Thread: Engine knock

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Well this isn't good. We have developed a knock in a 383 stroker. It sounds like piston slap. It only happens at very low idle or under load (stopped and in gear- it's an automatic). We did a hot compression test and found that #7 is running about 25 pounds higher than the rest. Most are running about 170, #7 is running at 200. We just ran it and started disconnecting plug wires one at a time to see if we could get the knock to stop. When we disconnected #7, it stopped knocking.

So the question is - why is #7 compression so high? Or why are the rest so low?

Sounds like a cracked piston, though I don't know why the compression would increase. How many miles on it? Cast or forged pistons?

Either way engine needs to come apart so whats the problem? youll know when you strip the motor.....

about 2500 miles. Pistons are Speed Pro Hyperutectic (H860CP).

It's not a deep knock like a rod, but more like a ping.

JE SRP FORGED!!!!!!!!!!!!!!!!!!! Thats the way to go in my opinion. i used speed pro's before but only the forged ones, who knows if its a cracked piston though, you wont know for sure till you tear the motor down....

i had a 327 in my 57 that had a slight ping noise at
#7 cylinder it was about 12 years old at the time so
i kept running it it lasted another 5 years before #7
rod went thru the oil pan getting on the free way,
it made alot of noise and i kept going on the side of
the free way untill i made it to a off ramp the i found
a gas station then flat bed home. now i have a 350 4bolt
and spent 190 bucks for balancing,i really think $190
was worth it it revs real fast and runs real smooth.

200 psi? Sounds like the exhaust valve isnt opening soon enough or enough in general. Also would explain the ping. Your mixing burned hot fuel with fresh. Check the lift and duration on that exhaust valve. May be a simple lobe on the camshaft.

drg84 may have hit the nail on the head. Never thought of exhaust valve causing HIGHER compression, but I have seen the other symptoms relating to cam lobe failure.

Thanks for all the input guys! I was suspecting the cam and was hoping to hear the same from someone else. I can't think of anything else that would effect compression like this, although I have difficulty understanding how it would effect it by 25 psi. Does the exhaust valve open before tdc on the compression stroke?

I drained the oil yesterday thinking I might find some metal if in fact we did knock a lobe off the cam. Didn't find anything though.

To check the lift on that exhaust valve can I just put an indicator on the lifter and compare it with another? Who knows maybe it will be easy to see if it is not opening at all.

I would sure like to think it's the cam - much easier to fix.

If it's the cam you'll see it easy with the cover off. The valve will barely move. Just put your hand on the rockers with the engine idling. Keep in mind that it could be a pulled stud or bent pushrod, but seeing that it's a SBC its probably the cam.

I thought about a rocker stud or maybe even the rocker nut backed out, but wouldn't that make additional rattle noise?

Well, we will find out soon enough. As soon as I get home we will pull the cover off and check it out. I'll reply with what we find.

If you have a hydralic cam the exhaust lifter might not be pumping up. Have seen that before.

All right, I could not wait to get home to check this out. It looks like we lost the INTAKE lobe on #7. It's only moving about 1/4 inch at the most. Exhaust looks fine. I forgot the heads have screw in studs with the locking nuts. Rocker arm looks good, as well as push rod. I sure never thought the intake would give us the symptons we were experiencing.

So now the question is - do I have metal in the bearings and empregnated in the pistons or would the oil do a good job at washing it down. I remember my dad had a 283 that lost a lobe, he just put a new cam in and it ran forever.

We got the cam out and sure enough the lobe is gone. Unfortunatley the lifter was damaged on the bottom enough so that it would not come out. With some magnets we tried getting it out the front through the cam bearings. It's now at the bottom of the oil pan.

A relief, only a lobe, and only 2500 mi. I might be tempted to pull the pan, if only to make sure there are no flakes or heavy filings.
Next question, why did the lobe go away?
At least with a fresh rebuild, it has to be clean to work on. Definately bummer on the cam, but much better than a piston.