[tornado-tech]

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

[william27103]

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

[tornado-tech]

Just looked through my paperwork from the engine builder and the Total Seal rings were gapped .019 on the top and .0145 on the second. Is that too tight of a ring gap for Keith Blacks?

[71ss]

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.

[bowtieorbust]

I got a set of KB flat's with 2 valve relief's and i ground my top ring's at .030, and the second ring and the oil scraper's at .016.

[1BAD80]

Here is their site, gaps are on it and explains like william27103

http://kb-silvolite.com/clearance_pop.php
I use 6" rods with dished pistons for a 10:5.1 with gapless rings on my 406 motors.

[camaroman7d]

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.as..._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

[tornado-tech]

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.

[coldknock]

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

[lluciano77]

You didn't have enough ring gap.

Keith Black recommends 32o total timing with their hypereutectic pistons. They say more than that will not improve performance.

[camaroman7d]

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

[tornado-tech]

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.

[arch]

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.

[glen242]

Originally posted by 1BAD80
Here is their site, gaps are on it and explains like william27103

http://kb-silvolite.com/clearance_pop.php
I use 6" rods with dished pistons for a 10:5.1 with gapless rings on my 406 motors.

What pistons, brand and part number, are you running with the 6" rods?

[tornado-tech]

As of right now, it has Keith Black 128040 2-valve flat-tops with Total Seal TSS rings.