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Old 11-30-2003, 05:47 PM
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Default pistons


well as always im looking for ways to make my yz250 go as fast as humanly possible and its got me thinking on a certain topic.

it might be that im mistaken , but piston rings are what keeps the compression in the cyclinder : , and are located at the top of the piston so you achieve maximum compression from each power stroke .

so why exactly is there about another 10cm of piston below the rings? wouldnt it just add weight , obviously the pin and bearing still need to be there .

what stops people from dimpling the sides of the piston to reduce the friction on the cyclinder? unless the piston wall plays a major part in compression it would give you better rpm

i just saw a dude on tv surfing and saw how they reduce drag by dimpling the underside of the surfboard somewhat.

(p.s techheads feel free to call me noob if im wrong lol)
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Old 11-30-2003, 05:56 PM
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Default Re: pistons

On a 2 stroke, it is there for support...alot of big holes that piston has to go over. Thats why on the new 4 strokes they can use slipper pistons...only enuf material to hold the rings and pin.
Very simply put.(Fingers are still numb from the cold races yeasterday!)
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Old 11-30-2003, 06:42 PM
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Default Re: pistons

in a two stroke, the piston skirt acts as a "valve", covering the intake ports when it is roughly halfway up, then it covers an exhaust port when juuust about at TDC. if you look at a modern MX 4-stroke piston, like a YZ/CRF/KXF/RMZ250F, the piston is only about an inch "thick", with only enough room for the rings and the wrist pin.

if you dimpled the piston skirt, it would eventually score the cylinder walls to fit THAT piston, and another piston wouldnt work. also, when you score the walls of the cylinder to "fit" the piston dimples, the perfectly round ring wouldn't seal at all, and compression would be lost, there would be ring blowby, and it would be a mess.

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Old 11-30-2003, 11:14 PM
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Default Re: pistons

The thing that has not been mentioned is heat transfer. take away too much and you lose too much of that. the drag is minimal since the piston is cam ground and or tapered in many instances,,,,(whcih if overdone can lose heat transfer as well)
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Old 12-02-2003, 01:29 AM
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Default Re: pistons

well they say a chinaman without a piston is like a pistonless chinaman
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Old 12-02-2003, 04:53 AM
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Default Re: pistons

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