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Pros, Cons on piston surface

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Posted by: SirHilton17

What are the pros, and Cons of having a polished, Crosshatched,or normal surface on a piston....

I can see the polished piston having less drag, crosshatched piston holding oil for better life

what are you guys views?


Posted by: QKENUF4U

if your talking about just the crown then i can give my .02$.
i had an old 81 YZ465 that i pulled the top end on to have my mech. check the ring end gap etc. so i could tell if it was wore out yet or not. while i had it apart i polished the piston and chamber in the head (1000/1500 grit w/ WD40 lube). about 3 months later i added a DG exhaust so i looked up the exhaust port to see how much carbon had built up. i had one quarter size carbon spot right in the middle of the piston (very light) while the rest looked like it did when i cleaned it up. chamber was very clean w/ NO carbon deposits etc.
plus it helpes prevent pre-ignition and detonation since there are no sharp edges to ignite the fuel.


Posted by: Jaybird

Quote:
Originally Posted by QKENUF4U
plus it helpes prevent pre-ignition and detonation since there are no sharp edges to ignite the fuel.
Did this pearl come from your mechanic?


Posted by: ellandoh

looks good on paper


Posted by: Masterphil

I remember reading a post by either Rich or Eric where they were experimenting on drilling dimples into the face of a piston to improve the mixture burning. I think it was a yzf444 piston, and they based the location of the dimples on where previous carbon build up was found.

The rougher the surface of the piston/combustion chamber, the more turbulence they will create during the combustion process. It's just a matter of wether it is a significant change or not.

I'm not buying that sharp edge pre-ignition crap. Think about the valve reliefs on a thumper piston. Yzf's have five, they must be some pre-igniting MOFO's, huh.

The bottom line is that I don't think that there really is anything to be gained from polishing these surfaces, besides reduced carbon buildup. Likely not any noticible power to be had.


Posted by: nephron

The pros in auto are still deburring valve reliefs and thermalcoating. Also, J&E makes a pre-dimpled piston...with no sharp edges, of course.


Posted by: Masterphil

When I was talking about sharp edges, I guess I was meaning more of a protrusion, rather than a casting flaw(burr). Pretty much any edge on a quality high-performance piston is going to be radiused or champhered, free from any such burrs.


Posted by: cujet

Short of ceramic coating (which I do not like) I am not convinced there is anything to be gained by minor changes in the piston surface. As cast or machined seems to be just fine in most applications.

Years ago I polished half of the piston top. Guess what? Carbon built up in the standard fashon and I could not determine any difference in the level of carbon buildup. While such a simple test is of course, not conclusive. At the time it was enough for me.

Chris


Posted by: QKENUF4U

all i know is that the BING BING BING (detonation) going down hills w/ throttle closed almost ceased and the carbon build up after 3 months was almost non-existant. as for the pre-ignition/detonation i was talking about 2-strokes more than 4.


Posted by: gwr

chris, do a study on dlc coated pistons. i have been using them in racing 4 strokes with good results, also tried one in an 85 cc ktm, was well worth the effort.



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