Landracing Forum
Tech Information => Technical Discussion => Topic started by: Jonny Hotnuts on June 28, 2009, 02:49:36 AM
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I made a homemade positive piston stop to degree the cams. I noticed today that it put a scratch in the top of one piston. The scratch is small but I am sure you could feel it with your finger if the head was off.
Is this anything I should be concerned with?
I dont want any hot spot to form......
~JH
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Jonny,
We don't know how bad the scratch is, but let me pose this question.
After all the time, money, and effort you've put into your car, how would you feel if the engine failed due to something you were aware of and didn't address?
It's frustrating, this having to make decisions.
Chris
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I'm not too fussy myself. I know pistons can be screwed up pretty hard and still make big power.
I have often punch marked piston crowns and haven't run into any problems, and that's deeper than a scratch, and hand cut valve reliefs with a power drill. Anything less than a 2-3 thou is getting filled in with carbon anyhow.
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The flame front rapidly heats the piston during the power stroke. At the end of the power stroke it starts cooling, and the incoming charge cools it further. The piston would rapidly fail if it was continually exposed to the high heat of combustion.
The scratch you describe can allow the heat to reach that area in a non-uniform pattern, but the amount with a small scratch isn't going to be much. If you leaned the engine out enough to melt the piston, the scratch isn't going to accelerate that point.
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Ignore it, if your subconscious lets you.
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Polish it out with a buffing wheel. That will take the sharp edge off.
Tom G.
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Has anyone actually seen a lower knock point with scratched piston? I remember when lots of guys were polishing pistons before they found it had either no effect, or a negative effect. Seems the spark plug electrode and exhaust valve lip will ignite the fuel way before you can ignite it with anything made of aluminum.
The only hard evidence I have is when I hand cut some LS6 pistons, which leaves a very large scratched surface finish. After tuning for max power, the timing before onset of KR (spark knock indicator) was pretty much the same. But that's not a Hayabusa, nor turbocharged.
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was the dome coated?
kent
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was the dome coated?
kent
No,
Off the shelf JE 81mm turbo pistons
(http://photos-c.ak.fbcdn.net/hphotos-ak-snc1/hs022.snc1/3071_1138983710829_1114496579_30437434_8167473_n.jpg)
Heres a pic after I remove the sharp edges and polished the duke out of them.
Pizzes me off 'cause I took so much time getten 'em shiney.
~JH
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BTW:
Motor fired for the first time on Sat......dry blocked it for a few seconds and without the blower....but the SOB sounds heathy and fired in the first 3 crank turns.
And there was no "clickety-clack"!
Would be a shame to have to pull the head off.....
~JH
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Jonny:
If you hadn't made them so nice and shiny you wouldn't notice the scratches! :evil: :-D
Pete
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Forgive me if I am beating a dead horse. The hottest object in the combustion chamber is the exhaust valve margin area. Aluminum readily convects heat and will sustain combustion temperatures that are well in excess of it's melting point if it's given enough time to transfer heat to the cylinder wall and the oil being squirted on the bottom by the piston cooler jet. It will be fine.
Keep in mind I have no experience with LSR vehicles and everything I know is from drag-racing. Some assembly required, batteries not included, your mileage may vary.