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Old 04-09-2006, 08:46 AM
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Cold knock issue with Gen III SBC`s...?

I`ve heard of it but dont know just what it is in these engines...? I have a 99 model truck with the 5.3ltr engine and it used to not do it but during colder weather it does some now... I was running 15-40 Royal Purple in it mainly because of the miles 150K+ but had switched to 10-30 RP and it helped or seems to a little...? Really just wondering what it is that causes the knock...? BTW will be putting another 5.3ltr in it later this summer I have one that has only 34 miles on it out of a 2002 truck...

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Old 04-09-2006, 10:27 AM
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they say its piston slap. the piston rocks in the bore a little, till it heats up. my 95 4.3 does it. the colder the more it does it.
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Old 04-09-2006, 12:25 PM
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Well might consider that but the engine does not have near enough time to warm up for this situation...? Also some do it from almost new... Why also would it not do it when the weather is warm...?

I run a BBC with clearance setup in the bores for the forged pistons and of course it sounds like a diesel when cold but gets much quieter when it reach`s operating temp (around 190F) this does not sound like that at all...

thanks on the reply though...
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Old 04-09-2006, 12:34 PM
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On these newer engines, the manufacturers have gone to a skirtless piston, which rocks very easily. Also, I believe they are forged, and this combined with the skirtless design contributes to a cold piston slap. Maybe the reason it does it worse when the weather is cold is because aluminium expands/contracts at a 3:1 rate as the cast iron block. IMO, if you've got that '02 engine already, run yours till she dies. It may suprise you.
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Old 04-09-2006, 12:36 PM
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If the knock is there for a short time while cold it is piston slap. The short (almost no ) skirted pistons may rock in the bore. Reduction in piston skirt reduces friction and thus helps with fuel mileage. Doesn't hurt longevity of the engine I've been told.
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Old 04-09-2006, 12:58 PM
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K... That explains it a little more and sounds reasonable... The current 5.3ltr thats in the truck runs super strong and does not use oil so it will stay in place for atleast a few more months...?

Now another question... The other engine I have (the 5.3ltr) how hard would it be to swap a cam out in it...? Was thinking of going with the hot cam setup...? Do you have to change springs or anything else...? Just havent researched this to much just yet... Also any kinda modd to the computer...? I was also going to add some nice coated headers and a better flowing exhaust... Just wanting to move the old ext. cab down the road a little better... After all it does have to pull the old 69 BBC truck to the track every so often...
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Old 04-09-2006, 02:16 PM
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[QUOTE=advanced design]If the knock is there for a short time while cold it is piston slap. The short (almost no ) skirted pistons may rock in the bore. Reduction in piston skirt reduces friction and thus helps with fuel mileage. Doesn't hurt longevity of the engine I've been told.[/QUOTE]

I am not sure i would believe that. Check this site, look at the pics.

http://www.pistonslap.com
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Old 04-09-2006, 02:23 PM
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Quote:
Originally Posted by Vortech 4.3
they say its piston slap. the piston rocks in the bore a little, till it heats up. my 95 4.3 does it. the colder the more it does it.
Yep, LT1s are the same way. No harm, just annoying. Chevy used really short piston skirts to chase down friction and power which allows them to rock a bit more.
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Old 04-09-2006, 02:48 PM
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I've got on of those friggin' rattley 5.3s in my 2000 Silverado. It's got 44K miles on it and it does it cold and a little hot, especially when you lean on it hard. I complained to the dealer about it ("It won't hurt anything......"). I complained to GM so they gave me a 100K mile warranty. I know darn well that if I did an engine for THEM that sounded like that they would B**** !

tom
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Old 04-09-2006, 02:58 PM
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This could also be contributed to too much advance timing, or, too lean condition in the motor, or more commonly antifreeze/coolant seeping through the porous aluminum cylinder heads...I have personally seen antifreeze come out of a 2.2 aluminum cylinder head outward to the atmosphere...there were no cracks...but, put under pressure, the aluminum cylinder failed...I don't know about the new BBC heads...but they definitely still have head gasket problems...Have 4, Mark V motors in fleet, all head gasket problems...Dealership changed gaskets...motors knocked when received them back, due to antifreeze being compressed in the chamber at one time or another (can not compress a liquid without something happening...) (O2 sensors did not even pick up the contaiments due to open loop mode)...refused the work...got new engines...All new SBC engines...have aluminum cyl heads...including Northstar engines...the type or quality of aluminum, GM uses, looks like a nodular "looking" aluminum...if you look very closely...it looks like open cell styrofoam compressed tightly...only aluminum..., very porous...it's not the same quality of aluminum that Porsche, Ferrari, Maserati, Aston Martin uses...These leaks are so small, not like a blown head gasket with heavy exhaust smoke of antifreeze...antifreeze overnight running down your short piston skirts doesn't do well by replacing the film of oil on you cylinder wall first thing in the morning...Just my 2 cents worth but it's the truth, as I see it...Cheers
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Old 04-09-2006, 03:10 PM
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Quote:
Originally Posted by nevermind
This could also be contributed to t...............Just my 2 cents worth but it's the truth, as I see it...Cheers
Nah, the problem with the SBCIIIs is simply a poorly designed piston. TOO SHORT! (Other makes are having similar problems for the same reason).

tom
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Old 04-09-2006, 03:18 PM
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So...your saying the other corporations, Chrysler, and Ford...are having same problems? Are all the engineers copying one another????Hmmmmm. I wonder.....
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Old 04-09-2006, 03:34 PM
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Quote:
Originally Posted by nevermind
So...your saying the other corporations, Chrysler, and Ford...are having same problems? Are all the engineers copying one another????Hmmmmm. I wonder.....
Yep. The biggest reason for shortening (and lightening) the piston is reduced friction and mass. They are all doing it to get the most from the engines (horsepower and mileage). Do a search for piston slap and you will find that amont others, the 5.4 Ford also has this problem, although it's not nearly as common as in the GM engines.

tom
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Old 04-09-2006, 04:12 PM
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Again that is a new head design...3 valve...again aluminum cyl heads...it could also be poor quality batch of heads causing the seepage...piston scuff as in previous pictures posted, don't just happen...look at the cylinder wall...the top of the piston near the Crown (above the rings) and the tip of the Skirt should be worn also (rocking)... this is not the case in those pictures, as in the engines that I have torn down...I feel that if the aluminum cylinder heads would be, bench pressure tested...they would be a contributing factor in the scuffing of the piston and the wall...This is my opinion, due to the many engines that I have torn down and rebuilt successfully over the years...4.6 fords DOHC also had the same problem due to the casting of the Heads...very complicated casting process...Cheers!


BTW, Machineshoptom...you are correct in the lightening of the pistons for economy, horsepower, and fuel mileage etc...

Last edited by nevermind; 04-09-2006 at 04:43 PM.
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Old 04-09-2006, 10:34 PM
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Hmmm, my '00 Silverado has the 6.0 with 85000 miles on it and it doesnt make a sound.

Bumpstick, if it got better going to the lighter weight oil maybe you should try going back to the 5W-30 the factory recommends and see what it does. I think those engines run tighter bearing clearances than the older designs, maybe even at 150k+ miles the oils you have been running are just too thick...??
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