Fellow Engine Builders: Low RPM Cold Knock

Trying to sort out a low rpm only, cold knock on a friend's newly built non-mopar L31 350ci V8. Looking for some opinions...

Symptoms:

Knocking sound occurs on cold start up below 1200 rpm.

Above 1200 rpm noise goes-away all together, even when cold.

When cold the lower the rpm the sound is more noticeable, like 800-1000rpm.

Knocking sound disappears completely when engine reaches 160 degrees.


Additional Info:
KB Hyper 12cc Dished Pistons, 9.7:1 Static Comp.
Hyd. Roller Street cam 224@.050" / .510"
Conventional 10-30 weight oil is being used, has instant oil pressure upon start up.
Factory oil bypass eliminated, running K&N Oil Filter with internal bypass

Engine Starts and Runs great outside of cold knock. Of course it could be piston slap, but it's very noticeable, seems a bit loud too me.

Looking for opinions, suggestions and similar experiences. Thanks!

What's the piston to wall clearance? IIRC, KB are pretty tight.

Pull one plug and start it and see if it is better or worse.

Do that to each cylinder.

Years ago I fixed a 4 cyl ford mustang the was making noise similar to what you described.
I found the skirt from one side of a piston in the pan.

I had the same but with a cracked skirt on a cast piston. Lasted 45k miles before it broke up. Everyone swore at times I had a rod knocking. Were also very loose in the bore. Hone/re ring.

Maybe I should have stated "only encouraging responses".

All kidding aside, I'm now hoping it's "only" piston slap. Yes, KB pistons, due to their silica percentages, expand less, which allows for a tighter wall clearance. Min required is .0015". I don't have the specs what these cylinders measured out at, but they may have been bored for a typical cast piston or even a forged piston which expand much more. If so, this would cause the KB piston to be "looser in the hole" than what is optimum or suggested.

If I put my hand on the throttle linkage to the carburetor when it's idling cold, I can "feel" the knock, so it is substantial.

Has anyone with a piston slapping engine noticed the slap going-away with some RPM even when it's cold?

As far as pulling plug wires one at a time....which I'm glad to try....Should I assume you're suggesting this to eliminate spark knock as the culprit? I don't think it could be spark knock as it goes away with temperature, even at idle, and I'd think it would get worse.

I appreciate the responses given.

Not a test for spark knock, just a test to confine the noise to one cylinder.

Thank you for the clarification Transman.
-Dan

it's the kb pistons. no offset in the pistons and are set up looser. i have a stock 440 with kb184's and it's pretty noisy when cold. my pistons shut up nicely at about 130f water temp and drive nicely quiet with some heat in the pistons. i also think the hardness of the hyper aluminum makes the noise worse. i have another car with soft forged pistons, kinda loose, and the noise they make is more of a dull thud rather than a sharp rattle, but shut up with warm up. nature of the beast.

Most everyone on the planet with a late 80s-early 90s Chevy 350. GM said it was "normal".

My '85 305 too, but when we tore it down the piston skirts were in the pan.

Exactly............ like he said....... “normal”.

piston noise should remain until engine gets some heat no matter the rpm. i'm not in love with it but i want to run pump gas.

Video Update:

After listening again while making this quick video, it doesn't sound like piston slap. The frequency of the noise at idle is inconsistent, seemingly not repetitive or rotational. I'm starting to think it could be something external, maybe the torque converter or something related to the starter? In person, it sounds like it's coming from the rear of the engine.

Video Link:

https://youtu.be/DpdM4FxTRWQ

Look for cracked flex plate, converter bolt backed out, maybe even a rock in there if there's no cover ?
Any are better - or more accurately, less worse - luck than a bad bearing or piston...

It isn't a header leak, is it?

have you tried a stethoscope to try to exactly pinpoint the location of the noise ?
they can be as simple as a small piece of rod or brake line held to the side of your ear [the semi-round part just before the opening]
be EXTRA careful when doing this, so as to not get tangled into moving parts !

Check those first. That isn't piston slap.

Had a buddy that had a "mystery knock" at the track. it was (just one of) the converter bolts

Cold piston slap is (to my ear) a very distinctive sound and it's generally more prevalent on strokers. Theres a reason Ma put those offset pins (and longer rods) in there.

Agree with everybody mentioning converter bolts. My Fury had an almost identical sound but it wasn’t quite as pronounced because of how loud my exhaust is. I tightened the converter bolts and the noise went away.

UPDATE!

First, thank you to all who offered suggestions. This really had me stumped. Many of you mentioned the flywheel / flex plate.

Below is a 2 minute video update showing exactly where the nose was coming from.


https://youtu.be/GZzx_uC7blU?si=cxSEXEHNUw8upIj_


For those who would prefer reading:

The flywheel bolts (not converter bolts) ended up being too long, but long enough that it wasn't noticeable to the eye. They torqued down to spec, but were actually bottoming out in the crankshaft and not pressing against the flywheel hard enough to secure it while running.

Thanks for the update! Glad you found it.

great find.

Never seen a Mopar without through holes on the crank flange.

They were working on Chevy, probably didn't need to post it on here.
I sincerely hope they repalced that flex plate, it had way much clearance and movement between the flexplate bolts bolt to flexplate fitment It should not rotate like it did

Id ask on a stockish looking build or parts, why not just use oem flywheel bolts vs aftermarket and costly. OEM stuff is pretty tough and proven.

Guilty as charged - not reading the first post.

Did the bolts solve the issue?? I've been following this thread for a couple weeks but just now registered to be able to see the replies, and I watched the vid and read the description. No idea if this is the issue I'm dealing with but it will be the first culprit once I get under the car to check things out. Thanks for the updates!!