Ceramic Coated Piston to Cylinder Clearance Problem

[cjpacitto]

I just had my pistons coated by Swain Tech and they no longer fit in the cylinders.

I wiggled and pushed them in and it scraped off a small amount of the skirt coating. However, it is still a tight fit once in.

I'm not quite sure what to do now. Can I simply install the piston and let the piston coating wear down to create the proper tolerance or am I risking significant damage before that happens?

Do I need to bore out the cylinders? (Alusil block on a 944) I have already lapped them with AN-30 but I suppose I could hone and re-lap if necessary.

Any thoughts???

[PFM]

Do not try to run them with no clearance. The coating is slippery but must have clearance. If you have the means to open up the bore that is an option. If not contact Swain about thinning the coating or removing it. I have seen the coatings thinned with scotch bright.

Good luck,

PFM

[jpnovak]

Is your clearance issue with the thermal barrier coating or the DFL treatment to the skirt? The DFL is designed to be buffed down using a fine scotch-brite or similar material to achieve the desired clearance. Contact Swain for more details.

Some DFL treatments are designed to slough-off during run-in. This means that the coating will self limit itself to achieve the desired clearance. I always buff my coatings down prior to assembly.

[Eagledriver]

I polished mine down with scotchbright to get the clearance I wanted. Measure with a feeler guage.

-Andy

[BURN-BROS]

I verified this with JE on a set of pistons. There is no problem running them as is. I pushed one thru the cylinder,w which was alamingly tight. By the third pass the piston was dropping thru with no problem.

[tadd]

Just a data point on the other side of the fence.

I was to have my 2.8rsr mahle pistons coated on the skirts and ring lands. I was late sending the parts and the coatings got missed. Charles (L&N) re-sent them back to Millenium for re-honing after they got coated.

Just a note: Charles is a wicked stand up guy and knows his stuff. His cylinders are things of beauty. I got A+ treatment even though I had gotten his nickies NOS second hand....

I would guess its what the coating people say...

[cjpacitto]

Update...

I sent the following to Swain Tech today:

Hello, I just received my pistons back from you after being coated and discovered that they no longer fit in the cylinders. They received PC-9 on the skirts.

Prior to sending the pistons in I took the following measurements:
Avg. Piston Diameter = 3.9364"
Avg. Cylinder Bore = 3.9376"
Avg. Piston to Cylinder Clearance = 0.0012"

Now that they have been coated, the average piston diameter is 3.9390".

The engine is a Porsche 944 with an alusil block. Factory specs require a minimum of 0.0003" (0.008 mm) clearance between the pistons & cylinders.

I spoke to someone at your shop and was told to use sandpaper to buff down the thickness of the PC-9 coating. If I do this I would end up with an average thickness of the PC-9 coating of just 0.0004" in order to get my required clearance. Is that ok? Is there a minimum thickness of PC-9 I should maintain? Would I be better off enlarging the cylinders? Any thoughts or advice you can share would be greatly appreciated. Also, I was told to use 400 grit or coarser sandpaper to buff the coating. Is that correct? It seems like it should be 400 grit or finer so as not to rough the coating up too much.

Thanks in advance,

And I got the following response:

You can go either way. If you sand down some coating you risk sanding right through the coating and ending up with bare aluminum again. If you hone the cylinder you have the time and expense of the cylinder work. If it were me I would open up the cylinder but you could go either way.

I would use 400 or finer to reduce chances or removing too much coating too fast and being right back to the aluminum. The down side of 400 or finer is the paper loads up with coating quickly. Either way is acceptable but I would lean towards 400 or finer.

Regards,
Richard
Swain Tech Coatings, Inc.

I think I will hone the cylinders out a little bit using a traditional 3-arm hone then re-lap the cylinders with AN-30 silicon compound. Once that is done and I remeasure the cylinders I'll do a very light wet sanding of each piston to get the piston to cylinder clearance just right.

There doesn't seem to be much info out there in regards to actually honing the cyldiners. The factory manual does say it can be bored/honed/lapped to allow for oversized pistons, so I'm assuming I should be ok with this approach.

[BURN-BROS]

How hard is the skirt coating? Generally the skirt coatings are rather soft and defomable. Once you hone and the skirt coating breaks in, what do you recon will be your piston/cylinder clearance will be?

My advice is to manually push the piston thru a few times before honing anything. If it starts to fall thru without your input then you are good to go.

[cjpacitto]

Here is info on the skirt coating:

PC-9™ Solid Film Lubricant: PC-9™ is Swain Tech Coatings' latest generation of piston skirt coating. PC-9™ has the same low friction properties of Poly Moly™, but PC-9™ is even tougher.

As part of Swain Tech Coatings' continued commitment to product development, PC-9™ has been engineered to meet the demands of today’s high revving, short piston skirt, and long rod motors. PC-9 works well with nickasil, chrome or iron cylinder bores.

It has to be a hard coating to work with an alusil or nikasil cylinder. It will be serving the same role as the factory iron plating on the piston skirts.

Factory spec says minimum piston to cylinder clearance of 0.0003" (0.008 mm) and I'll try to be slightly larger but as close to that as possible.

I've pushed pistons through numerous times now. They do slide through but aren't anywhere close to "free fall".

[cjpacitto]

Originally Posted by Eagledriver

I polished mine down with scotchbright to get the clearance I wanted. Measure with a feeler guage.
-Andy

The factory manual says both cylinder diameter and piston diameter should be measured 61 mm below its top, which seems to be the widest part of the piston and is near the bottom of the piston skirt.

