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Location:
QLD
Registered:
October 2004
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question on pistons
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Sun, 28 August 2005 09:00
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hey guys i got 1 quik question about forged pistons.
say if u were to get 20thou oversized. how much bigger would that be in cm's than standard. also would u need to bore the block? and why are forgies so great i was told they eliminate piston knoc. any info would be appreciated
thanks
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Location:
Montrose, VIC
Registered:
May 2002
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Re: question on pistons
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Sun, 28 August 2005 09:13
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20 thou = 20/1000 inches. You can work the rest out...
Naturally a bigger piston will be wanting a bigger hole to go in. As for 'eliminating piston knock', presuming that someone means that lovely loose rattling sound when could, well no, forgies generally make this problem worse - they need too be fitted with more clearance, as they expand more with heat.
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Location:
On your mum!
Registered:
May 2002
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Re: question on pistons
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Sun, 28 August 2005 09:20
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I agree with mrshin re piston knock. Forged pistons are tougher and can withstand detonation better than a cast item.
20 thou = close enough to 1/2mm (I have never met anyone who speaks in cm when talking ANYTHING engineering related).
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Location:
QLD
Registered:
October 2004
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Re: question on pistons
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Sun, 28 August 2005 09:21
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thanks for that
i was told that arias have a silicon conpound or some crap like that whick eliminates that but im probably wrong
thanks
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Location:
South Australia
Registered:
July 2002
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Re: question on pistons
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Mon, 29 August 2005 01:43
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Do forged pistons usually come with a recommended clearance (between skirt and bore)?
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Toymods Vice President
Location:
Sydney
Registered:
May 2002
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Re: question on pistons
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Mon, 29 August 2005 02:24
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Yes aftermarket pistons will come with a spec sheet that indicates the relevant bore measurements for skirt clearance.
Yes Arias do have a range of pistons with higher silicon content which makes them harder and reduces the thermal expansion rate. This means they can be run with less bore clearance when cold because they don't grow as much when hot. On the other hand "hard" is not really what you're looking for in a piston, it makes them more brittle and likely to crack.
Taking an 86mm bore as an example most forged pistons specify a 3.5 thou skirt clearance. My Arias pistons specified 2.5. All of the newer Arias pistons I have seen though specify 3.5 thou. They changed the design to be more like the other cheap forgies like JE etc.
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Location:
South Australia
Registered:
July 2002
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Re: question on pistons
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Mon, 29 August 2005 02:35
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Yeah i have a set of JE pistons for my 3S-GTE but they came second hand with no spec sheet  Will have to email them to find out what clearance they recommend.
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