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Old 03-25-2008, 09:28 PM
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ring end gap question

Hi All,
Heres my problem....
Car tube Vega foot brake car after 35 years away i got this bracket car...put togather enough parts to get it going...engine 383 040. over flat top with KB clamier pistons...vortec heads that i ported ....car has 30 passes...has run a 10.97 best...never had a problem until last time out....the car would stumble of the line....today i pulled the plugs,much to my surprise number one plug had the gap nearly closed...it also showed some dings...knowing this not good i pulled the head one fourth of the top ring land on number one is gone.....I did not build the short block local engine shop did...I did the heads and installed and degreed the cam....my question is do you think was ring butting...I have no idea what they where set at I do know they knew they file to fit....another clue is the other 7 pistons had a wire edge on the top ring...my thought is there were seeing ring butt but wore in...very heavy wear at that. Are these pistons junk or???? The engine has never been over 200 degrees and was running with 110 race gas blended 50% with 91 octane pump gas total advance was 36 degrees...all KB could offer was they thought that was to much advance....car is motor only no gas or other....any help??? I am in the process of building what i thought would be a better engine 406 with brodix heads more cam and etc....same piston dont want another 30 pas engine.....

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Old 03-25-2008, 09:46 PM
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Quote:
Originally Posted by 32hibo
Hi All,
Heres my problem....
Car tube Vega foot brake car after 35 years away i got this bracket car...put togather enough parts to get it going...engine 383 040. over flat top with KB clamier pistons...vortec heads that i ported ....car has 30 passes...has run a 10.97 best...never had a problem until last time out....the car would stumble of the line....today i pulled the plugs,much to my surprise number one plug had the gap nearly closed...it also showed some dings...knowing this not good i pulled the head one fourth of the top ring land on number one is gone.....I did not build the short block local engine shop did...I did the heads and installed and degreed the cam....my question is do you think was ring butting...I have no idea what they where set at I do know they knew they file to fit....another clue is the other 7 pistons had a wire edge on the top ring...my thought is there were seeing ring butt but wore in...very heavy wear at that. Are these pistons junk or???? The engine has never been over 200 degrees and was running with 110 race gas blended 50% with 91 octane pump gas total advance was 36 degrees...all KB could offer was they thought that was to much advance....car is motor only no gas or other....any help??? I am in the process of building what i thought would be a better engine 406 with brodix heads more cam and etc....same piston dont want another 30 pas engine.....
KB hyper pistons (claimers included) MUST run a much larger ring end gap (.028" to .030" would be close) than normal. These pistons are designed to run much hotter at the top, necessitating a larger end gap. IN THE BOX that the pistons come in, there are instructions that indicate this fact and how much end gap to run for the particular application. The machine shop that did the machine work and assembly dropped the ball on this one.

tom
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Old 03-26-2008, 12:37 AM
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Are these pistons junk

They are now

On my S &S 406 with KB's went with a .033" top ring end gap.
Bigger is better. At least when it comes to ring end gap on KB pistons.

KB also says you don't need quite so much spark lead with their pistons.
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Old 03-26-2008, 05:54 AM
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pistons

Take one of the top rings from a cylinder that does not have a broken ring land and check the gap in the bore it came out of.

let use know what that gaps is.

Then we can go forward with the diagnosis.

Keith
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Old 03-26-2008, 07:03 AM
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I would say as long as you are building another motor you may want to change the piston you are using. It is my experience that the KB pistons don't hold their tolerances very well. I went through 3 sets of BBC pistons then I gave up and went with SRP forged pistons.

The problem was that they was a .002-.003 taper from the top to the bottom of the pistons so while you could hold the correct tolerance at the bottom. By the time you got to the top it was way out of spec.
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Old 03-26-2008, 09:00 AM
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First thing I did

I thought that would be a good first check so I did exactly that and it was 035 on a ring from a good piston....however not sure this is a good number as I mentioned the rings all have rolled wire edge....the other pistons could have butted and woren there own clearance..... The top rings all look scuffed whereas the 2nd ring does not, another indication of butting...I understand KB says 2 to 4 degrees less timing however less than what?....Some run sbc chevys with far more total timing than 36 degrees......I just dont know...I have one new piston coming am going to gap the rings and asm myself....should done that to start with....if its something else,other than ring gap I am not sure what to do.....
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Old 03-26-2008, 11:01 AM
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It is possible that the piston just broke...they are hyper pistons right? You were drag racing, what was your shift point?
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Old 03-26-2008, 11:19 AM
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Quote:
Originally Posted by Double_v23
...........

The problem was that they was a .002-.003 taper from the top to the bottom of the pistons so while you could hold the correct tolerance at the bottom. By the time you got to the top it was way out of spec.
Were they bigger at the top (below the oil ring groove) that at the bottom? Or were they smaller near the top?

tom
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Old 03-26-2008, 11:57 AM
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I guess I don't remember which way they were tapered but when I sent them back they checked them, apologized, then drop shipped me another set, which I sent back, then they apologized and sent me another set, this repeated until they were on "national backorder" then I went with another brand.

They did say that the pistons were out of spec and never charged me any additional shipping but what a pain it was!!!

I think they put them on national backorder because they realized they had a problem.
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Old 03-26-2008, 03:03 PM
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Quote:
Originally Posted by 32hibo
I thought that would be a good first check so I did exactly that and it was 035 on a ring from a good piston....however not sure this is a good number as I mentioned the rings all have rolled wire edge....the other pistons could have butted and woren there own clearance..... The top rings all look scuffed whereas the 2nd ring does not, another indication of butting...I understand KB says 2 to 4 degrees less timing however less than what?....Some run sbc chevys with far more total timing than 36 degrees......I just dont know...I have one new piston coming am going to gap the rings and asm myself....should done that to start with....if its something else,other than ring gap I am not sure what to do.....
the best way to see if your end gaps are to tight in the engine after it has been run, is to inspect the end gaps to see if they are rubbing together, if they are shiney i would say not enough gap.

sam-missle
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Old 03-26-2008, 07:50 PM
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got the new piston today

start to reasm.....changes are....bigger jets in the carb and will back the timing off to 32 degrees total....also when the new rings arrived I discovered the moly top ring had been installed in the second position by the shop that did the asm....also the shift point is 6,000 very low in my opinion....thats street motor RPM....engine should have run a long time......set the top ring gap at 32 and the second at 24.....thats the high end of the spec....
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