thing, but the sound was coming right out of the fuel pump. I ended up changing the fuel rod and it did go away. Good luck

If you don't find it by assisted listening, kill one cylinder at a time and see if the knock changes.

A bad piston, rod, or crank bearing will change when the cylinder is killed. A valve noise, fuel pump, etc, won't change.