Thread: egine knock?

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hey guys,

I have a newly rebuilt 355 (all new parts on the inside) with about 500 miles on it. I have noticed a very slight knock when egine is cold and idling at 700 rpms. As soon as you idle it up (say to 8 or 900rpm no more noise. And then once the engine idles for 2 minutes or so it goes away for good. This was not a backyard rebuild as I had a machine shop do the work. Any ideas of what exactly would be doing the knocking.

PS. Cant really take it back for warranty as the build was done two years ago, just got it into the vehicle last month.

thanks for any responce.

what weight of oil are you using?

Crankrattle/walk perhaps?

Sounds like piston slap. One or more pistons are a few thousandths loose.

try tracking it down with a mechanics stethoscope,long extension small diameter pipe,whatever's handy.fuel pumps,feedback carbs,hei distributers all can make funny noises. Try ro get some idea where the noise originates.

what kind of knock is it a really deep on or more of a light metalic

thanks for the replies.

What is piston slap and how bad is it???


the oil is a 10w30. It sounds like its coming from the bottom, but the knock is more of a "tink...tink...tink". It doesn't happen on every stroke, but probably once per second only at Idle. Which if my math is correct is only about 1 per 12rpms (Idle at 700rpm). And like I said, as soon as I idle it up a bit, it goes away.

Any other thoughts?

Cracked flexplate.

Not frequent enough for pistons. It could be something loose on the outside. Sometimes the heat riser valve in the exhaust manifold will rattle like that. The flex plate idea might be right, too, but thats usually faster.

Might also want to check and see if one of your torque converter bolts are hitting the cover......

In my mere 40 years in the automotive repair business I have seen this hundreds of times.

1. A cracked flexplate don't go away.
2. a wrist pin knocks worse when hot than cold
3. Too light of oil allow clearence problems to be heard while an engine is in the warm up.

This symptom has all the right signs of being a poorly fit piston(piston slap). As long as the knock goes away fairly quickly, it will not be a problem.

I have seen the vast majority of these engines go well over 100,000 miles with adequate warm up.

IMO, I would take things apart to find and cure the knock. Got to do something to protect your investment............. Guess it boils down to how lucky you feel........

could be any of those things, but it sounds like a piston knock to me. most pistons that knock, knock when you crank the motor, goes away after the motor gets hot, wrist pins don't, flex plates don't and make dif. noises at dif. rpm's.

Change to a straight 30W oil and report back results