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Discussion Starter · #1 ·
Had oil consumptions problems on pretty new 408sbc. Thought it might be the valve guides. I just finished removing the heads and found 3 pistons with the top ring land broke. I got the pistons with 3/16" ring lands. Have any of you guys had this same problem with KB's? Since have to rebuild the motor and getting new pistons, what pistons would you recommend for a 6" rod 400 that sees a max of 6000rpm? Am looking a 2-valve flat top to keep the compression around 10:1 and not really wanting to go with KB's again. This is also a non-nitrous motor
 

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i don't really have a suggestion as to what pistons to use, but the ring lands more than likely broke because the wasn't enough end gap on the rings. They have to be larger on SOME of the keith black pistons because the top ring land is so close to the top of the piston and it gets alot hotter, causing them to break. I've even heard of the whole top of the piston popping off without enough end gap on some of the KB pistons.

anyway, just a thought

Will
 

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Yes man this hypereutetic pistons are weak, you need more end gap in first ring as much as 24 posibly, and one thing you must not forget this pistons must be used in an engine with the total timing reduced to 36 or 34 degrees, i have broken 6 of this in diferent aplications so be care, and if posibly run good fuel.
 

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Yes man this hypereutetic pistons are weak, you need more end gap in first ring as much as 24 posibly, and one thing you must not forget this pistons must be used in an engine with the total timing reduced to 36 or 34 degrees, i have broken 6 of this in diferent aplications so be care, and if posibly run good fuel.
Non-sense!! If you are breaking that many pistons you are not doing something right. Also there is no difference in the timing for KB hyperutectic pistons.

KB pistons are fine BUT, the ring end gap MUST be right. Looking at what your ring gaps were set at I would have to say they are too tight. The link 1bad posted gives you the "ring end gap factor" you multiply that by your bore size for a given application and this is your ring gap.

I have run KB's a ~500HP 385ci SBC and they are still in that engine today, this is after over 7 years of use and abuse.

If you are looking for a different piston and a better piston take a look at Wiseco Pro-Tru Forged pistons. I am running them in my 6" rod, blown 388.

I found the best deal through www.dirttrackthunder.com they are good people to deal with and have great prices.

For your application you need part number PT020H and the price is $449.00 including hastings moly rings (complete set). They are flat top 2 valve relief forged pistons (complete with pins and locks). Here are the details.

400 V-8
FLAT TOP 2-VALVE RELIEF
BORE 4.125"
STROKE 3.75"
ROD LENGTH 6"
COMP HT. 1.125
PIN DIAMETER .9272
Head Vol.-Comp Ratio
64cc-11.4
76cc-10.0

Pro-Tru pistons by Wiseco are an excellent option for a forged racing piston. They come complete with Hastings Plasma Moly rings, chrome moly wrist pins and spiro loxs.

Here is a link to the page these pistons are on (top of the page) http://www.parts123.com/PartFrame.a...dirttrackthunder.com&TITLE=Dirt_Track_Thunder
Hope that helps.

Royce

tornado-tech,
I just noticed you said you want to keep the compression at 10:1. Were you running flat top KB's? If so what size are the chambers on your heads? I don't see how you are getting 10:1 with true flat top pistons. This could be the reason you broke the pistons as well, hyperutectics do not like detonation and that is a sure way to break them.


Royce
 

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Discussion Starter · #8 ·
I run 76cc s/r torquer heads and run 91 octane all the time. I only run 33 degrees total timing (18 initial and 15 mechanical). The compression should be about 10:1 with the .051 head gaskets I use.
 

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Your initial timing was probably a big factor, along with the ring gap, in breaking the ring lands. Try 8-12 degrees intital and 20-24 mechanical. It might help the pistons live a little longer.

Larry
 

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Well if you are using a head gasket that thick, you have no quench. This will be even more prone to detonation. I am not sure how far in the hole your pistons are but, with the .051 head gasket they need to be .006 out of the hole to give you any quench.

Since you have to buy new pistons, may I suggest something? Buy a "D" dish piston that will give you a true 9.5 -10:1 with a tighter quench, run a head gasket that will put your piston .035-.045 from the head. This will help prevent detonation and you will have a better performing engine.

Royce
 

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Discussion Starter · #12 ·
Ok. If I went with the Wiseco flat-tops and a .044 head gasket which should give me better quench, would I need to rebalance the entire assembly again if the former balance job had everything weight matched for the heavier Keith Blacks. This is an internal balance motor. I just bought a set of the old Summit Sportsman II's (68cc, 2.05/1.6) for little money from racing friend that look identical on the outside to my s/r's. I know I will raise compression but plan on running Sunoco 110 to avoid future problems.
 

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Yup, needs to be rebalanced. Also make sure you have the proper quench this time, like camaroman said. And proper ring gap for the pistons you are using, like everybody else said.
 

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Just want to ditto on the ring gap. I cant see how KB pistons would negate the common timing practices of performance SBC engines. 34-36 degrees is typically what an engine likes depending on the total package. It seems to me that KB is just trying some CYA since the hypers are much more sensitive to detonation than forged pistons.

I have also delt with dirttrackthunder and they are very good to work with as well as knowledgable about the parts they sell. You could also look into a set of SRP pistons. I am running them with CA Zgap rings in my turbo engine and I must say they are very nice. They are a little more than the wiescos if you count ring costs, but are basically JE pistons in a different box from what I have been told.

Chris
 

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Here is where the Kieth Black states below:

"Ignition timing on most KB installations should be 30-34 degrees total with full mechanical advance dialed in. More advance may feel better off the line, but the engine lays down as the combustion chamber components come up to temerature. "

Any thoughts?
 

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I agree with Chris. KB is just covering their butt.

Not really worth discussing or arguing over. The fact is ring gap is the biggest risk to those pistons after that it is detonation. If you had a combo that liked 40* of timing and didn't detonate the KBs would work fine. Changing the type of piston has little to do with your timing curve/amount (IMO).

I think the reason they put that *note in there is because these pistons will mostly be used in street cars or cars that run on pump gas for the most part. With todays pump gas you can't get away with a bunch of timing so they are trying to keep people out of detonation. Not many pump gas engines are running more than 34* of timing anyway.

Royce
 
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