Piston skirt treatment study

What are you trying to achieve by coating pistons - less friction or better fit and more compression ? One thing I never worry about unless I am using methanol fuel, is excessive bore clearance. With methanol fuel is common to use the motor without warming it up for long enough to avoid the pistons rattling and potentially becoming damaged. However new piston rings and a good valve job are very important.

Tighter fit without risk of seizing means less piston rocking, which means longer ring life. Doesn't have any effect on compression as long as rings are still good. Also not much effect on friction, which is mostly from the rings. Probably helpful for safe break-in too. Less cold piston clatter is nice too, but not really so significant from a performance standpoint. Seems to me like it would be particularly useful for a street bike that expects high mileage. JE offers similar coatings as an option, but I've so far not tried them.

Ken

There is a conflict of interests here. JE says that a piston cam shape that is more out of round and protrudes more in the center of the skirt (so that the rest of the piston does not rub the cylinder) creates more power but that could be at some sacrifice to longevity.

And, Jim, for someone in your position, working along with customers in reading multiple sets of pistons, the exact dimensional shapes - barrel, taper, roll - can be deduced from different engines, according to piston type - forged/cast, piston design - slipper/traditional/pin boss shapes, piston alloy -hi/lo silicon, and other piston design factors such as pin types and locations, ring types and locations, and so on.

Neat stuff

On the other hand -

Part 3:
"A Study of Friction and Lubrication Behavior for Gasoline Piston Skirt Profile Concepts"

Kwang-soo Kim, Paras Shah,
Federal-Mogul Corporation
Masaaki Takiguchi / Shuma Aoki,
Musashi Institute of Technology

Copyright © 2009 SAE International


Can't figure how to copy my pdf of that paper to here - it is about skirt surface profiles but not coatings; conclusion is that a 30-40 micron (3-4 thou) rectangular recess below the rings on the thrust face coupled with a similar square recess in the center of the thrust face results in least noise and least friction across all engine speeds and temperatures; the recesses apparently create a captured patch of thicker oil film.

Test engine is a 86x86 500 single at 5 thou clearance with conventional rings. I failed to note of who referred me to the article.


Edit - found a link
http://www.federalmogul.com/en-US/Media/Documents/HighqualitySAEpaper200909PFL1163manuscript2.pdf

We've come full circle on this one.
The link was provided by hobot, and I thought he was laying claim to an uncanny ability to use a hand file for carving a pattern in a piston to a depth of only 1.5 millionths of an inch.
This was due to a math error on my part, and my inability to delete the first post, where I quote him and the link.

xbacksideslider said:

And, Jim, for someone in your position, working along with customers in reading multiple sets of pistons, the exact dimensional shapes - barrel, taper, roll - can be deduced from different engines, according to piston type - forged/cast, piston design - slipper/traditional/pin boss shapes, piston alloy -hi/lo silicon, and other piston design factors such as pin types and locations, ring types and locations, and so on.

Neat stuff

Click to expand...

I worked with JE in choosing what I think is the best shape for the lightweight pistons. There are different barrel shapes.