Weiseco's or JE's????

[Reaper1]

Supposedly the coating adheres to the aluminum at a molecualr level, so even thought the "visable" coating is gone, it is still effective because it is bonded to the aluminum...just info I've gathered over time from different sources...

[DirectConnection]

Quote:

Originally Posted by all-trac I've seen and measured skirt coatings that were .0005" thick per side, resulting in a total increase of .001". Most coaters can vary the thickness of the coating on the skirt, and can make it extra thick so it is .001" per side. That being said, I've never seen a skirt coating that didn't wear off exactly where the piston skirt rides on the cylinder bore. Be it Swain or an OEM coating on a piston. I would not personally take into account the thickness of the coating for bore clearances.

Hmm... Anyone want to buy some new 2.5 JE's with the Swain skirt coating?

[puppet]

Why not machine teflon plates/pads/buttons into the skirts?

Teflon buttons? That's what the top fuel guys use to do to try and save the piston skirts. This invisble coating that has bonded to the aluminum, anyway to see that under a microscope? Or scrape the skirt and send it out to be analyzed? Or maybe the surface finish could be checked and see if it is any different?

I have my personal doubts about the invisible coating. The aluminum/alloy would have to be brought to such a temperature that the coating and the alloy were chemically fused. If this were done, the color of the alloy would not remain the same after the outer layer was removed. I've heard horror stories when coating places use to heat the piston to apply the coating and the alloy would lose it's hardness, resulting in soft pistons. But hey, I'm no scientist.

[TRRBOJEEP]

There is one other piece of the puzzle that hasn't been mentioned yet. Is the piston to bore clearance measured from the top of the piston or the bottom (skirt)? The reason I mention this is the Ross pistons I have in my 2.5 are actually machined on a taper. (the diameter of the piston at the top is 3.8974" and at the bottom of the skirt they measure 3.9010" I'm assuming they're machined this way because the top of the piston sees more heat and expansion than the bottom (skirt) does? And, therefore, using that logic, I'm assuming the bore clearances would be set up using the diameter of the top of the piston. Anyway here is Ross's recomendations for their forged pistons bore clearances for noted applications:

Blown gas street................................. .010"
Blown fuel & blown gas street........ 012 - .014"
Normally aspirated street cars................ .007"
Blown alcohol...................................... .011"
Modified type engines, including drags,
circle track and road race............ .008 - .009"

I've got 10K miles of hammering so far, and all is good with the Ross's. Because they obviously "grow" quite a bit in the cylinders, I try to always have the motor warmed up fully before laying into it.

The piston is measured is .500" up from the bottom of the skirt. Those clearances that Ross list looks like recommendations from a long time ago, those clearances would be on the extremely loose side. Ross does machine their skirts on a straight taper. You are correct that the top of the piston is smaller than the skirt for heat expansion from the combustion chamber. You can run a Ross with .004"-.005" clearance with no problem, but they will never recommend them that tight.

[Andre Parker]

The coating on the skirts fills in the voids in the metal. While it may look smooth to the naked eye, look at the surface under a microscope and the surface looks like the moon. The coatings fill in between the low spots and the metal on the high spots is still between the low spots. There have been extensive tests for wear reduction for most of these coatings. I have had good luck with them and will continue to use them.

Seems like you could still see the coating without a microscope if it is just in the low spots. I was taught that a skirts slightly rough surface was to carry oil. I've had great luck without the coatings. I've never seen any extensive or unbiased tests, just praise from the person selling the coating.

[puppet]

Those fine ridges/valleys in the piston carry the oil. Coating the skirts has to fill those up. Seems like you'd be trading one ability for another. .. better oil retention replaced by slick coating.
What happens when the coating wears off the ridge? You've got neither ability. ...poor oil retention because the little valleys are plugged with coating. The coating is worn off the little ridges which are now rubbing against the cylinder walls with less lubrication present.

[Koreth]

what about the hone on the cyl walls? does oil not stay on the cyl walls because of all the pits and valleys the hone creates?

[Reaper1]

The coating absorbs some oil...that's why it works so well for cold start up...

[puppet]

Quote:

Originally Posted by Koreth what about the hone on the cyl walls? does oil not stay on the cyl walls because of all the pits and valleys the hone creates?

I'm sure it does to some degree. Seems that the engineers are moving away from the more agressive honing of the past though. With todays parts the trend is to more of a finer cylinder wall finish.

[turbovanman]

Based on this info and other variables, If the price is right, I am going to get it done. Might as well, the pistons are brand new.