I was measuring cylinder bore and piston diameter separately then comparing the two. How exactly do you go about using the feeler gauge? Seems like you'd have to slide it all they way past the piston in the bore to measure at the prescribed spot.

[BURN-BROS]

Originally Posted by cjpacitto

Here is info on the skirt coating:


It has to be a hard coating to work with an alusil or nikasil cylinder. It will be serving the same role as the factory iron plating on the piston skirts.

Factory spec says minimum piston to cylinder clearance of 0.0003" (0.008 mm) and I'll try to be slightly larger but as close to that as possible.

I've pushed pistons through numerous times now. They do slide through but aren't anywhere close to "free fall".

Fair enough, The stuff I am familiar with is pretty soft.

I agree with honing the cylinders then. Good luck with your build!

[911pcars]

If 400 grit paper clogs, use wet-dry paper ...... with water.

Sherwood

[Steve@Rennsport]

JMHO, but I'm not sure I'd hone the Ausils.

These left the factory after being etched to allow the silicon crystals to stand proud of the aluminum bore and done for wear protection. Naturally, this was done along with the iron-coating on the pistons.

It seems to me that if you take a hone (stones or diamond) to the bore, you will be taking off those very hard silicon crystals that are the primary anti-wear surface and expose the bores to high rates of wear. BTDT, and its not good.

Although you might not wish to hear this, I would have the block honed and then plated with Nikasil to give you a good anti-wear surface that will last a very long time. The plating can be done to give you the desired piston-to-cylinder wall clearances. US Chrome does this all the time.

[Eagledriver]

The measurement with the feeler guage was done on my 911 so I just stuck the blade in from the bottom. Might be harder to do with the 944 block. You could try measuring the clearance with the pistons upside down (skirt up). The piston skirt is the tightest part, so that is where to measure. I set my clearance to .0015 inches.

-Andy

[911pcars]

Originally Posted by Eagledriver

The measurement with the feeler guage was done on my 911 so I just stuck the blade in from the bottom. Might be harder to do with the 944 block. You could try measuring the clearance with the pistons upside down (skirt up). The piston skirt is the tightest part, so that is where to measure. I set my clearance to .0015 inches.

-Andy

IMHO, trying to accurately measure 0.0015" clearance is best done with inside/outside micrometers and/or dial gauge. A feeler gauge will only provide a ball park figure. For engine longevity, it's also important to know where to measure the cylinder, especially if used.

Sherwood

[kenikh]

IIRC, the proper process to 'hone' alsusils requires machine honing, then etching to expose the silicon as Steve mentions. It's not something that I can think can be done and stay in spec, which is why most simply rering alusils and hope for the best. I don't think you can readily get oversize alusil pistons and am not sure who can coat aftermarket pistons to be compatible or even what coating today would approximate the iron based coating used in the past. As most new motors today are Lokasil type (pretty much identical to alusil), there must be a solution...I just don't know what it is.

[cjpacitto]

I wanted to add this info to address some of the questions posed and as reference material in case anyone finds it in the future while dealing with the same issue....

Stock pistons are 100.0mm. Porsche also offers 100.5mm and 101.0mm pistons for these engines. The factory manual discusses boring, honing, and "lapping" (or "finishing") proceedures. Several Pelicans have researched this and developed a pretty good method for going about the final lapping. It uses Sunnen AN-30 silicon paste and a 3 arm hone covered with felt. Info can be found at these links:
Re-ring Alusils?
Project 924 S engine replacement

Alusil is the actual alloy that makes up the engine block. This means it has the silicon particles throughout the block. It is not a surface coating like nikasil. This means that after honing you can simply "lapp" the cylinder to expose the silicon particles currently at the surface. That's really it in a nutshell thought it's actually got some fine points you have to watch out for... so again, if you are ever considering doing this, please read the links above thoroughly before doing so.

SwainTech does specifically say their PC-9 coating works well with nikasil cylinders, so I'd say it does the same thing as the iron coating and possibly even holds up better.

[lindy 911]

I'd have to disagree with the statement that the PC-9 would work in the Alusil bore. The piston coating is designed to be soft, that's why it can be sanded. The pistons for the Alusil bore are hard coated not soft coated like the PC-9 material. When you run this coating in an Alusil bore I think it will be worn off very quickly by the silicon crystal embedded in the cylinder wall. The original design was for one component to be hard and the other component to be soft. Your application has two soft components working together.

SwainTech is no different than the other coating companies out there and the piston coating they offer is designed to be used with an iron bore or nickle chrome plating, both of which are very hard. I would rethink the use of these pistons with the PC-9 coating.

Lindy

[fumanchu]

Just a data point here.

I rebuilt a 951 motor 6 years ago. I had the stock pistons coated with Swaitech's PC-9 treatment. The PC-9 treatment is a usuable replacement for the stock iron coating. It is hard unlike some coatings. Is it better than factory iron?? I dont know. When finished, that motor ran great, I must mention my piston to cylinder clearance was greater than cjpaccito's, so I was not suffering the tight clearance after the coating application.

I have talked with the current owner of the car and it is now producing 374 rwp (286 rwp at the time of build)and currently racing in canada. It has the same bottom end as before.

YMMV... to each his own I guess.

I'm gonna try it with my 911 eventually too. Someone has to.

[lindy 911]

I stand corrected.

Lindy