<U>Engine Builders</U>

How does skirt coating affect piston to wall clearance? CP calls for .0035" clearance. Right now my pistons are 3.3060" with my Swaintech PC-9 coating and my bores are 3.3085" leaving me with only .0025" piston to wall clearance instead of the recommended .0035".

Will .0025" be enough to run on a high boost car? If not, I can open up the bores another thousandths but then what happends if/when the skirt coating wears off? Then my clearance will be too much. The coating is approx .001" thick on each side.

Someone also suggested with the gold coat thermal coating my pistons tops recieved, they may not heat up and expand as much. Meaning they will not need as much piston to wall clearance.

 

You might talk to swain tech and ask them how the coating will react to being run/heat. If i had to choose w/o talking to them, i'd bore the sleeves to get the clearances right. If the coating wears, you'll be a bit on the loose side, which is MUCH better than being on the tight side!!

 

Your right Id rather be too loose than too tight. But Im getting conflicting information. My engine shop says I'll be fine with what I have but they're just some local domestic shop.

Swain's tech said if CP recommends .0035" he suggested .0030". But it sounded like he was just pulling a number out of his ***. Should I seriously pay to have my block re-honed just for half a thousandths? Everything is ready to assemble now.

 

Sounds like you have .001 difference right now, that's a ton in this situation. Pull it now and bore it, I promise you it's easier to to now than it will be in a month, along with buying new pistons...

 

Alright I will. Thanks for the advice Tony.

 

i didnt think those coating went on that thick......

i would bore/hone to .0030 and once the coating wears off you will probably land right at where it should be.

 

Swaintech says their "PC-9" coating is applied extra thick with the intention that it is supposed to be permanent.

If Im going to go through the hassle of having it honed again Im going to have a full thousandths removed.

 

i didnt think something like this, meant to help keep the headaches away is causing more stress...not bashing, just stating that i would be going nuts if i where in your shoes.

you want something that keeps your pistons cooler and what not...then you have to worry about the coating coming off...kinda sucks

good luck tho...kinda makes me rethink about coating my pistons

 

the only reason the coating will come off is if you run w/o an air filter and get dirt in the motor. as long as it stays clean you wont have any problems.

 

If the coating is .001 per side it will wear down and bed in and you will lose that .002 in clearance(well actually you will gain it).
On my coatings when I do the thermal dome and dry film skirt coating I advise no adjustment in bore change and ask for the piston bores to be finished before I coat. Reason being is that when I prep the skirt I lightly sandblast the skirt which is actually making it smaller , then I apply the coating fully knowing that it will bed down to a tenth or so and the piston to wall will increase(dry films actually impregnate into the piston itself so when it visually buffs off its actually still there doing it's job) The piston as stated earlier will also expand alot less since the domes are coated keeping the heat in the chamber and not absorbed into the pistons causing the expansion.
One of the benifits of coating the piston is in fact that you can run tighter clearances.
THIS INFO IS ON WHAT I USE, so theirs might be different.
I'll give you an example though, my pistons were .002 once coated and put into my motor, which is going on 3 years old and well over 300 dyno pulls.

Now to mess you up even more(sorry) When I coat Cp pistons I usually tell the customer to have the final bore be a few tenths more then normal because I have seen them expand worse then any other piston I have coated. if thats not possible I make a change in my process slightly to accomode those few tenths.

 

The coating is probably hard anodized coating. It's probably 2-3 mils (.002-.003") thick. The coating is almost as strong as the steel sleeve which is better than the regular aluminum in other pistons. I almost hard anodized my pistons (SRP) just for the hell of it. I'm a chemist that works with aluminum extrusions all the time. Your clearance is probably good (.0025"), but if CP recommends the 0.0035 then I'd do that. Go ahead and bore becasue your pistons will still expand due to heat long with the hard coating. Those should last longer than the rest of your life if everything is right.

 

I HIGHLY doubt his pistons are hard anodized. They appear to be dry film moly B coated.
When you try to hard anodize a piston you run into a problem, the ring grooves, you cant hard ano a skirt and keep it off the ring area and the rest of th piston without adding a ton of expense to the process.
If you add .002 to a ring grove your screwed.

