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Old 10-21-2008, 03:07 PM
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another burned piston..

Hi guys, looking for some help with a sbc 350 that keeps lifting/melting top ringland.its a circle track car (1/3 mile)
stock crank and rods
10:1 flat tops
1.94 camel humps
crane hyd.flat tappet
.500 lift. int and exh. 246 dur @.050
performer int.
HEI stock with accel mod.and coil
stock holly 4412 2v
this season we burned 5 pistons,one at a time.the only thing i can think would cause our problem is detonation.i've looked for vac leaks,rejetted carb,backed off the timing,changed heads,intake man.,ck float level several times,taken carb apart looking for problems,unable to locate cause but suspect a vac. leak under the intake.3 out of 5 burned pistons came out of cyl.#6,the other 2 from #5.we dont even get 1 night of racing in after a rebuild and burn a piston.(getting expensive).when the intake comes off,it looks as if oil is getting into the runners in the heads and there is oily substance on all pistons.int. gaskets look to be getting compression but are also very wet with oil around all int.runners.could int. gaskets be wicking oil?
we run 35* total advance timing
sunoco 98 oct. race gas unleaded
any help would be appreciated

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Old 10-21-2008, 03:56 PM
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another burned piston..

are you running to lean ?
does the headers glow cheery red ?
what intake you running ?
if your melting the top ring , this will cause the blow by of oil .
are the pistons hypertectic's
and have you checked the cam lobes for wear on the intake lobes of number 5 and 6 .
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Old 10-21-2008, 04:19 PM
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What about crankcase ventilation?? How is your engine vented?? Alot of oil into the mixture really messes up the fuel/air ratio. I'd probably make sure that thing isn't building up a bunch of crankcase pressure or you aren't dumping that oil laden crankcase pressure into the intake somewhere.......sounds like you already addressed the usual areas of concern so now you have to start looking at the odd stuff.

I'm assuming ring end gaps and all that basic stuff is ok.
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Old 10-21-2008, 04:39 PM
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intake is a edelbrock performer 2101 dual plane with a 1" adapter for a 2 barrel.The exh. has never gotten "red" and water temp stays between 190*and 205* at least until the piston comes apart,then the w/temp rises alittle.The crankcase is vented thru 2- 3" breathers filters on the top of the left valve cover. The cam and lifters are out of the block and look like new......ring gap set at .020..pistons are forged TRWs
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Old 10-21-2008, 05:16 PM
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wow, sounds like you have the bases covered.

The engine isn't all that radical to be causing too much of a problem, sounds like a normal claimer motor.

Years ago we ran a 360 sprinter that had a tendency to fill the right valve cover with oil after about 10 laps, we had to restrict the oil a bit more and open up the drainback holes. That was a solid lifter motor with alloy race heads, but with your iron heads and hydraulic lifters that shouldn't be happening.

This boils down to high combustion chamber temperatures and I suspect a lean condition more than anything. The piston crown "rolling" the edge and melting at the ring land isn't too characteristic of detonation. I've seen a few of the new GEN III engines hurt just like this when power adders or a bunch of boost is installed....just too much heat.

You originally suspected a vacuum leak and I think that might still be it......if you are convinced you are getting enough fuel. Gotta be a lean condition.
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Old 10-21-2008, 05:23 PM
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Right after it burns a piston did you remove the carb? if so did you see oil all over the plenum? this indicates a classic intake leak. Have your intake checked by a machine shop for warpness. I once had a edelbrock performer so badly warped even after they removed .020 it was still off so I threw it away. If that checks out okay I too agree it`s going extreme lean.
When I had a bad intake leak I could remove the intake with the carb still on it and watch the oil drip out of the runners, if your intake does this, it`s got a leak.
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Old 10-21-2008, 10:05 PM
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I'd look at an intake leak also. Has the block been decked and / or the heads milled?

I had an engine that I couldn't get the intake to seal though the gaskets "looked" like they had been compressed. All of the surfaces checked for flatness so it had me stumped. I set the manifold on the engine with no gaskets and I'll be darned, there it was, plain as day. The top was touching but the bottom had a good .060" gap. The manifold would come tight on the top but barely compress the gasket on bottom causing a leak along the entire bottom edge. The heads had been angle milled at some point in their life.

I see that you changed the heads and intake so it would have to be the block, which is unlikely but you never know, stuff happens. Mock it up "dry" and see how it looks. It's a long shot but it won't cost anything!
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Old 10-22-2008, 12:51 AM
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This wiki article was written for this exact scenario.....
http://www.crankshaftcoalition.com/w...ak%2C_internal
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Old 10-22-2008, 05:13 AM
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pistons

What brand and type of material are the pistons???

Can you post a picture or 2 of the broken pistons??

What is the top ring gap??

Keith
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Old 10-22-2008, 05:52 AM
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Along with Keiths question what part number intake gasket are you using and are you using gaskets on the end rails or RTV?
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Old 10-22-2008, 06:48 AM
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Are you using a power valve in the carb?What rating is it? 4412 carbs are known to pull enough vacuum at the end of the straightaway to close the power valve causing a lean condition right at the wrong time due to the small carb size.
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Old 10-22-2008, 10:00 PM
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All i know of the pistons is that they are 10:1 forged trw 2 relief flat tops
The ring gaps for the top and second rings are both .020
the gaskets are the stock port fel-pro blue that come with the perma-torque blue engine gasket set
power valve is a 5 and at this point the jets are 75s
I never use the end seals,always use rtv

these pistons went in the holes spotless clean and shiny,as you see them here,they have about 45 laps or 15 miles on them.
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Last edited by pearlman55; 10-22-2008 at 10:12 PM.
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Old 10-22-2008, 10:54 PM
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Think about this.

Cylinders 5 & 6 are on the same 'crank' rod journal.

Has this 'crank' been 'stroked'?

This 'crank' rod journal, could have been ground "offset" higher compaired to the others.
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Old 10-22-2008, 11:20 PM
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Stock crank and rods..
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Old 10-22-2008, 11:45 PM
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We build a ton of circle track engines and we don't see that problem and after looking at your pistons I would have to say its detonation from to much timing.

1 Is your balancer degreed ?

2 Is there a 36 degree mark on on your balancer?

3 I seen a guy have this problem and after seeing him use a dial back we found the light was off 7 degrees, We always use the one wire light from Flaming River and run the engine up to full adavancement and bring the 36 degree mark up to the pointer.

(note 36 degrees is what we have seen on the dyno on our engines with this similar set up and was only using it as a base number)

Those aren't Hyper pistons are they???

Useally to much timing makes the exhaust runn cooler and late timing or to much fuel will make the exhaust run hotter.

If you want PM me your number and I will try to work through this with you.
Carl
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