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Fellow Engine Builders: Low RPM Cold Knock
#3159471
07/13/23 08:23 PM
07/13/23 08:23 PM
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Joined: Jul 2005
Posts: 707
California
BigDaddy440
OP
super stock
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OP
super stock
Joined: Jul 2005
Posts: 707
California
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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!
1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: BigDaddy440]
#3159485
07/13/23 10:16 PM
07/13/23 10:16 PM
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Joined: Jan 2003
Posts: 20,179
Park Forest, IL
slantzilla
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 20,179
Park Forest, IL
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What's the piston to wall clearance? IIRC, KB are pretty tight.
"Everybody funny, now you funny too."
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: B1duster]
#3159500
07/13/23 11:42 PM
07/13/23 11:42 PM
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Joined: Aug 2019
Posts: 9,468
Super Spudsville
Mr PotatoHead
Half Baked
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Half Baked
Joined: Aug 2019
Posts: 9,468
Super Spudsville
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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.
STOP POTATO HATE!
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: Mr PotatoHead]
#3159501
07/14/23 12:31 AM
07/14/23 12:31 AM
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Joined: Jul 2005
Posts: 707
California
BigDaddy440
OP
super stock
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OP
super stock
Joined: Jul 2005
Posts: 707
California
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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.
Last edited by BigDaddy440; 07/14/23 12:32 AM.
1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: BigDaddy440]
#3159504
07/14/23 01:40 AM
07/14/23 01:40 AM
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Joined: Mar 2011
Posts: 3,076
Michigan
A727Tflite
master
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master
Joined: Mar 2011
Posts: 3,076
Michigan
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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.
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: A727Tflite]
#3159505
07/14/23 01:52 AM
07/14/23 01:52 AM
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Joined: Jul 2005
Posts: 707
California
BigDaddy440
OP
super stock
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OP
super stock
Joined: Jul 2005
Posts: 707
California
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Thank you for the clarification Transman.
-Dan
1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: BigDaddy440]
#3159512
07/14/23 06:38 AM
07/14/23 06:38 AM
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Joined: Jun 2008
Posts: 3,916
usa
lewtot184
master
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master
Joined: Jun 2008
Posts: 3,916
usa
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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.
Last edited by lewtot184; 07/14/23 06:44 AM.
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: BigDaddy440]
#3159517
07/14/23 08:42 AM
07/14/23 08:42 AM
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Joined: Jan 2003
Posts: 20,179
Park Forest, IL
slantzilla
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 20,179
Park Forest, IL
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Has anyone with a piston slapping engine noticed the slap going-away with some RPM even when it's cold?
Most everyone on the planet with a late 80s-early 90s Chevy 350. GM said it was "normal". 
"Everybody funny, now you funny too."
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: bobby66]
#3159616
07/14/23 04:29 PM
07/14/23 04:29 PM
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Joined: Jan 2003
Posts: 14,499
So. Burlington, Vt.
fast68plymouth
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 14,499
So. Burlington, Vt.
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Has anyone with a piston slapping engine noticed the slap going-away with some RPM even when it's cold?
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”. 
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: slantzilla]
#3160031
07/16/23 06:48 PM
07/16/23 06:48 PM
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Joined: Jul 2005
Posts: 707
California
BigDaddy440
OP
super stock
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OP
super stock
Joined: Jul 2005
Posts: 707
California
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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
1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: topside]
#3160116
07/16/23 09:59 PM
07/16/23 09:59 PM
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Joined: Jan 2003
Posts: 20,179
Park Forest, IL
slantzilla
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 20,179
Park Forest, IL
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It isn't a header leak, is it?
"Everybody funny, now you funny too."
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: slantzilla]
#3160600
07/18/23 02:23 PM
07/18/23 02:23 PM
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Joined: Sep 2003
Posts: 19,408
north of coder
moparx
"Butt Crack Bob"
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"Butt Crack Bob"
Joined: Sep 2003
Posts: 19,408
north of coder
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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 ! 
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: topside]
#3160867
07/19/23 09:32 AM
07/19/23 09:32 AM
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Joined: Jan 2003
Posts: 13,365
Marion, South Carolina [><]
an8sec70cuda
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 13,365
Marion, South Carolina [><]
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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...  Check those first. That isn't piston slap. 
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: an8sec70cuda]
#3161067
07/19/23 04:02 PM
07/19/23 04:02 PM
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Joined: Jan 2003
Posts: 9,875
Weddington, N.C.
Streetwize
master
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master
Joined: Jan 2003
Posts: 9,875
Weddington, N.C.
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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.
Last edited by Streetwize; 07/19/23 04:07 PM.
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Re: Fellow Engine Builders: Low RPM Cold Knock
[Re: Streetwize]
#3161196
07/19/23 10:07 PM
07/19/23 10:07 PM
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Joined: Jan 2003
Posts: 8,165
Plymouth, MI
Blusmbl
master
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master
Joined: Jan 2003
Posts: 8,165
Plymouth, MI
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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.
'18 Ford Raptor, random motorcycles, 1968 Plymouth Fury III - 11.37 @ 118
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