 

You also wouldn't want the piston to be stronger then steel because when it comes in contact with the wall it would tear it up. The piston and coating should always be softer then the cyl.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tbone &raquo;</TD></TR><TR><TD CLASS="quote">If the coating is .001 per side it will wear down and bed in and you will lose that .002 in clearance(well actually you will gain it).
On my coatings when I do the thermal dome and dry film skirt coating I advise no adjustment in bore change and ask for the piston bores to be finished before I coat. Reason being is that when I prep the skirt I lightly sandblast the skirt which is actually making it smaller , then I apply the coating fully knowing that it will bed down to a tenth or so and the piston to wall will increase(dry films actually impregnate into the piston itself so when it visually buffs off its actually still there doing it's job) The piston as stated earlier will also expand alot less since the domes are coated keeping the heat in the chamber and not absorbed into the pistons causing the expansion.
One of the benifits of coating the piston is in fact that you can run tighter clearances.
THIS INFO IS ON WHAT I USE, so theirs might be different.
I'll give you an example though, my pistons were .002 once coated and put into my motor, which is going on 3 years old and well over 300 dyno pulls.

Now to mess you up even more(sorry) When I coat Cp pistons I usually tell the customer to have the final bore be a few tenths more then normal because I have seen them expand worse then any other piston I have coated. if thats not possible I make a change in my process slightly to accomode those few tenths.

</TD></TR></TABLE>

Good info. Now, how long will it typically take to "bed down" the coating on a piston skirt? This could change the way someone with a coating breaks their motor in.

 

Thanks for the conflicting information everyone! lol Some say Im fine, some say Im not.

Darius from CP Pistons had this to say "When using a coating we do not put in any extra clearance, you will be fine at your present bore size."

 

I don't know how he can say something like that when different coatings are different thickensses?? Bottom line, you now have .0025" clearance, right? CP recommmends .0035", right?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">I don't know how he can say something like that when different coatings are different thickensses?? Bottom line, you now have .0025" clearance, right? CP recommmends .0035", right?</TD></TR></TABLE>

It's the same thing that TBone said sooo

 

No he didn't. Blaze the Chemi said that.

Tbone said with the heat coating the piston wont expand as much allowing for tighter clearnaces. Also Tbone says the material becomes impregnanted into the piston skirt when it wears down, increasing clearance over time.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">No he didn't. Blaze the Chemi said that.

Tbone said with the heat coating the piston wont expand as much allowing for tighter clearnaces. Also Tbone says the material becomes impregnanted into the piston skirt when it wears down, increasing clearance over time.</TD></TR></TABLE>

AH lemme reitterate what I am saying, TBone agrees to have your block bored and honed to the manufacturers specs and not take into account the coating, I KNOW since I had my pistons, well actually I have had 5-6 sets of pistons sent to TBone for coating and we have always bored and honed the block per the manufacturers specs then had the pistons coated

 

Ok thats a clearer answer. But what happends when the coating wears down? Your clearance will increase. This sucks, I shouldnt have even got them coated.

 

yea they increase...but bigger gap is bettter than tighter...but your still at least at the cp spec when you bore.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tbone &raquo;</TD></TR><TR><TD CLASS="quote">You also wouldn't want the piston to be stronger then steel because when it comes in contact with the wall it would tear it up. The piston and coating should always be softer then the cyl.</TD></TR></TABLE>

When did I say that?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by blaze the chemi &raquo;</TD></TR><TR><TD CLASS="quote">The coating is probably hard anodized coating. It's probably 2-3 mils (.002-.003") thick. <U>The coating is almost as strong as the steel sleeve which is better than the regular aluminum in other pistons.</U> I almost hard anodized my pistons (SRP) just for the hell of it. I'm a chemist that works with aluminum extrusions all the time. Your clearance is probably good (.0025"), but if CP recommends the 0.0035 then I'd do that. Go ahead and bore becasue your pistons will still expand due to heat long with the hard coating. Those should last longer than the rest of your life if everything is right. </TD></TR></TABLE>

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">Ok thats a clearer answer. But what happends when the coating wears down? Your clearance will increase. This sucks, I shouldnt have even got them coated.</TD></TR></TABLE>

What did you have them coated with? What process? Some of my largest clients are Ford, Diamler-Chrystler, and companies that process this stuff we are talking about, or similar. When they have a problem, they call me up.

 

Swaintech PC-9 is a Dry Film. They tell me it is extremely tough and is intended to be a permanent coating. However if the piston were to expand enough to stick, the coating would be sacrificed. Unless something extreme happends they told me the coating will last as long as the pistons.

I also had them apply their Gold Coat Cermanic coating to the tops.

 

I'm not sure what that means, because I don't work for them. Dry film could be anything because some places name different processes different things. If you knew what it was coated with exactly, not Swaintech PC-9, we might be able to help you with the expansion rate.

 

if you just did an agressive and prolonged hone job, would it open it up enough to measure?


or would you have to bore it out?